Michael W Riggs

Interests

Teaching

Comparative Pathology

Research

Immunobiology and control of cryptosporidiosis - drug discovery, vaccines, immunotherapeutics

Courses

Scholarly Contributions

Chapters

• Riggs, M. W., & Schaefer, D. A. (2019). Calf Clinical Model of Cryptosporidiosis for Efficacy Evaluation of Therapeutics.. In Methods and Protocols in Cryptosporidium Research, Lab Protocol Series on Methods in Molecular Biology, Springer Nature(pp 253-282). Springer Nature (e-published ahead of print August 27, 2019).
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  Invited book chapter. Riggs M.W., Schaefer D.A. Calf Clinical Model of Cryptosporidiosis for Efficacy Evaluation of Therapeutics. In: Mead J., Arrowood M. (eds) Cryptosporidium. Springer Nature Methods in Molecular Biology, Chapter 15, vol 2052: 253-282. Humana, New York, NY. e-published ahead of print August 27, 2019.

Journals/Publications

• Riggs, M. W., Voorhis, W. C., Arnold, S. L., Tam, P. I., Betzer, D. P., Schaefer, D. A., Joseph, S., Woods, A. K., Chi, V., McNamara, C. W., Love, M. S., & Zhang, C. X. (2022). Pharmacokinetics and Pharmacodynamics of Clofazimine for Treatment of Cryptosporidiosis. Antimicrobial Agents and Chemotherapy. doi:10.1128/aac.01560-21
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  Infection with Cryptosporidium spp. can cause severe diarrhea, leading to long-term adverse impacts and even death in malnourished children and immunocompromised patients. The only FDA-approved drug for treating cryptosporidiosis, nitazoxanide, has limited efficacy in the populations impacted the most by the diarrheal disease, and safe, effective treatment options are urgently needed. Initially identified by a large-scale phenotypic screening campaign, the antimycobacterial therapeutic clofazimine demonstrated great promise in both in vitro and in vivo preclinical models of Cryptosporidium infection. Unfortunately, a phase 2a clinical trial in HIV-infected adults with cryptosporidiosis did not identify any clofazimine treatment effect on Cryptosporidium infection burden or clinical outcomes. To explore whether clofazimine's lack of efficacy in the phase 2a trial may have been due to subtherapeutic clofazimine concentrations, a pharmacokinetic/pharmacodynamic modeling approach was undertaken to determine the relationship between clofazimine in vivo concentrations and treatment effects in multiple preclinical infection models. Exposure-response relationships were characterized using Emax and logistic models, which allowed predictions of efficacious clofazimine concentrations for the control and reduction of disease burden. After establishing exposure-response relationships for clofazimine treatment of Cryptosporidium infection in our preclinical model studies, it was unmistakable that the clofazimine levels observed in the phase 2a study participants were well below concentrations associated with anti-Cryptosporidium efficacy. Thus, despite a dosing regimen above the highest doses recommended for mycobacterial therapy, it is very likely the lack of treatment effect in the phase 2a trial was at least partially due to clofazimine concentrations below those required for efficacy against cryptosporidiosis. It is unlikely that clofazimine will provide a remedy for the large number of cryptosporidiosis patients currently without a viable treatment option unless alternative, safe clofazimine formulations with improved oral absorption are developed. (This study has been registered in ClinicalTrials.gov under identifier NCT03341767.).
• Zhang, C., Love, M., McNamara, C., Chi, V., Woods, A., Joseph, S., Schaefer, D. A., Betzer, D. P., Riggs, M. W., Pui-Ying, T., Van Voorhis, W., & Arnold, S. (2022). Pharmacokinetics and pharmacodynamics of clofazimine for treatment of cryptosporidiosis.. Antimicrobial Agents and Chemotherapy, 66, 1-17.
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  Submitted 5 August 2021. Returned for modification 19 September 2021 Accepted 1 November 2021. Published January 2022 Volume 66 Issue 1 e01560-21 Antimicrobial Agents and Chemotherapy aac.asm.org 1, EXPERIMENTAL THERAPEUTICS, 18 January 2022 Zhang C., Love M., McNamara C., Chi V., Woods A., Joseph S., Schaefer D., Betzer D., Riggs M., Pui-Ying T, Van Voorhis W. and S Arnold. 2021. Pharmacokinetics and pharmacodynamics of clofazimine for treatment of cryptosporidiosis.
• Winzer, P., Winzer, P., Whitman, G. R., Whitman, G. R., Voorhis, W. C., Voorhis, W. C., Vidadala, R. S., Vidadala, R. S., Tzipori, S., Tzipori, S., Schaefer, D. A., Schaefer, D. A., Sanchez-sanchez, R., Sanchez-sanchez, R., Riggs, M. W., Riggs, M. W., Ortega-mora, L. M., Ortega-mora, L. M., Ojo, K. K., , Ojo, K. K., et al. (2021). One health therapeutics: Target-Based drug development for cryptosporidiosis and other apicomplexa diseases.. Veterinary parasitology, 289, 109336. doi:10.1016/j.vetpar.2020.109336
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  This is a review of the development of bumped-kinase inhibitors (BKIs) for the therapy of One Health parasitic apicomplexan diseases. Many apicomplexan infections are shared between humans and livestock, such as cryptosporidiosis and toxoplasmosis, as well as livestock only diseases such as neosporosis. We have demonstrated proof-of-concept for BKI therapy in livestock models of cryptosporidiosis (newborn calves infected with Cryptosporidium parvum), toxoplasmosis (pregnant sheep infected with Toxoplasma gondii), and neosporosis (pregnant sheep infected with Neospora caninum). We discuss the potential uses of BKIs for the treatment of diseases caused by apicomplexan parasites in animals and humans, and the improvements that need to be made to further develop BKIs.
• Arnold, S. L., Voorhis, W. C., Woods, A. K., Tam, P. Y., Schaefer, D. A., Riggs, M. W., Mcnamara, C. W., Love, M. S., Joseph, S. B., Hermann, D., Conrad, T., Chi, V., Betzer, D. P., & Barrett, L. K. (2020). Predicting the impact of infectious diarrhea pathophysiology on drug disposition. The FASEB Journal, 34(S1), 1-1. doi:10.1096/fasebj.2020.34.s1.07532
• Choi, R., Hulverson, M. A., Wang, W., Vidadala, R. S., Whitman, G. R., Barrett, L. A., Schaefer, D. A., Betzer, D. P., Riggs, M. W., Doggett, J. S., Hemphill, A., Ortega-Mora, L. M., McCloskey, M. C., Arnold, S. M., Hackman, R. C., Marsh, K. C., Lynch, J. J., Leroy, B. E., Freiberg, G. M., , Kempf, D. J., et al. (2020). Bumped Kinase Inhibitors as Therapy for Apicomplexan Parasitic Diseases: Lessons Learned. International Journal for Parasitology., 50, 413-422. doi:doi.org/10.1016/j.ijpara.2020.01.006
• O'Connor, R. M., Nepveux, F. J., Abenoja, J., Bowden, G., Reis, P., Beaushaw, J., Relat, R., Driskell, I., Gimenez, F., Riggs, M. W., Schaefer, D. A., Schmidt, E. W., Lin, Z., Distel, D. L., Clardy, J., Ramadhar, T., Allred, D. R., Fritz, H., Rathod, P., , Chery, L., et al. (2020). A symbiotic bacterium of shipworms produces a compound with broad spectrum anti-apicomplexan activity.. PLOS Pathogens., 16, 1-21. doi:https://doi.org/10.1371/journal.ppat.1008600
• Schaefer, D. A., & Riggs, M. W. (2020). Calf Clinical Model of Cryptosporidiosis for Efficacy Evaluation of Therapeutics.. Methods in molecular biology (Clifton, N.J.), 2052, 253-282. doi:10.1007/978-1-4939-9748-0_15
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  Cryptosporidiosis, caused by the apicomplexan parasite Cryptosporidium parvum, is a moderate-to-severe diarrheal disease now recognized as one of the leading causes of morbidity and mortality in livestock globally, and in humans living in resource-limited parts of the world, particularly those with AIDS or malnourished individuals. This recognition has fueled efforts for the discovery of effective therapeutics. While recent progress in drug discovery has been encouraging, there are presently no acceptably effective parasite-specific drugs for the disease. The urgent need for new drug discovery or drug repurposing has also increased the need for refined animal models of clinical disease for therapeutic efficacy evaluation. Here, we describe an acute model of cryptosporidiosis using newborn calves to evaluate well-defined clinical and parasitological parameter outcomes, including the effect on diarrhea severity and duration, oocyst numbers produced, and multiple measures of clinical health. The model is highly reproducible and provides unequivocal direct measures of treatment efficacy on diarrhea severity and parasite replication.
• Van Voorhis, W. C., Choi, R., Wang, W., Arnold, S. L., Schaefer, D. A., Betzer, D. P., Vidadala, R. S., Lee, S., Whitman, G. R., Barrett, L. K., Maly, D. J., Riggs, M. W., Fan, E., Kennedy, T. J., Tzipori, S., Doggett, J. S., Winzer, P., Anghel, N., Imhoff, D., , Muller, J., et al. (2020). One Health Therapeutics: Target-Based Drug Development for Cryptosporidiosis and Other Apicomplexa Diseases.. Veterinary Parasitology., Available online 15 December 2020.. doi:doi.org/10.1016/j.vetpar.2020.109336
• VanDussen, K. L., Funkhouser-Jones, L. J., Akey, M. E., Schaefer, D. A., Ackman, K., Riggs, M. W., Stappenbeck, T. S., & Sibley, L. D. (2020). Gut metabolites influence susceptibility of neonatal mice to cryptosporidiosis.. mBio, 11, 1-16. doi:doi.org/10.1128/mBio.02582-20
• Heo, I., Dutta, D., Schaefer, D. A., Iakobachvili, N., Artegiani, B., Sachs, N., Bowden, G., Hendrickx, A., Willems, R., Peters, P. J., Riggs, M. W., O'Connor, R. C., & Clevers, H. (2018). Modeling Cryptosporidium infection in human small intestinal and lung organoids. Nature Microbiology, 3, 814-823. doi:https://doi-org.ezproxy1.library.arizona.edu/10.1038/s41564-018- 0177-8
