New Statistical Model to Evaluate Dewormer Efficacy

New Statistical Model to Evaluate Dewormer Efficacy

When dewormer resistance occurs on a farm, it will be evident in several horses, as they all share the same parasite population.

Photo: Kristen M. Janicki, MS, PAS

Research teams from the University of Kentucky Gluck Equine Research Center and George Mason University have defined a new advanced statistical model to evaluate anthelmintic dewormer efficacy. The researchers set out to illustrate sources of variability in fecal egg count reduction tests in horses and to develop a model to identify biological factors such as age, gender, and farm management that affect dewormer efficacy.

Fecal egg count reduction tests to evaluate dewormer efficacy is the most common method to determine drug resistance to horse parasites. In cases where the efficacy is either very high or very low, there are few statistical challenges to consider, and results are often clear without complicated data analysis.

Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, assistant professor at the Gluck Center, said when results are close to the chosen cutoff value for determining resistance, the variability plays an important role and can lead to misclassification of some farms.

“The statistical model we have developed accounts for various biological sources of egg count variability and provides a more reliable result,” he said. “We chose to study the efficacy of pyrantel pamoate (a commonly used dewormer). Efficacy data generated with this drug is known to have a great deal of variability. The overall aim of the study is to differentiate between a true egg count reduction and an apparent reduction due to chance variability in the data.”

The team performed their study on 64 Danish horse farms of different breeds. Of 1,644 horses, 614 had egg counts more than 200 eggs per gram and were treated. They collected fecal samples before treatment and again two weeks after pyrantel pamoate treatment.

The data showed the statistical model could be organized. Because horses on the same farm share the same management, soil type, and vegetation, their worm burdens are likely to be more alike when compared to horses from other farms, Nielsen said. There will often be differences between farms, but farms in the same geographic area are likely to be alike due to similar soil and vegetation types, and perhaps even management habits.

“In our data, the hierarchy is horses within farms, and farms within defined geographical areas. If these sources of variability are well-controlled, we can generate a more reliable result. Our analyses illustrated this point,” he said.

Nielsen said that golden rule is that whenever dewormer resistance occurs, it will be evident in several horses, as they all share the same parasite population.

“Nonetheless, we often observe that farms are classified with dewormer resistance because of just one horse having a very low egg count reduction,” Nielsen said.

Shaila Sigsgaard is an editorial assistant for the Bluegrass Equine Digest.

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