6.1. Shortened ERP: Interpretation For many years it was assumed, without evidence, that shortened ERP is a sign of emerging resistance (Sangster, 1999). However, this is not necessarily the case, and recent data suggest that selection of parasites with life cycles of shorter duration may have occurred (Nielsen et al., 2022a). The practical consequence of reduced ERPs is that egg output suppression is not maintained for extended periods following treatment. This can lead to a significant increase in egg shedding onto the pastures leading to a reduction in the level of parasite control. A computer simulation study suggested that parasite burdens may increase up to several fold when ERPs are reduced to 4–5 weeks, depending on age group, climatic conditions, and treatment protocols (Nielsen et al., 2023).
6.2 Determining ERP The World Association for the Advancement of Veterinary Parasitology (WAAVP) has recently published guidelines for determining ERP (Nielsen et al., 2022b). The determination is based on fecal egg counting reductions (FECRs), so many of the principles outlined for the fecal egg count reduction test (FECRT) apply to ERP determination as well. While best practice would entail sampling a group of horses on a weekly basis for several weeks following treatment, a more realistic and pragmatic approach would be to “spot-check” ERP by determining FECR at a strategically chosen time interval, such as 4, 6, or 8 weeks. Egg reappearance for the macrocyclic lactone class occurs when the upper 90% confidence interval for the calculated FECR falls below a threshold of 90% reduction. This will provide useful information about the performance of ivermectin and moxidectin.
7. Larvicidal Treatment
The term larvicidal treatment is used for anthelmintics with activity against encysted cyathostomin larvae. Currently, there are two anthelmintics with registered efficacy against these stages: moxidectin and fenbendazole. However, contrary to commonly held beliefs, neither of these can be expected to perform with high larvicidal efficacy. Several recent studies have demonstrated that both compounds reduce larval burdens by less than 85% (Nielsen, 2022). For the five-day regimen of fenbendazole, this is a pronounced reduction from historically reported efficacy levels and fulfills the criteria for anthelmintic resistance. For moxidectin, the larvicidal efficacy has always been variable with most studies reporting efficacies in the 50–70% range against developing larval stages (Nielsen, 2022). Thus, moxidectin appears to have maintained this larvicidal efficacy level, although a recent study suggested that the reduction in encysted larval counts may be short-lived among horses kept on pasture (Nielsen et al., 2022a).
There are currently no data demonstrating a clinical benefit of larvicidal treatment. Larval cyathostominosis, the clinical syndrome caused by encysted cyathostomins, appears to be extremely rare in North American horses, and the efficacy of routine larvicidal treatments to reduce its risk is unknown. This makes assessing the prophylactic value of larvicidal treatments in populations of horses very challenging. Furthermore, the expected efficacy of a larvicidal treatment is substantially below 100%, with significant proportions of encysted larvae expected to survive treatment regardless of the anthelmintic. Larval reductions are also transient, with larval counts rebounding within 5 weeks post-treatment (Nielsen et al., 2022a).
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