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  Returned for revision August 24, 2017. Revisions completed and manuscript resubmitted February 12, 2018. Accepted for publication 3/27/2018. Published 6/25/2018.
• Arnold, S. S., Choi, R., Hulverson, M. A., Schaefer, D. A., Vinayak, S., Vidadala, R. S., McCloskey, M. C., Whitman, G. R., Huang, W., Barrett, L. K., Ojo, K. K., Fan, E., Maly, D., Riggs, M. W., Striepen, B., & Van Voorhis, W. C. (2017). Necessity of Bumped Kinase Inhibitor Gastrointestinal Exposure in Treating Cryptosporidium Infection.. J. Infectious Diseases, 216, 55-63.
• Huang, W., Choi, R., Hulverson, M. A., Zhang, Z., McCloskey, M. C., Schaefer, D. A., Barrett, L. K., Vidadala, R. S., Riggs, M. W., Maly, D. J., Van Voorhis, W. C., Ojo, K. K., & Fan, E. (2017). 5-Aminopyrazole-4-carboxamide based compounds prevent the growth of Cryptosporidium parvum. Antimicrobial Agents and Chemotherapy, 61, 1-6.
• Hulverson, M. A., Choi, R., Arnold, S. M., Schaefer, D. A., Hemphill, A., McCloskey, M. C., Betzer, D. P., Muller, J., Vidadala, R. R., Whitman, G. R., Rivas, K. L., Barrett, L. K., Hackman, R. C., Love, M. S., McNamara, C. W., Shaughnessy, T. K., Kondratiuk, A., Kurnick, M., Banfor, P. N., , Lynch, J. J., et al. (2017). Advances in bumped-kinase inhibitors for human and animal therapy of cryptosporidiosis. International Journal for Parasitology, 47, 753-763.
• Hulverson, M. A., Vinayak, S., Choi, R., Schaefer, D. A., Castellanos-Gonzalez, A., Vidadala, R. S., Brooks, C. F., Herbert, G. T., Betzer, D. P., Whitman, G. R., Sparks, H. N., Arnold, S. L., Rivas, K. L., Barrett, L. K., White, A. C., Maly, D. J., Riggs, M. W., Striepen, B., Van Voorhis, W. C., & Ojo, K. K. (2017). Bumped-Kinase Inhibitors for Therapy of Cryptosporidiosis. Journal of Infectious Diseases, 215, 1275-84.
• Van Voorhis, W. C., Doggett, S. J., Parsons, M., Hulverson, M. A., Choi, R., Arnold, S., Riggs, M. W., Hemphill, A., Howe, D. K., Mealey, R., Lau, A. O., Merritt, E. A., Maly, D. J., Fan, E., & Ojo, K. K. (2017). Extended-Spectrum Antiprotozoal Bumped Kinase Inhibitors. Experimental Parasitology, 180, 71-83.
• Schaefer, D. A., Betzer, D. P., Smith, K. D., Millman, Z. G., Michalski, H. C., Menchaca, S. E., Zambriski, J. A., Ojo, K. K., Hulverson, M. A., Armold, S. L., Rivas, K. L., Vidadala, R. S., Huang, W., Barrett, L. K., Maly, D. J., Fan, E., Van Voorhis, W. C., & Riggs, M. W. (2016). Novel Bumped Kinase Inhibitors Are Safe and Effective Therapeutics in the Calf Clinical Model for Cryptosporidiosis. Journal of Infectious Diseases, 214, 1856-64 plus supplementary online.
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  Submitted as a MAJOR ARTICLE 8-9-2016. Accepted 10-10-2016. Published 12-15-2016.
• Viswanathan, V. K., Vedantam, G., Riggs, M. W., Wilbur, J. S., Byrd, W., Ramamurthy, S., Ledvina, H. E., Khirfan, K., & Boedeker, E. C. (2015). The Secreted Effector Protein EspZ Is Essential for Virulence of Rabbit Enteropathogenic Escherichia coli. Infection and Immunity, 83(3), 1139-1149. doi:10.1128/iai.02876-14
• Wilbur, J. S., Byrd, I. W., Ramamurthy, S., Ledvina, H., Khirfan, K., Riggs, M. W., Boediker, E., Vedantam, G., & Viswanathan, V. (2015). The secreted effector protein EspZ is Essential for virulence of rabbit enteropathogenic Escherichia coli. Infection and Immunity, 83(3), 1139-49. doi:10.1128
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  Attaching and effacing (A/E) pathogens adhere intimately to intestinal enterocytes and efface brush border microvilli. A key virulence strategy of A/E pathogens is the type III secretion system (T3SS)-mediated delivery of effector proteins into host cells. The secreted protein EspZ is postulated to promote enterocyte survival by regulating the T3SS and/or by modulating epithelial signaling pathways. To explore the role of EspZ in A/E pathogen virulence, we generated an isogenic espZ deletion strain (ΔespZ), and corresponding cis-complemented derivatives, of rabbit enteropathogenic Escherichia coli, and compared their ability to regulate the T3SS and influence host cell survival in vitro. For virulence studies, rabbits infected with these strains were monitored for bacterial colonization, clinical signs and intestinal tissue alterations. Consistent with earlier reports, espZ-transfected epithelial cells were refractory to infection-dependent effector translocation. Also, compared to the parent and complemented strains, ΔespZ induced greater host cell death. In the rabbit infections, fecal ΔespZ levels were ten-fold lower than the parent strain one day post-infection, while the complemented strain was recovered at intermediate levels. In contrast to the parent and complemented mutants, ΔespZ fecal carriage progressively decreased on subsequent days. ΔespZ-infected animals gained weight steadily over the infection period, failed to show characteristic disease symptoms, and displayed minimal infection-induced histological alterations. TUNEL staining of intestinal sections revealed increased epithelial cell apoptosis on Day 1 post-infection with ΔespZ, as compared to animals infected with the parent or complemented strains. Thus, EspZ-dependent host-cell cytoprotection likely prevents epithelial cell death and sloughing and, thereby, promotes bacterial colonization.
• Peace, A. C., & Riggs, M. W. (2014). Gossypiboma-induced Abdominal Fibrosarcoma in a German Shepherd Dog.. Veterinary Medicine, 109(11), 346-350.
• Thivierge, K. S., Cotton, S., Schaefer, D. A., Riggs, M. W., To, J., Lund, M. E., Robinson, M. W., Dalton, J. P., & Donnelly, S. (2013). Cathelicidin-like Helminth Defence Molecules (HDMs): Absence of Cytotoxic, Anti-microbial and Anti-protozoan Activities Imply a Specific Adaptation to Immune Modulation. PLOS Neglected Tropical Diseases, 7(7), 2307-2321.
• Thivierge, K., Cotton, S., Schaefer, D. A., Riggs, M. W., Joyce, T. o., Lund, M. E., Robinson, M. W., Dalton, J. P., & Donnelly, S. M. (2013). Cathelicidin-like Helminth Defence Molecules (HDMs): Absence of Cytotoxic, Anti-microbial and Anti-protozoan Activities Imply a Specific Adaptation to Immune Modulation. PLoS Neglected Tropical Diseases, 7(7).
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  PMID: 23875042;PMCID: PMC3708846;Abstract: Host defence peptides (HDPs) are expressed throughout the animal and plant kingdoms. They have multifunctional roles in the defence against infectious agents of mammals, possessing both bactericidal and immune-modulatory activities. We have identified a novel family of molecules secreted by helminth parasites (helminth defence molecules; HDMs) that exhibit similar structural and biochemical characteristics to the HDPs. Here, we have analyzed the functional activities of four HDMs derived from Schistosoma mansoni and Fasciola hepatica and compared them to human, mouse, bovine and sheep HDPs. Unlike the mammalian HDPs the helminth-derived HDMs show no antimicrobial activity and are non-cytotoxic to mammalian cells (macrophages and red blood cells). However, both the mammalian- and helminth-derived peptides suppress the activation of macrophages by microbial stimuli and alter the response of B cells to cytokine stimulation. Therefore, we hypothesise that HDMs represent a novel family of HDPs that evolved to regulate the immune responses of their mammalian hosts by retaining potent immune modulatory properties without causing deleterious cytotoxic effects. © 2013 Thivierge et al.
• Carryn, S., Schaefer, D. A., Imboden, M., Homan, E. J., Bremel, R. D., & Riggs, M. W. (2012). Phospholipases and cationic peptides inhibit Cryptosporidium parvum sporozoite infectivity by parasiticidal and non-parasiticidal mechanisms. Journal of Parasitology, 98(1), 199-204.
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  PMID: 21787211;Abstract: The apicomplexan parasite Cryptosporidium parvum is an important cause of diarrhea in humans and cattle, and it can persistently infect immunocompromised hosts. No consistently effective parasite-specific pharmaceuticals or immunotherapies for control of cryptosporidiosis are presently available. The innate immune system represents the first line of host defense against a range of infectious agents, including parasitic protozoa. Several types of antimicrobial peptides and proteins, collectively referred to herein as biocides, constitute a major effector component of this system. In the present study, we evaluated lactoferrin, lactoferrin hydrolysate, 5 cationic peptides (lactoferricin B, cathelicidin LL37, indolicidin, β-defensin 1, β-defensin 2), lysozyme, and 2 phospholipases (phospholipase A2, and phosphatidylinositol-specific phospholipase C) for anti-cryptosporidial activity. The biocides were evaluated either alone or in combination with 3E2, a monoclonal antibody (MAb) against C. parvum that inhibits sporozoite attachment and invasion. Sporozoite viability and infectivity were used as indices of anti-cryptosporidial activity in vitro. All biocides except lactoferrin had a significant effect on sporozoite viability and infectivity. Lactoferrin hydrolysate and each of the 5 cationic peptides were highly parasiticidal and strongly reduced sporozoite infectivity. While each phospholipase also had parasiticidal activity, it was significantly less than that of lactoferrin hydrolysate and each of the cationic peptides. However, each phospholipase reduced sporozoite infectivity comparably to that observed with lactoferrin hydrolysate and the cationic peptides. Moreover, when 3 of the cationic peptides (cathelicidin LL37, β-defensin 1, and β-defensin 2) were individually combined with MAb 3E2, a significantly greater reduction of sporozoite infectivity was observed over that by 3E2 alone. In contrast, reduction of sporozoite infectivity by a combination of either phospholipase with MAb 3E2 was no greater than that by 3E2 alone. These collective observations suggest that cationic peptides and phospholipases neutralize C. parvum by mechanisms that are predominantly either parasiticidal or non-parasiticidal, respectively. © American Society of Parasitologists 2012.
