5. Anthelmintic Resistance Status
Current levels of anthelmintic resistance in equine parasites are summarized in Table 4. The occurrence of resistance is consistent in managed horses across the world, but notable differences can be found between individual farms, and resistance cannot be concluded on any given farm without proper testing. It is important to remember that a given regional prevalence only tells you what to expect across many farms/stables; it will not tell you what the resistance status is for a specific farm/stable. Thus, Table 4 only presents which parasite groups are most likely to show resistance to which drug class.
Table 4. Current levels of resistance documented in peer-reviewed studies in equine cyathostomin, pinworm, ascarid, and tapeworm parasites to the four anthelmintic classes in managed horse herds. Generally, these are worldwide trends that have also been reported in several US surveys.
Drug class
Benzimidazoles Pyrimidines
Macrocyclic lactones Praziquantel
Cyathostomins Widespread Widespread
Pinworms None None
Early indications Widespread -
-
Parascaris spp. Early indications
Widespread: reported on multiple continents with high farm prevalences often above 80% Early indications: few single farm cases of reduced efficacy
5.1. Tapeworms Recent data suggest treatment failure of praziquantel and pyrantel pamoate against A. perfoliata in Central Kentucky (Nielsen, 2023; Finnerty et al., 2024). Veterinarians should consider periodically screening for the presence of tapeworm eggs in fecal samples 14 days post-treatment to ensure that the treatment had adequate apparent efficacy. A standardized FECRT protocol has not been developed for equine tapeworms at this stage, but recent data suggest that even a routine FEC screening technique can provide indication of treatment efficacy, when counts are determined before and 14 days after administration of the anthelmintic to a group of horses (Nielsen, 2023). However, it should be emphasized that, due to the randomness of finding tapeworm eggs in feces, a high level of variation can be expected and diagnostic sensitivity may be poor. Thus, as with testing only a few horses for strongyle resistance, data indicating poor efficacy will be more reliable than data indicating high efficacy, which might occur by chance.
Given these challenges, group sizes are important. In the recently published study, yearling groups were in the range of 10-16 horses (Nielsen, 2023), which was enough to produce treatment efficacy information. Using a technique such as the modified centrifugation-based technique mentioned above for detection and enumeration of equine tapeworm eggs in fecal samples (e.g., Proudman and Edwards, 1992) will count more eggs (Anderson et al., 2024) and will, thus, provide more useful indications of treatment efficacy. In general, both pyrantel pamoate (13.2 mg base/kg) and praziquantel (1.0-2.5 mg/kg) should be expected to reduce anoplocephalid egg counts by at least 95% at 14 days post treatment.
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Tapeworms -
Early indications Early indications Widespread -
- Early indications
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