4.1. Resistance testing Guidelines for anthelmintic resistance testing using the FECRT have been thoroughly revised by a committee of experts appointed by the World Association for the Advancement of Veterinary Parasitology (WAAVP). These guidelines were published in 2023 (Kaplan et al., 2023), and veterinarians can read these for more detailed guidance. Note that the guidelines manuscript is published as open-access and is available free of charge at the publisher’s website. A general description of this new approach for anthelmintic resistance screening is outlined in the following paragraphs.
There are three new principles introduced within the WAAVP guidelines:
1) Thresholds for expected treatment efficacy are now based on historic efficacy data recorded for the anthelmintics when the products were first introduced to the equine market. These thresholds differ slightly between the different anthelmintic classes and parasite categories, and these differences impact the optimal protocol for testing each of the drug classes and/or parasite types.
2) Recommendations are based on an “eggs counted” principle, which refers to the number of eggs counted under the microscope before the conversion to eggs per gram (EPG) of feces. Defining the minimum number of eggs counted pre-treatment ensures adequate statistical power for the test. This also provides flexibility in the choice of FEC method depending on the mean FEC of the group of horses.
3) The conclusions are no longer based solely on calculating mean percent egg count reductions, but instead on statistical confidence limits, which consider the variation observed in the levels of egg count reduction seen between horses.
The guidelines have two efficacy thresholds for interpreting the results: an upper threshold, which represents the expected level of efficacy (when there is no resistance) of the product based on historic information; and a lower threshold, below which the efficacy can be interpreted as significantly reduced and indicative of drug resistance. The interval between the two thresholds is considered a grey zone, where more information is needed before a conclusion of resistance or susceptibility can be drawn. These principles are described in Figure 1.
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