• Imboden, M., Schaefer, D. A., Bremel, R. D., Homan, E. J., & Riggs, M. W. (2012). Antibody fusions reduce onset of experimental Cryptosporidium parvum infection in calves. Veterinary Parasitology, 188(1-2), 41-47.
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  PMID: 22455725;PMCID: PMC3387522;Abstract: Cryptosporidium parvum is one of the main causes of diarrhea in neonatal calves resulting in significant morbidity and economic losses for producers worldwide. We have previously demonstrated efficacy of a new class of antimicrobial antibody fusions in a neonatal mouse model for C. parvum infection. Here, we extend efficacy testing of these products to experimental infection in calves, the principal target species. Neonatal calves were challenged with C. parvum oocysts and concomitantly treated with antibody-biocide fusion 4H9-G1-LL37 over the course of four days. This resulted in reduced severity of the disease when compared to control animals. Overall clinical health parameters showed significant improvement in treated animals. Oocyst shedding was reduced in treated when compared to control animals. Control of oocyst shedding is a prerequisite for breaking the cycle of re-infection on dairy farms. Antibody-biocide fusion products thus have the potential to reduce the impact of the infection in both individual animals and in the herd. © 2012 Elsevier B.V.
• Imboden, M., Riggs, M. W., Schaefer, D. A., Homan, E. J., & Bremel, R. D. (2010). Antibodies fused to innate immune molecules reduce initiation of Cryptosporidium parvum infection in mice. Antimicrobial Agents and Chemotherapy, 54(4), 1385-1392.
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  PMID: 20086143;PMCID: PMC2849385;Abstract: At present no completely effective treatments are available for Cryptosporidium parvum infections in humans and livestock. Based on previous data showing the neutralizing potential of a panel of monoclonal antibodies developed against C. parvum, and based on the fact that innate immune peptides and enzymes have anticryptosporidial activity, we engineered several of these antibodies into antibody-biocide fusion proteins. We hypothesized that the combination of high-affinity antibody targeting with innate immune molecule-mediated killing would result in a highly effective new antiprotozoal agent. To test this hypothesis, we expressed antibody-biocide fusion proteins in a mammalian cell culture system and used the resulting products for in vitro and in vivo efficacy experiments. Antibody-biocide fusion proteins efficiently bound to, and destroyed, C. parvum sporozoites in vitro through a membrane-disruptive mechanism. When antibody-biocide fusion proteins were administered orally to neonatal mice in a prophylactic model of cryptosporidiosis, the induction of infection was reduced by as much as 81% in the mucosal epithelium of the gut, as determined on the basis of histopathological scoring of infectious stages. Several versions of antibody fusion proteins that differed in antigen specificity and in the biocide used had strong inhibitory effects on the initiation of infection. The results lay the groundwork for the development of a new class of antimicrobials effective against Cryptosporidium. Copyright © 2010, American Society for Microbiology. All Rights Reserved.
• Riggs, M. W., Imboden, M., Schaefer, D. A., Homan, E. J., & Bremel, R. D. (2010). Antibodies Fused to Innate Immune Molecules Reduce Initiation of Cryptosporidium parvum Infection in Mice. Antimicrobial Agents and Chemotherapy, 54(4), 1385-1392. doi:10.1128/aac.00754-09
• Wyatt, C. R., Riggs, M. W., & Fayer, R. (2010). Cryptosporidiosis in Neonatal Calves. Veterinary Clinics of North America - Food Animal Practice, 26(1), 89-103.
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  PMID: 20117545;Abstract: Cryptosporidiosis in calves is an ongoing problem, primarily because of the high prevalence and high morbidity associated with the infection. This article summarizes current knowledge of the host/parasite interactions associated with cryptosporidiosis. The infection process in intestinal mucosa, the pathophysiology of the disease process, and the immune responses initiated in the calf to control the infection are discussed. Methods for diagnosing C. parvum infection, treatments that have been tried, and management controls are also examined. © 2010 Elsevier Inc.
• Sturbaum, G. D., Schaefer, D. A., Jost, B. H., Sterling, C. R., & Riggs, M. W. (2008). Antigenic differences within the Cryptosporidium hominis and Cryptosporidium parvum surface proteins P23 and GP900 defined by monoclonal antibody reactivity. Molecular and Biochemical Parasitology, 159(2), 138-141.
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  PMID: 18400317;Abstract: The biological basis for the specificity of host infectivity patterns of Cryptosporidium spp., in particular C. hominis and C. parvum, has yet to be fully elucidated. Comparison of the C. parvum and C. hominis P23 and GP900 predicted amino acid sequences revealed 3 differences in P23 and 4 and 17 differences in GP900 domains 1 and 5, respectively. Using monoclonal antibodies developed against the surface (glyco)proteins P23 and GP900 of the C. parvum Iowa isolate, solubilized glycoprotein from three C. hominis isolates was screened for reactivity using Western immunoblots. One of ten P23 MAbs and three of 21 GP900 MAbs were not reactive with any of the three C. hominis isolates. The non-reactive P23 MAb binds to a peptide epitope, while the non-reactive GP900 MAbs bind to either carbohydrate/carbohydrate-dependent or peptide epitopes of C. parvum. These results demonstrate phenotypic differences between C. hominis and C. parvum within two (glyco)proteins that are involved in parasite gliding motility and attachment/invasion. © 2008 Elsevier B.V. All rights reserved.
• Priest, J. W., Mehlert, A., Arrowood, M. J., Riggs, M. W., & A., M. (2003). Characterization of a Low Molecular Weight Glycolipid Antigen from Cryptosporidium parvum. Journal of Biological Chemistry, 278(52), 52212-52222.
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  PMID: 14557271;Abstract: Cryptosporidium parvum, an Apicomplexan parasite of the mammalian gut epithelium, causes a diarrheal illness in a wide range of hosts and is transmitted by contamination of food or water with oocyst-laden feces from an infected animal. We have identified a glycosylinositol phospholipid from the sporozoite stage of the parasite that is frequently recognized by serum antibodies from human cryptosporidiosis patients. The humoral immune response is dominated by IgG1 subclass antibodies but can also include IgA and IgM antibodies. The glycosylinositol phospholipids were purified by butanol extraction of a Triton X-114-soluble fraction followed by octyl-Sepharose column chromatography and preparative high performance TLC and were shown to include at least 5 species. By using mass spectrometry and radiolabeled neutral glycan analysis, we found that the structure of the dominant glycosylinositol phospholipid antigen contained a C18:0 lyso-acylglycerol, a C16:0-acylated inositol, and an unsubstituted mannose3-glucosamine glycan core. Other diacyl species were also identified, most notably a series of glycosylinositol phospholipids having an acyl-linked C20:0 to C28:0 lipid on the inositol ring. Less abundant species having three acyl-linked fatty acids and species with an additional 1-3 hexoses linked to the mannose core were also observed. We are currently working to determine the role that these glycolipids may play in the development of disease and in the clearance of infection.
• Riggs, M. W. (2002). Recent advances in cryptosporidiosis: The immune response. Microbes and Infection, 4(10), 1067-1080.
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  PMID: 12191657;Abstract: An increased understanding of host immune responses to Cryptosporidium parvum which are responsible for clearance of primary infection and resistance to reinfection, and characterization of the parasite molecules to which they are directed, are essential for discovery of effective active and passive immunization strategies against cryptosporidiosis. In this article, recent advances in knowledge of humoral and cellular immune responses to C. parvum, their antigen specificities, and mechanisms of protection are briefly reviewed. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
• Riggs, M. W., Schaefer, D. A., Kapil, S. J., Barley-Maloney, L., & Perryman, L. E. (2002). Efficacy of Monoclonal Antibodies against Defined Antigens for Passive Immunotherapy of Chronic Gastrointestinal Cryptosporidiosis. Antimicrobial Agents and Chemotherapy, 46(2), 275-282. doi:10.1128/aac.46.2.275-282.2002
• Riggs, M. W., Schaefer, D. A., Kapil, S. J., Barley-Maloney, L., & Perryman, L. E. (2002). Efficacy of monoclonal antibodies against defined antigens for passive immunotherapy of chronic gastrointestinal cryptosporidiosis. Antimicrobial Agents and Chemotherapy, 46(2), 275-282.
