5.2. Pinworms Apparent resistance to ivermectin and moxidectin has been described in O. equi in Europe, New Zealand, and both North and South America (Nielsen, 2022). However, testing for resistance in O. equi is challenging and FECRT cannot be used. Published studies have made use of the Scotch tape test for evaluating treatment efficacy (Nielsen, 2022). If macrocyclic lactone resistance is suspected based on lack of clinical response, the available evidence suggests benzimidazoles should be given priority over pyrantel salts due to better historic efficacy levels (Reinemeyer and Nielsen, 2014).
5.3. Other parasites Some veterinarians report an apparent lack of response to macrocyclic lactone treatment of lesions suspected to be caused by Habronema spp. larvae. However, no such reports exist in the peer-reviewed literature, so it is not possible to evaluate the evidence underlying these claims. Furthermore, it should be emphasized that no anthelmintic formulation has ever been scientifically evaluated for efficacy against Habronema spp. larvae present in skin or mucosal lesions, no products are registered with label claims against these, and there is no practical means to measure efficacy against these larvae in a farm environment. Thus, it is scientifically impractical to determine if these observations are due to anthelmintic resistance.
6. Strongyle Egg Reappearance Periods
The term egg reappearance period (ERP) describes the amount of time it takes from the date of an effective deworming in a group of horses with positive strongyle fecal egg counts until eggs can be found in the feces again. In other words, this is a measure of how long the treatment, if effective, can suppress strongyle egg shedding, which is a meaningful additional measure of performance of the anthelmintic. While ERP can be measured for all anthelmintics, it is generally only relevant to ivermectin and moxidectin. This is because resistance is widespread to the two other anthelmintic classes, which means that they most often do not effectively reduce strongyle FECs to begin with. Furthermore, even if they are effective, there is not a lot of useful information that can be gained from the ERP of these drug classes.
Over the past three decades, ERP following both ivermectin and moxidectin treatment has decreased dramatically (Table 5). In the 1990s, moxidectin performed with ERPs in the 12–16- week range, which was substantially longer than any other anthelmintic. In comparison, ivermectin ERPs were in the 8–10-week range (Nielsen, 2022). However, since 2017, several studies have reported ERPs to be 4–5 weeks for both drugs (Nielsen, 2022).
Table 5. Strongyle egg reappearance periods reported for ivermectin and moxidectin in the 1990s compared to several studies conducted since 2017. 1990s
Now
Ivermectin Moxidectin
Data derived from Nielsen, 2022 18
8–10 weeks 12–16 weeks
4–5 weeks 4–5 weeks
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