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Control, Transmission, and Prevalence of Natural Infections of Internal Parasites of Equids and Ruminants
Department of Veterinary Sciences
Effects of internal parasites of domestic animals range from causing no perceivable clinical problems to death of these hosts. Historically, many species of internal parasites have shown resistance to chemical compounds after a period of usage. Research is necessary to continuously monitor the level of activity of the drugs. The purpose is to provide up-to-date information to animal owners and veterinarians about the the most active or inactive compounds for parasite control.
2010 Project Description
Most of the research has been on continuing parasite studies in equids. This is in regard especially to drug resistant parasites. The probable reason for return of small strongyle eggs currently much sooner than after initial treatment with ivermectin was found to be because of lessened drug activity on immature and young adult stages in the large intestine. This same phenomenon seems evident for the closely related compound moxidectin.
Extensive field studies have verified the common occurrence of drug resistance of ascarids to ivermectin and lowered activity of pyrantel pamoate to these parasites. These studies also have revealed that oxibendazole is now the most active compound for removal of ascarids, with fenbendazole being somewhat less efficacious. Analysis of over 50 years of parasite data from horses in Kentucky was done. Results showed that there is no absolute relationship to numbers of ascarids and small strongyles and egg/larval counts. There was closer correlation of egg counts less than 500.
Three previously unrecognized species of small strongyles were identified in Kentucky horses. New data on the approximate prepatent period, i.e. time from infection to egg-laying was obtained for several species of small strongyles
The results from the various studies have been published and also disseminated to horse persons, veterinarians and researchers. Practical value of the research is that these multi-year studies allow following and keeping up to the ever-changing pattern of efficacy of compounds against internal parasites. All of the parasiticides on the market for equids are showing reduced or virtually immeasurable activity against several species of nematodes. It is known that classes of these drugs initially were highly active on nematodes but less so after extensive usage. Preliminary studies have been shown that profiles of strongyle eggs per gram of feces (EPGs) can be determined especially for older horses. Thus, EPG values which are very low indicate unnecessary treatment of these horses.
Lyons, E.T., Tolliver, S.C., and Collins, S.S. 2009. Probable reason why small stongyle EPG counts are returning early after ivermectin treatment of horses on a farm in Central Kentucky. Parasitology Research 104:569-574.
Ionita, M., Howe, D.K., Lyons, E.T., Tolliver, S.C., Kaplan, R.M., Mitrea, I.L., and Yeargan, M. 2010. Use of a reverse line blot assay to survey small strongyle (Strongylida: Cyathostominae) populations in horses before and after treatment with ivermectin. Veterinary Parasitology 168:332-337.
Rossano, M.G., Smith, A.R., and Lyons, E.T. 2010. Shortened strongyle-type egg reappearance periods in naturally infected horses treated with moxidectin and failure of a larvicidal dose of fenbendazole to reduce fecal egg counts. Veterinary Parasitology 173:349-352.
Lyons, E.T., Tolliver, S.C., Kuzmina, T.A., and Collins, S.S. 2010. Critical tests evaluating efficacy of moxidectin against small strongyles in horses from a herd for which reduced activity had been found in field tests in Central Kentucky. Parasitology Research 107:1495-1498.
Lyons, E.T., Tolliver, S.C., Collins, S.S., Ionita, M., Kuzmina, T.A., and Rossano, M. Field tests demonstrating reduced activity of ivermectin and moxidectin against small strongyles in horses on 14 farms in Central Kentucky in 2007 to 2009. Parasitology Research (Published On Line on September 23,2010).
Nielsen, M.K., Baptiste, K.E., Tolliver, S.C., Collins, S.S., and Lyons, E.T. 2010. Analysis of multiyear studies in horses in Kentucky to ascertain whether counts of eggs and larvae per gram of feces are reliable indicators of numbers of strongyles and ascarids present. Veterinary Parasitology 174:77-84.
Lyons, E.T., and Tolliver, S.C. Macrocyclic lactones for parasite control in equids. Current Pharmaceutical Biotechnology (accepted for publication, 2010).
Lyons, E.T., Ionita, M., and Tolliver, S.C. 2010. Important gastrointestinal parasites. In McKinnon, AO, Squires, EL, Vaala, WE and Varner, DD (Eds), Equine Reproduction. Blackwell Publishing, pp. 292-301.