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  PMID: 11796330;PMCID: PMC127040;Abstract: Cryptosporidium parvum is an important cause of diarrhea in humans and calves and can persistently infect immunocompromised hosts. Presently, there are no consistently effective parasite-specific drugs for cryptosporidiosis. We hypothesized that neutralizing monoclonal antibodies (MAbs) targeting the apical complex and surface antigens CSL, GP25-200, and P23 could passively immunize against cryptosporidiosis. We recently reported that a formulation of MAbs 3E2 (anti-CSL), 3H2 (anti-GP25-200), and 1E10 (anti-P23) provided significant additive prophylactic efficacy over that of the individual MAbs in neonatal ICR mice. In the present study, these MAbs were evaluated for therapeutic efficacy against persistent infection in adult gamma interferon-depleted SCID mice. 3E2 demonstrated the most significant and consistent therapeutic effect, reducing intestinal infection in two experiments. In one experiment, 3E2 plus 3H2 and 3E2 plus 3H2 plus 1E10 also significantly reduced infection; however, no significant increase in efficacy over 3E2 alone was apparent. The results indicate that anti-CSL MAb 3E2 has highly significant efficacy in reducing, but not eliminating, persistent C. parvum infection.
• Wyatt, C. R., Barrett, W. J., Brackett, E. J., Schaefer, D. A., & Riggs, M. W. (2002). Association of IL-10 expression by mucosal lymphocytes with increased expression of Cryptosporidium parvum epitopes in infected epithelium. Journal of Parasitology, 88(2), 281-286.
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  PMID: 12053998;Abstract: The objective of this study was to determine whether changes in the ileal intraepithelial lymphocyte (IEL) phenotype and function occurred prior to development of diarrhea in Cryptosporidium parvum - infected calves. Calves were orally inoculated with 108 oocysts and maintained in enteric pathogen-free conditions until their use in experiments. Age-matched uninfected calves were used for comparisons. Ileal IELs were isolated and phenotyped to determine whether changes in lymphocyte population dynamics had occurred by 3 days postinoculation (PI). Ex vivo reverse transcriptase-polymerase chain reaction of messenger ribonucleic acid (mRNA) from IELs from infected calves was compared with controls to determine whether changes in cytokine expression had occurred by 3 days PI. No significant changes in lymphocyte population dynamics were documented; however, IELs isolated from 4 out of 8 infected calves, but not from 8 out of 8 control calves, expressed mRNA for interleukin-10 (IL-10). IL-10 expression by IELs was associated with the expression of a significantly larger (P < 0.001) proportion (0.75) of monoclonal antibody - defined C. parvum epitopes within infected ileal epithelium, as compared with a much smaller proportion (0.30) of epitopes with IL-10-lymphocytes. The results suggest that a temporal association exists between the expression of IL-10 by ileal IELs and the expression of C. parvum antigens in infected calf epithelium prior to development of cryptosporidiosis.
• Langer, R. C., Schaefer, D. A., & Riggs, M. W. (2001). Characterization of an intestinal epithelial cell receptor recognized by the Cryptosporidium parvum sporozoite ligand CSL. Infection and Immunity, 69(3), 1661-1670.
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  PMID: 11179341;PMCID: PMC98070;Abstract: The protozoan parasite Cryptosporidium parvum is a leading cause of diarrhea in humans and neonatal calves. The absence of approved parasite-specific drugs, vaccines, and immunotherapies for cryptosporidiosis relates in part to limited knowledge on the pathogenesis of zoite attachment and invasion. We recently reported that the C. parvum apical complex glycoprotein CSL contains a zoite ligand for intestinal epithelial cells which is defined by monoclonal antibody (MAb) 3E2. In the present study, the host cell receptor for CSL was characterized. For these studies, a panel of epithelial and mesenchymal cell lines was examined for permissiveness to C. parvum and the ability to bind CSL. Cells of epithelial origin were significantly more permissive and bound significantly greater quantities of CSL than cells of mesenchymal origin. Caco-2 intestinal cells were selected from the epithelial panel for further characterization of the CSL receptor. Immunoelectron microscopy demonstrated that CSL bound initially to the surface of Caco-2 cells and was rapidly internalized. The molecule bound by CSL was identified as an 85-kDa Caco-2 cell surface protein by radioimmunoprecipitation and CSL affinity chromatography. Sporozoite incubation with the isolated 85-kDa protein reduced binding of MAb 3E2. Further, attachment and invasion were significantly inhibited when sporozoites were incubated with the 85-kDa protein prior to inoculation onto Caco-2 cells. These observations indicate that the 85-kDa protein functions as a Caco-2 cell receptor for CSL. CSL also bound specifically to intestinal epithelium from calves, indicating receptor expression in a second important host species. Molecular characterization of the CSL receptor may lead to novel avenues for disrupting ligand-receptor interactions in the pathogenesis of C. parvum infection.
• Riggs, M. W., Langer, R. C., & Schaefer, D. A. (2001). Characterization of an Intestinal Epithelial Cell Receptor Recognized by the Cryptosporidium parvum Sporozoite Ligand CSL. Infection and Immunity, 69(3), 1661-1670. doi:10.1128/iai.69.3.1661-1670.2001
• Riggs, M. W., Schaefer, D. A., Kapil, S. J., Barley-Maloney, U., Ferryman, L. E., & McNeil, M. R. (2001). Targeted disruption of CSL ligand - Host cell receptor interaction in treatment of Cryptosporidium parvum infection. Journal of Eukaryotic Microbiology, 48(SUPPL.), 44S-46S.
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  PMID: 11906075;
• Schaefer, D. A., Auerbach-Dixon, B. A., & Riggs, M. W. (2000). Characterization and formulation of multiple epitope-specific neutralizing monoclonal antibodies for passive immunization against cryptosporidiosis. Infection and Immunity, 68(5), 2608-2616.
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  PMID: 10768951;PMCID: PMC97466;Abstract: The coccidian parasite Cryptosporidium parvum causes diarrhea in humans, calves, and other mammals. Neither immunization nor parasite-specific pharmaceuticals that are consistently effective against this organism are available. While polyclonal antibodies against whole C. parvum reduce infection, their efficacy and predictability are suboptimal. We hypothesized that passive immunization against cryptosporidiosis could be improved by using neutralizing monoclonal antibodies (MAbs) targeting functionally defined antigens on the infective stages. We previously reported that the apical complex and surface-exposed zoite antigens CSL, GP25-200, and P23 are critical in the infection process and are therefore rational targets. In the present study, a panel of 126 MAbs generated against affinity-purified CSL, GP25-200, and P23 was characterized to identify the most efficacious neutralizing MAb formulation targeting each antigen. To identify neutralizing MAbs, sporozoite infectivity following exposure to individual MAbs was assessed by enzyme-linked immunosorbent assay. Of 126 MAbs evaluated, 47 had neutralizing activity. These were then evaluated individually in oocyst- challenged neonatal mice, and 14 MAbs having highly significant efficacy were identified for further testing in formulations. Epitope specificity assays were performed to determine if candidate MAbs recognized the same or different epitopes. Formulations of two or three neutralizing MAbs, each recognizing distinct epitopes, were then evaluated. A formulation of MAbs 3E2 (anti-CSL [αCSL]), 3H2 (αGP25-200), and 1El0 (αP23) provided highly significant additive efficacy over that of either individual MAbs or combinations of two MAbs and reduced intestinal infection by 86 to 93%. These findings indicate that polyvalent neutralizing MAb formulations targeting epitopes on defined antigens may provide optimal passive immunization against cryptosporidiosis.
• Adjei, A. A., Jones, J. T., Riggs, M. W., & Enriquez, F. J. (1999). Evidence of thymus-independent local and systemic antibody responses to Cryptosporidium parvum infection in nude mice. Infection and Immunity, 67(8), 3947-3951.
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  PMID: 10417160;PMCID: PMC96676;Abstract: Differences in susceptibility to persistent cryptosporidial infection between two strains of adult athymic nude mice prompted us to investigate the immune mechanism(s) that may control resistance to infection in these T- cell-deficient mice. We studied fecal oocyst shedding, serum and fecal parasite-specific antibody responses, and fecal immunoglobulin levels in athymic C57BL/6J nude and athymic BALB/cJ nude mice following oral inoculation with Cryptosporidium parvum oocysts at 8 to 9 weeks of age. C57BL/6J nude mice had significantly higher fecal parasite-specific immunoglobulin A (IgA) (days 27, 31, 35, and 42 postinoculation) and IgM (days 10, 17, 24, 28, 31, 38, 42, and 48 postinoculation) levels than BALB/cJ nude mice (P < 0.05) and significantly higher serum parasite-specific IgA levels at 63 days postinoculation (P < 0.03). Moreover, C57BL/6J nude mice shed significantly fewer C. parvum oocysts than BALB/cJ nude mice from days 52 to 63 postinoculation (P < 0.05). In contrast, BALB/cJ nude mice had higher levels of non-parasite-specific IgA (days 38 to 63 postinoculation) and IgM (days 24, 35, 38, and 52 postinoculation) than C57BL/6J nude mice in feces (P < 0.05). These data suggest that parasite-specific fecal antibodies may be associated with resistance to C. parvum in C57BL/6J nude mice.
