The University of Georgia College of Veterinary Medicine

Georgia Veterinary Scholar Program

2003 Scholars

Georgia Veterinary Scholar	Faculty Mentor

Jennifer Neiss University of Georgia Class of 2005	Dr. Ray Kaplan

**Moxidectin Resistance in Haemonchus contortus in Goats: In Vivo and In Vitro Diagnosis**

Gastrointestinal nematode parasites are the single greatest threat to the health and productivity of goats in the southern United States (US). Although many species of parasites contribute to the overall problem of gastrointestinal parasitism, Haemonchus contortus is by far the most prevalent, pathogenic, and economically important. Control programs for H. contortus typically rely on frequent treatment with anthelmintics, but the intensive use and virtual total reliance on these drugs has led to the development of anthelmintic-resistant nematode populations, which are now recognized globally as the single greatest threat to grazing small ruminant production. Recent surveys in Georgia goat herds suggest that ivermectin resistance is highly prevalent; but moxidectin, a closely related drug, retains efficacy against these resistant worms. However, previous studies have demonstrated that the superior efficacy of moxidectin is due only to its higher potency; comparative efficacy studies between sensitive and resistant nematode strains demonstrate that ivermectin-resistant worms are also technically moxidectin-resistant. Therefore, it is expected that farms with ivermectin-resistant nematodes will rapidly develop moxidectin resistance if frequent treatment with moxidectin is applied. Additionally, currently available in vitro assays for detecting anthelmintic resistance only include tests for ivermectin, but not moxidectin resistance. Since both of these drugs have the same mechanisms of action and resistance, with sufficient data it should be possible to use the in vitro data for ivermectin to predict resistance to moxidectin. In this study, groups of goats on 5 farms previously diagnosed with ivermectin-resistant and moxidectin-sensitive H. contortus will be retested with varying concentrations of moxidectin to determine an in vivo dose response. DrenchRite larval development assays will also be performed on theses farms, and in vivo LD50s for moxidectin will then be correlated with in vitro LC50s for ivermectin. Since approximately 2 years have passed since anthelmintic efficacy was tested on these farms and moxidectin has been used extensively over this time, it is expected that on 1 or more farms moxidectin resistance will be diagnosed. If moxidectin resistance is diagnosed, this will be the first report of such resistance in the in US. Furthermore, it is expected that these data will enable us to modify interpretation of DrenchRite results for ivermectin so that this assay could also be used to diagnose moxidectin resistance and/or predict occurance of moxidectin resistance in the near future. This project will provide a variety of animal handling and laboratory experiences. All goats in this study will be treated with anthelmintic, have blood and fecal specimens collected, and will be examined and scored using the FAMACHA system. Laboratory techniques will include performance of fecal egg counts, fecal cultures, larval identifications, DrenchRite assays, measurement of PCV and data analysis.