• Langer, R. C., & Riggs, M. W. (1999). Cryptosporidium parvum apical complex glycoprotein CSL contairσ a sporozoite ligand for intestinal epithelial cells. Infection and Immunity, 67(10), 5282-5291.
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  PMID: 10496907;PMCID: PMC96882;Abstract: Cryptosporidiosis, caused by the apicomplexan parasite Cryptosporidium parvum, has become a well-recognized diarrheal disease of humans and other mammals throughout the world. No approved parasite-specific drugs, vaccines, or immunotherapies for control of the disease are currently available, although passive immunization with C. parvum-specific antibodies has some efficacy in immunocompromised and neonatal hosts. We previously reported that CSL, an ~1,300-kDa conserved apical glycoprotein of C. parvum sporozoites and merozoites, is the antigenic species mechanistically bound by neutralizing monoclonal antibody 3E2 which elicits the circumsporozoite precipitate (CSP)-like reaction and passively protects against C. parvum infection in vivo. These findings indicated that CSL has a functional role in sporozoite infectivity. Here we report that CSL has properties consistent with being a sporozoite ligand for intestinal epithelial cells. For these studies, native CSL was isolated from whole sporozoites by isoelectric focusing (IEF) following observations that the ~1,300-kDa region containing CSL as seen by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was comprised of approximately 15 molecular species (pI 3 to 10) when examined by two-dimensional (2-D) electrophoresis and silver staining. A subset of six ~1,300-kDa species (pI 4.0 to 6.5) was specifically recognized by 3E2 in 2-D Western immunoblots of IEF-isolated CSL. Isolated native CSL bound specifically and with high affinity to permissive human intestinal epithelial Caco-2 cells in a dose-dependent, saturable, and self-displaceable manner. Further, CSL specifically bound to the surface of live Caco-2 cells inhibited sporozoite attachment and invasion. In addition, sporozoites having released CSL after incubation with 3E2 and occurrence of the CSP-like reaction did not attach to and invade Caco-2 cells. These findings indicate that CSL contains a sporozoite ligand which facilitates attachment to and invasion of Caco-2 cells and, further, that ligand function may be disrupted by CSL-reactive monoclonal antibody. We conclude that CSL is a rational target for passive or active immunization against cryptosporidiosis.
• Riggs, M. W., & Langer, R. C. (1999). Cryptosporidium parvum Apical Complex Glycoprotein CSL Contains a Sporozoite Ligand for Intestinal Epithelial Cells. Infection and Immunity, 67(10), 5282-5291. doi:10.1128/iai.67.10.5282-5291.1999
• Riggs, M. W., McNeil, M. R., Perryman, L. E., Stone, A. L., Scherman, M. S., & O'Connor, R. M. (1999). Cryptosporidium parvum sporozoite pellicle antigen recognized by a neutralizing monoclonal antibody is a β-mannosylated glycolipid. Infection and Immunity, 67(3), 1317-1322.
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  PMID: 10024577;PMCID: PMC96463;Abstract: The protozoan parasite Cryptosporidium parvum is an important cause of diarrhea in humans, calves, and other mammals worldwide. No approved vaccines or parasite-specific drugs are currently available for the control of cryptosporidiosis. To effectively immunize against C. parvum, identification and characterization of protective antigens are required. We previously identified CPS-500, a conserved, neutralization-sensitive antigen of C. parvum sporozoites and merozoites defined by monoclonal antibody 18.44. In the present study, the biochemical characteristics and subcellular location of CPS-500 were determined. CPS-500 was chloroform extractable and eluted with acetone and methanol in silicic acid chromatography, consistent with being a polar glycolipid. Following chloroform extraction and silicic acid chromatography, CPS-500 was isolated by high-pressure liquid chromatography for glycosyl analysis, which indicated the presence of mannose and inositol. To identify which component of CPS-500 comprised the neutralization-sensitive epitope recognized by 18.44, the ability of the monoclonal antibody to bind CPS-500 treated with proteases, or with α- or β-glycosidases, was determined. Monoclonal antibody 18.44 did not bind antigen treated with β- D-mannosidase but did bind antigen treated with α-D-mannosidase, other α- or β-glycosidases, or a panel of proteases. These data indicated that the target epitope was dependent on terminal β-D-mannopyranosyl residues. By immunoelectron microscopy, 18.44 binding was localized to the pellicle and an intracytoplasmic tubulovesicular network in sporozoites. Monoclonal antibody 18.44 also bound to antigen deposited and released onto substrate over the course travelled by gliding sporozoites and merozoites. Surface localization, adhesion and release during locomotion, and neutralization sensitivity suggest that CPS-500 may be involved in motility and invasion processes of the infective zoite stages.
• Enriquez, F. J., & Riggs, M. W. (1998). Role of immunoglobulin a monoclonal antibodies against P23 in controlling murine Cryptosporidium parvum infection. Infection and Immunity, 66(9), 4469-4473.
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  PMID: 9712802;PMCID: PMC108540;Abstract: Cryptosporidium parvum is an important diarrhea-causing protozoan parasite of immunocompetent and immunocompromised hosts. Immunoglobulin A (IgA) has been implicated in resistance to mucosal infections with bacteria, viruses, and parasites, but little is known about the role of IgA in the control of C. parvum infection. We assessed the role of IgA during C. parvum infection in neonatal mice. IgA-secreting hybridomas were developed by using Peyer's patch lymphocytes from BALB/c mice which had been orally inoculated with viable C. parvum oocysts. Six monoclonal antibodies (MAbs) were selected for further study based on indirect immunofiuorescence assay reactivity with sporozoite and merozoite pellicles and the antigen (Ag) deposited on glass substrate by gliding sporozoites. Each MAb was secreted in dimeric form and recognized a 23-kDa sporozoite Ag in Western immunoblots. The Ag recognized comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with P23, a previously defined neutralization-sensitive zoite pellicle Ag. MAbs were evaluated for prophylactic or therapeutic efficacy against C. parvum, singly and in combinations, in neonatal BALB/c mice. A combination of two MAbs given prophylactically prior to and 12 h following oocyst challenge reduced the number of intestinal parasites scored histologically by 21.1% compared to the numbers in mice given an isotype-matched control MAb (P < 0.01). Individual MAbs given therapeutically in nine doses over a 96-h period following oocyst challenge increased efficacy against C. parvum infection. Four MAbs given therapeutically each reduced intestinal infection 34.4 to 42.2% compared to isotype-matched control MAb-treated mice (P < 0.05). One MAb reduced infection 63.3 and 72.7% in replicate experiments compared to isotype-matched control MAb-treated mice (P < 0.0001). We conclude that IgA MAbs directed to neutralization-sensitive P23 epitopes may have utility in passive immunization against murine C. parvum infection.
• Riggs, M. W., Stone, A. L., Yount, P. A., Langer, R. C., Arrowood, M. J., & Bentley, D. L. (1997). Protective Monoclonal Antibody Defines a Circumsporozoite-Like Glycoprotein Exoantigen of Cryptosporidium parvum Sporozoites and Merozoites. Journal of Immunology, 158(4), 1787-1795.
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  PMID: 9029117;Abstract: The apicomplexan protozoan parasite Cryptosporidium parvum causes a diarrheal disease in human and other mammals for which specific therapy and immunoprophylaxis are unavailable. Passive immunization with Abs against whole C. Parvum organisms has variable efficacy in immunocompromised or neonatal hosts. Because apical and surface-exposed zoite Ags of the Apicomplexa are critical to infectivity and targets of protective immunity, we examined the ability of mAbs generated against such Ags in C. parvum sporozoites to passively protect against infection and identify biologically relevant parasite molecules. A panel of mAbs was produced against affinity-purified native Ags using sporozoite apical- and surface-reactive mAb C4A1 as binding ligand. One resulting mAb, designated 3E2, elicited prominent morphologic changes in sporozoites and merozoites characterized by rapid and progressive formation, posterior movement, and release of membranous Ag-mAb precipitates. These changes has a striking resemblance to the malarial circumsporozoite precipitate (CSP) reaction. Sporozoite infectivity was completely neutralized after in vitro exposure to 3E2 and the CSP-like reaction. Furthermore, orally administered 3E2 completely prevented or markedly reduced infection in neonatal BALB/c mice. 3E2 bound to apical complex and surface molecules of zoites and was demonstrated in membranous precipitates by immunoelectron microscopy. In Western blots, 3E2 recognized multiple 46 to ∼770 kDa sporozoited Ags and an ∼1300-kDa Ag designated CSL, also expressed by merozoites. CSL was characterized as a soluble glycoprotein exoantigen released by infectious sporozoites. Further, CSL was determined to be the molecular species mechanistically involved in the CSP-like reaction by its identification in SDS-PAGE gels and Western blots of purified membranous precipitates. These findings indicate the CSL has a functional role in sporozoite infectivity and is a candidate molecular target for passive or active immunization against cryptosporidiosis.
• Riggs, M. W., Stone, A. L., Yount, P. A., Langer, R. C., Arrowood, M. J., & Bentley, D. L. (1997). Protective monoclonal antibody defines a circumsporozoite-like glycoprotein exoantigen of Cryptosporidium parvum sporozoites and merozoites.. The Journal of Immunology, 158(4), 1787-1795. doi:10.4049/jimmunol.158.4.1787
• Langer, R. C., & Riggs, M. W. (1996). Neutralizing monoclonal antibody protects against Cryptosporidium parvum infection by inhibiting sporozoite attachment and invasion. Journal of Eukaryotic Microbiology, 43(5), 76S-77S.
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  PMID: 8822871;
• Perryman, L. E., Jasmer, D. P., Riggs, M. W., Bohnet, S. G., McGuire, T. C., & Arrowood, M. J. (1996). A cloned gene of Cryptosporidium parvum encodes neutralization-sensitive epitopes. Molecular and Biochemical Parasitology, 80(2), 137-147.
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  PMID: 8892291;Abstract: Two mAb, C6B6 and 7D10, each significantly reduced infection of mice by Cryptosporidium parvum and reacted with a 23-kDa glycoprotein (p23) of geographically disperse C. parvum isolates. The antibodies were used to identify plaques in a cDNA library prepared from C. parvum sporozoite mRNA. cDNA insert sequences from positive plaques were determined and used to isolate additional clones encoding p23 coding sequences. A consensus open reading frame of 333 base pairs, encoding 111 amino acids, was identified in this collection of cDNAs. The predicted amino acid sequence contained one N-glycosylation site, but lacked hydrophobic membrane spanning regions. Epitope mapping revealed that mAb 7D10 defines the linear epitope QDKPAD which occurs twice in the C terminal region of the peptide encoded by the ORF. This same C terminal peptide region contains a non-linear epitope bound by mAb C6B6. Serum from mice immunized with synthetic C terminal peptide reacted with sporozoite p23. The occurrence of neutralization-sensitive epitopes encoded by defined regions of the C. parvum genome suggests that recombinant proteins or synthetic peptides containing these epitopes may prove useful for inducing immune responses that diminish infection.
• Riggs, M. W., Yount, P. A., Stone, A. L., & Langer, R. C. (1996). Protective monoclonal antibodies define a distinct, conserved epitope on an apical complex exoantigen of Cryptosporidium parvum sporozoites. Journal of Eukaryotic Microbiology, 43(5), 74S-75S.
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  PMID: 8822870;
• Riggs, M. W., Noon, T. H., Bradley, G. A., Metcalf, H. C., Reggiardo, C., Bicknell, E. J., Lozano-Alarcon, F., & Reed, R. E. (1995). Neuroaxonal Degeneration in Sheep GrazingSorghumPastures. Journal of Veterinary Diagnostic Investigation, 7(2), 229-236. doi:10.1177/104063879500700211
• Riggs, M. W., Cama, V. A., Leary Jr., H. L., & Sterling, C. R. (1994). Bovine antibody against Cryptosporidium parvum elicits a circumsporozoite precipitate-like reaction and has immunotherapeutic effect against persistent cryptosporidiosis in SCID mice. Infection and Immunity, 62(5), 1927-1939.
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  PMID: 8168959;PMCID: PMC186444;Abstract: Control of cryptosporidiosis is currently hampered by the absence of drugs or vaccines proven consistently effective against Cryptosporidium parvum. On the basis of observations that anti-C. parvum antibody has therapeutic effect against cryptosporidiosis, cows were immunized with C. parvum to produce hyperimmune colostral antibody. An antibody-rich fraction was prepared and differentiated from control (nonhyperimmune) antibody by enzyme-linked immunosorbent assay, immunofluorescence assay, immunoelectron microscopy, and in vitro neutralizing titer against DEAE-cellulose-isolated C. parvum sporozoites. Oocyst, purified sporozoite, and merozoite antigens recognized by hyperimmune antibody were defined by Western blot (immunoblot). Hyperimmune antibody recognized antigens common to oocysts, sporozoites, and merozoites, as well as stage-specific antigens. Upon incubation with hyperimmune antibody, sporozoites underwent distinct morphologic changes characterized by progressive formation and eventual release of membranous sporozoite surface antigen-antibody complexes, similar to the malaria circumsporozoite precipitate reaction. The infectivity of sporozoites having undergone this reaction was neutralized. The reaction was minimal or absent on sporozoites incubated with control antibody. To determine therapeutic effect in vivo, persistent C. parvum infection was established in adult severe combined immune-deficient (SCID) mice by oral inoculation with 107 oocysts. At 5 weeks postinfection, infected mice were treated for 10 days with hyperimmune or control antibody by inclusion in drinking water and daily gavage. Fecal oocyst shedding and infection scores in the gastrointestinal tract and gall bladder/common bile duct in hyperimmune antibody-treated mice were significantly lower than those in the control antibody-treated mice. Hyperimmune bovine antibody prepared against C. parvum may provide a first- generation therapy for control of cryptosporidiosis. Additionally, the defined antigens can be evaluated as subunit immunogens to produce better- characterized polyclonal antibody for control of cryptosporidiosis or as targets for monoclonal antibody-based immunotherapy.
• Riggs, M. W., Cama, V. A., Leary, H. L., & Sterling, C. R. (1994). Bovine antibody against Cryptosporidium parvum elicits a circumsporozoite precipitate-like reaction and has immunotherapeutic effect against persistent cryptosporidiosis in SCID mice. Infection and Immunity, 62(5), 1927-1939. doi:10.1128/iai.62.5.1927-1939.1994
• Uhl, E. W., O'Connor, R., Perryman, L. E., & Riggs, M. W. (1992). Neutralization-sensitive epitopes are conserved among geographically diverse isolates of Cryptosporidium parvum. Infection and Immunity, 60(4), 1703-1706.
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  PMID: 1372299;PMCID: PMC257052;Abstract: Isolates of Cryptosporidium parvum from New York, Florida, Brazil, Mexico, and Peru were examined for the presence of two sporozoite surface epitopes originally identified in an Iowa isolate by neutralizing monoclonal antibodies (MAbs) 18.44 and 17.41. Immunofluorescence microscopy and immunoblotting demonstrated the presence of both epitopes on all isolates. Incubation of DEAE-cellulose-purified sporozoites of the New York, Florida, Brazil, and Mexico isolates with MAb 18.44 or 17.41 significantly neutralized their infectivity for 4- to 6-day-old BALB/c mice. The results indicate that two neutralization-sensitive epitopes are conserved on geographically diverse C. parvum isolates.
• Bjorneby, J. M., Hunsaker, B. D., Riggs, M. W., & Perryman, L. E. (1991). Monoclonal antibody immunotherapy in nude mice persistently infected with CRyptosporidium parvum. Infection and Immunity, 59(3), 1172-1176.
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  PMID: 1997419;PMCID: PMC258384;Abstract: Three groups of congenitally athymic nude mice were persistently infected following oral administration of 2 x 107 Cryptosporidium parvum oocysts. Two groups were treated once daily for 10 days with either neutralizing monoclonal antibody (MAb) 17.41 or an isotype control MAb. The third group received no treatment. Intestinal-infection scores were significantly decreased in nude mice treated with MAb 17.41 compared with isotype control MAb-treated and nontreated control groups (P < 0.005). biliary and pancreatic crytosporidial-infection cores were similar for the MAb 17.41-treated and isotype control MAb-treated groups (P > 0.05).
• Anusz, K. Z., Mason, P. H., Riggs, M. W., & Perryman, L. E. (1990). Detection of Cryptosporidium parvum oocysts in bovine feces by monoclonal antibody capture enzyme-linked immunosorbent assay. Journal of Clinical Microbiology, 28(12), 2770-2774.
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  PMID: 2280009;PMCID: PMC268271;Abstract: A monoclonal antibody enzyme-linked immunosorbent assay (ELISA) was developed to detect Cryptosporidium parvum oocysts in bovine feces. Fecal oocysts were concentrated by centrifugation through formalin-ethyl acetate solution and captured with monoclonal antibody 18.280.2 reactive with C. parvum oocysts. Captured oocysts were detected with goat anti-oocyst serum, following the addition of a peroxidase conjugate of rabbit anti-goat immunoglobulin and O-phenylenediamine substrate. The assay was specific for Cryptosporidium sp. oocysts and did not detect oocysts of Eimeria auburniensis, Eimeria bovis, Eimeria ellipsoidalis, or Eimeria zuernii. Assay sensitivity allowed detection of 3 x 105 oocysts per ml of feces, compared with 1 x 106 oocysts per ml detected by examination of acid-fast-stained fecal smears and 1 x 103 oocysts per ml detected by indirect immunofluorescence. The capture ELISA was suitable for diagnostic analysis of bovine fecal samples and for assessment of oocyst shedding in experimentally infected calves. This assay may also prove useful for diagnostic assessment of humans in which cryptosporidiosis is suspected.
• Bjorneby, J. M., Riggs, M. W., & Ferryman, L. E. (1990). Cryptosporidium parvum merozoites share neutralization-sensitive epitopes with sporozoites. Journal of Immunology, 145(1), 298-304.
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  PMID: 2193057;Abstract: Sporozoites and merozoites are stages in the life cycle of Cryptosporidium parvum that can cyclically infect intestinal cells, causing persistent infection and severe diarrhea in immunodeficient patients. Infection by sporozoites can be neutralized by surface-reactive mAb. We show that merozoite infectivity can also be neutralized by surface-reactive mAb. To do this, viable C. parvum merozoites were isolated by differential and isopycnic. centrifugation, and distinguished from sporozoites by transmission electron microscopy. Differential reactivity with a panel of seven mAb was used to determine the amount of sporozoite contamination in isolated merozoite preparations. The isolated merozoites were distinguished from sporozoites (p < 0.0001) by four sporozoite-specific mAb (16.332, 16.502, 17.25, and 18.357) in an indirect immunofluorescence assay. Three mAb (16.29, 17.41, and 18.44) consistently reacted with both merozoites and sporozoites. Isolated merozoites were infectious for neonatal mice when administered by intraintestinal injection. Infectivity for mice was significantly neutralized (p < 0.05) when 1 to 2 × 105 merozoites were incubated with sporozoite-neutralizing mAb 17.41 or 18.44, before inoculation. Merozoites incubated with an isotype control mAb remained infectious for neonatal mice. We conclude that C. parvum merozoites share neutralization-sensitive epitopes with sporozoites.
• Dhein, C. R., Prieur, D. J., Riggs, M. W., Potter, K. A., & Widders, P. R. (1990). Suspected ciliary dysfunction in Chinese Shar Pei pups with pneumonia.. American journal of veterinary research, 51(3), 439-446.
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  PMID: 2316922;Abstract: Chronic pneumonia was investigated in a litter of young Chinese Shar Pei in which 4 of 6 dogs were affected. Serum immunoglobulin concentrations (IgA, IgG, IgM) determined by radial immunodiffusion varied over time, but were not consistently lower in affected dogs, compared with control dogs. Two dogs that died had hydrocephalus and lymphoid depletion, in addition to severe broncho-pneumonia. Evaluation of ciliary ultrastructure in 2 affected dogs revealed random orientation of adjacent respiratory tract or oviductal cilia and a greater number of microtubular disarrangements, compared with control dogs. In vivo tracheal mucociliary clearance of 99mtechnetium macroaggregated albumin was absent in 1 dog examined. The ciliary abnormalities were suspected to have resulted in an inefficient mucociliary transport system predisposing to the development of pneumonia. Further evaluation of 1 Chinese Shar Pei revealed lymphocyte mitogenesis results that were not consistently less than those of a control dog, normal total hemolytic complement values, and normal blood neutrophil chemotaxis.
• Perryman, L. E., Riggs, M. W., Mason, P. H., & Fayer, R. (1990). Kinetics of Cryptosporidium parvum sporozoite neutralization by monoclonal antibodies, immune bovine serum, and immune bovine colostrum. Infection and Immunity, 58(1), 257-259.
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  PMID: 2294054;PMCID: PMC258439;Abstract: Monoclonal antibodies, immune bovine serum, and immune bovine colostral whey neutralized infectivity of Cryptosporidium parvum sporozoites for mice in a time-dependent manner. Immune colostral whey neutralized sporozoites more rapidly and completely than immune serum, monoclonal antibody (MAb) 18.44, or a combination of MAb 18.44 and MAb 17.41. Mice were partially protected against oral challenge with C. parvum oocysts when treated with immune colostral whey, MAb 17.41, or a combination of MAb 17.41 and MAb 18.44.
• Stauber, E., Robinette, J., Basaraba, R., Riggs, M., & Bishop, C. (1990). Mast cell tumors in three ferrets.. Journal of the American Veterinary Medical Association, 196(5), 766-767.
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  PMID: 2106501;Abstract: Pruritic open skin lesions (5 mm diameter) on the thorax, neck, or head of 3 ferrets were diagnosed as mast cell tumors. Diagnosis of the tumors was based on histologic examination of biopsy specimens. Histologic diagnosis was necessary because the lesions had been treated as mycotic or bacterial ulcers and did not respond to conventional treatment. Excised mast cell tumors did not recur, and malignant behavior of the tumors was not observed.
• Fayer, R., Perryman, L. E., & Riggs, M. W. (1989). Hyperimmune bovine colostrum neutralizes Cryptosporidium sporozoites and protects mice against oocyst challenge. Journal of Parasitology, 75(1), 151-153.
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  PMID: 2783966;Abstract: Activity of colostral whey, produced by a cow immunized with oocysts of Cryptosporidium parvum and found to provide prophylaxis against cryptosporidiosis in calves, was tested in 2 experiments. In one experiment BALB/c mice were given the immune whey (HW), whey from a nonimmunized cow (CW), or a balanced salt solution (HBSS) before, during, and after oral inoculation with oocysts of C. parvum. Significantly fewer (P < 0.05) C. parvum were found in mice that received HW (undiluted, 1:20 or 1:50) than in those treated with similarly diluted CW or with HBSS. In the second experiment it was determined that protection was mediated by specific anti-sporozoite activity when significantly fewer (P < 0.05) C. parvum were found in mice that received sporozoites treated with HW diluted 1:20 or 1:50 compared with mice that received sporozoites treated with similarly diluted CW or with HBSS.
• Riggs, M. W., McGuire, T. C., Mason, P. H., & Perryman, L. E. (1989). Neutralization-sensitive epitopes are exposed on the surface of infectious Cryptosporidium parvum sporozoites. Journal of Immunology, 143(4), 1340-1345.
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  PMID: 2473130;Abstract: Cryptosporidiosis is a diarrheal disease of humans, calves, and other mammals caused by the coccidian parasite Cryptosporidium parvum. Immune bovine serum and two surface-reactive anti-sporozoite mAb with neutralizing activity were used to identify sporozoite surface Ag by radioimmunoprecipitation/SDS-PAGE and immunoblotting. When isolated sporozoites were incubated with mAb 18.44, 12 to 25 times the ID50 for mice was completly neutralized. This mAb binds diffusely to the sporozoite surface and recognizes a sporozoite surface Ag that eluted in the void volume of a Bio Gel A column with an exclusion limit of 500,000 daltons. The Ag recognized by mAb 18.44 was not radiolabeled with 125I or [35S]methionine, migrated with the dye front in SDS-PAGE, and was insensitive to proteinase K digestion, suggesting a non-protein composition. mAb 17.41 significantly neutralized 25 times the ID50 of sporozoites for mice. This mAb binds multifocally to the sporozoite surface and recognizes [35S]methionine-labeled sporozoite surface Ag of 28,000 m.w., 55,000 m.w., and 98,000 m.w. Immune bovine serum immunoprecipitated [35S]methionine- or 125I-labeled sporozoite Ag ranging from < 14,300 m.w. to > 200,000 m.w., including surface Ag of 28,000 m.w. and 55,000 m.w. The results indicate that two different molecules capable of inducing neutralizing antibody are exposed on the surface of C. parvum sporozoites.
• Barbee, D. D., Allen, J. R., Grant, B. D., Riggs, M. W., Crawley, G. R., & Sande, R. D. (1987). Detection by computed tomography of occult osteochondral defects in the fetlock of a horse.. Equine veterinary journal, 19(6), 556-558.
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  PMID: 3504768;
• Riggs, M. W. (1987). Evaluation of foals for immune deficiency disorders.. The Veterinary clinics of North America. Equine practice, 3(3), 515-528.
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  PMID: 3322524;Abstract: Currently recognized equine immune deficiency disorders include failure of passive colostral immunoglobulin transfer, combined immunodeficiency, selective IgM deficiency, transient hypogammaglobulinemia, and agammaglobulinemia. Additional equine immune deficiency disorders probably exist. Immune deficiency should be suspected in any horse experiencing persistent or recurrent infections that are not responsive to conventional therapy.
• Riggs, M. W., & Perryman, L. E. (1987). Infectivity and neutralization of Cryptosporidium parvum sporozoites. Infection and Immunity, 55(9), 2081-2087.
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  PMID: 3623693;PMCID: PMC260660;Abstract: Cryptosporidiosis, a diarrheal disease of calves and humans caused by the coccidian parasite Cryptosporidium parvum, is terminated in hosts with normal immune systems. To assess the mechanisms of immunity in cryptosporidiosis, it is necessary to isolate and quantitate sporozoites, the infective stage of Cryptosporidium spp. Here we report the (i) separation of infective C. parvum oocysts from calf feces by either extraction, sieving, and hypochlorite treatment; (ii) separation of viable C. parvum sporozoites from intact and excysted oocysts by anion-exchange chromatography; and (iii) quantitation of sporozoite infectivity in vivo by direct intraintestinal injection of isolated sporozoites in 7-day-old BALB/c mice. When isolated sporozoites were incubated with heat-inactivated immune bovine serum, 25 times the 50% infective dose for 7-day-old mice was completely neutralized. Sporozoites incubated with preimmune bovine serum were infectious for 7-day-old mice.

Presentations

• Arnold, S., Conrad, T., McNamara, C., Love, M., Chi, V., Woods, A., Joseph, S., Riggs, M., Schaefer, D., Betzer, D., Barrett, L., Van Voorhis, W., Tam, P., & Hermann, D. (2020, spring). Predicting the impact of infectious diarrhea pathophysiology on drug disposition.. Experimental Biology Conference 2020. San Diego, California.
• Funkhouser-Jones, L. J., Akey, M. E., Wilke, G., VanDussen, K. L., Schaefer, D. A., Ackman, K., Riggs, M. W., Stappenback, T. S., & Sibley, L. D. (2020, summer). Gut metabolites influence susceptibility to Cryptosporidium infection. 31st Annual Woods Hole Molecular Parasitology Meeting.
• Ojo, K. K., Choi, R., Huang, W., Hulverson, M. A., Whitman, G. R., Arnold, S. L., Barrett, L. K., Schaefer, D. A., Riggs, M. W., & al, E. a. (2019, fall). 5-Aminopyrazole-4-carboxamide-based Bumped-Kinase Inhibitors 1770 and 1708: moving to a preclinical candidate for cryptosporidiosis.. Apicowplexa: 5 th International Meeting on Apicomplexan Parasites in Farm Animals. Berlin, Germany.
• Van Voorhis, W. C., Arnold, S. L., Ojo, K. K., Hulverson, M. A., Choi, R., Whitman, G. R., Barrett, L. K., Riggs, M. W., & al, E. a. (2019, fall). Lessons from BKIs: Why it may be hard to get a drug that treats Cryptosporidiosis and Cystoisoporiasis but also treats Toxoplasmosis, Sarcocystosis, and Neosporosis. Apicowplexa: 5 th International Meeting on Apicomplexan Parasites in Farm Animals. Berlin, Germany.
• Riggs, M. W. (2018, November). Adaptive Immune Responses Relevant to Cryptosporidium Vaccine Development. Advances in Cryptosporidium Research, an international scientific conference sponsored by the Bill and Melinda Gates Foundation. November 29 - December 1, 2018.. Tufts University School of Veterinary Medicine, North Grafton, Massachusetts.: The Bill and Melinda Gates Foundation..
• Viswanathan, V. K., Campos, S. K., Vedantam, G., & Riggs, M. W. (2018, May). Dissection of the Contributions of Enteropathogenic Escherichia Coli Secreted Effector Protein ESPZ to Virulence. Digestive Diseases Week. Washington DC: American Gastroenterological Association.
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  Invited oral presentation for DDW conference
• Riggs, M. W. (2016, Jan 22). The neonatal calf clinical model for cryptosporidiosis. Symposium on Animal Models and Drug Discovery for Cryptosporidiosis. San Francisco, CA: Bill and Melinda Gates Foundation.
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  Invited presentation.
• Riggs, M. W. (2016, June 21-22). Neonatal Mouse and Calf Models for Evaluation of Candidate Anti-Cryptosporidials. Structure Guided Drug Discovery for Tuberculosis and Malaria. Montreal, Quebec, Canada: Bill and Melinda Gates Foundation.
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  Invited seminar.
• Riggs, M. W. (2015, May). Lessons from Cryopreservation of Other Protozoa. Cryptosporidium Cryopreservation Workshop. Washington DC: Bill & Melinda Gates Foundation.
• Van Voorhis, W. C., Maly, D., Ojo, K. K., Fan, E., White, A. C., Castellanos, A., Riggs, M. W., Isoheranan, N., & Doggett, S. (2015, March). Bumped Kinase Inhibitors Targeting Cryptosporidium Calcium-dependent Protein Kinase1: Structure-based Drug Development to Proof of Concept in Calves.. 16th Drug Design & Development Seminar. Berlin Germany: Koch Institute.
• Van Voorhis, W. C., Schaefer, D. A., Barrett, L. K., DeRocher, A., Parsons, M., White, A. C., Riggs, M. W., Hemphill, A., Ortega Mora, L., & Dubey, J. P. (2015, November). Bumped Kinase Inhibitors areTherapeutics for Cryptosporidium, Toxoplasma, Neospora, and Sarcocystis infections.. Annual Meeting of the German Society of Parasitology.
• Van Voorhis, W. C., Schaefer, D. A., Rivas, K. L., Fan, E., Barrett, L. K., Zhang, Z., Ojo, K. K., & Riggs, M. W. (2015, June). A 5-aminopyrazole-4-carboxamide cryptosporidium therapeutic alleviates diarrhea in the calf-challenge model.. American Society of Tropical Medicine and Hygeine Annual Meeting.
• Riggs, M. W. (2014, 2014-11-14). Invited Seminar: Novel Strategies for Treating Cryptosporidiosis – A Re-emerging Disease of Humans and Animals.. Center of Biomedical Research Excellence in Immunobiology Seminar Series, Vermont Center for Immunology & Infectious Diseases.. Burlington, VT: University of Vermont.
• Riggs, M. W. (2014, 2014-11-16). Protective Immune Responses in Cryptosporidiosis: Considerations Relevant to Vaccine Design.. First meeting of the International Scientific Advisory Group for Cryptosporidium Strategy.. Washington, DC: Bill and Melinda Gates Foundation.
• Riggs, M. W. (2014, 2014-23-5). Invited Speaker: Immune Responses, Immunotherapy and Immunoprophylaxis in Cryptosporidiosis.. A One-Health Approach to Cryptosporidiosis: Exploring Synergies between Research on Human & Animal Infections.. London, UK: The Wellcome Trust and Bill & Melinda Gates Foundation.
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  Invited seminar presented at international scientific conference.
• Van Voorhis, W. C., Riggs, M. W., Lendner, M., Isoheranan, N., Schaefer, D. A., Ojo, K. K., Fan, E., Maly, D., Vidadala, R. R., Zhang, Z., Neckerman, G., & De Hostos, E. L. (2014, 2014-11-4). Bumped Kinase Inhibitors Treat Cryptosporidium Infection in the Calf Model. American Society of Tropical Medicine and Hygiene 63rd Annual Meeting. New Orleans LA: ASTMH.

Poster Presentations

• Choi, R., Hulverson, M. A., Huang, W., Whitman, G. R., Schaefer, D. A., Betzer, D. P., Riggs, M. W., & al, E. a. (2019, Fall). Bumped Kinase Inhibitors for the treatment of apicomplexan infections.. Conference for Research Workers in Animal Disease (CRWAD). Chicago, IL.
• Schaefer, D. A., & Riggs, M. W. (2018, November). Calf Clinical Model of Cryptosporidiosis for Efficacy Evaluation of Therapeutics.. Advances in Cryptosporidium Research, an international scientific conference sponsored by the Bill and Melinda Gates Foundation.. Tufts University School of Veterinary Medicine, North Grafton, Massachusetts.: Bill and Melinda Gates Foundation..
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  D. A. Schaefer presented this poster. M. W. Riggs co-author.
• Van Voorhis, W. C., Ojo, K. K., Arnold, S. L., Hulverson, M. A., Choi, R., Vidadala, R. S., Huang, W., Barrett, L. K., Schaefer, D. A., Riggs, M. W., Fan, E., & Maly, D. (2017, June). Bumped-Kinase Inhibitors for Therapy of Cryptosporidium Infection and Diarrhea. Invited Participant, Cryptosporidium Drug Accelerator Workshop at the Second International Symposium on Medicinal Chemistry for Global Health. Tres Cantos, Spain: Bill and Melinda Gates Foundation.
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  1. Poster co-author as above.2. M.W. Riggs, Invited participant, Cryptosporidium Drug Accelerator Workshop at the Second International Symposium on Medicinal Chemistry for Global Health, , Bill and Melinda Gates Foundation, Tres Cantos, Spain, June 19-23-2017.
• Ojo, K. K., Fan, E., Hemphill, A., Maly, D. J., Van Voorhis, W. C., Howe, D. K., Doggett, J. S., Lau, A. O., White, C., Ortega Mora, L. M., Lendner, M., & Riggs, M. W. (2014, 2014-11-4). Dual use of lead bumped kinase inhibitors as potential treatment for human and veterinary parasitic infectious diseases.. American Society of Tropical Medicine and Hygiene 63rd Annual Meeting. New Orleans LA: ASTMH.
• Thivierge, K., Cotton, S., Schaefer, D. A., Riggs, M. W., To, J., Lund, M. E., Robinson, M. W., Dalton, J. P., & Donnelly, S. (2013, July). Cathelicidin-like Helminth Defence Molecules (HDMs): Absence of Cytotoxic, Anti-microbial and Anti-protozoan Activities Imply a Specific Adaptation to Immune Modulation. American Society of Parasitologists. Quebec City, Canada.

Others

• Riggs, M. W. (2017, October). Invited International Scientific Meeting Participant, Annual Grand Challenges Meeting on Global Health.
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  M.W. Riggs, Invited Participant, Annual Grand Challenges Meeting on Global Health, co-sponsored by the United States Agency for International Development, Bill and Melinda Gates Foundation, Wellcome Trust and Grand Challenges Canada. Washington, D.C., October 1 - 4, 2017.