While drug resistance has been known to be common in equine gastrointestinal parasites for several decades, this has been restricted to three parasite categories: the small strongyles (cyathostomins), the ascarids (Parascaris spp.) and the pinworms (Oxyuris equi), all of which belong to the nematode family. However, in the past two years, we have seen mounting evidence suggesting that anthelmintic resistance could be emerging in the equine tapeworm species, Anoplocephala perfoliata, which belongs to the flatworm family, and is a well-documented cause of colic in horses.
Starting in early 2022, local veterinarians in Central Kentucky were reporting that they were observing tapeworm eggs in routine fecal samples analyzed two weeks post-deworming with products containing praziquantel. Given that this is one of two currently available anthelmintics with documented efficacy against tapeworms, this was a concerning finding. Our laboratory received some of these samples and we were able to confirm the findings. Since this finding, we have evaluated the efficacy of different praziquantel products as well as the only available treatment alternative – a double dose of pyrantel pamoate – and documented a lack of clearance of tapeworm eggs in all cases. These observations were primarily made in groups of yearlings on two farms; we were also able to evaluate a praziquantel product in one group of mares with a similar apparent lack of efficacy.
Any age group can harbor these parasites and the occurrence and infection intensity does not decline with age.
Historic field efficacy trials with these anthelmintic products had all demonstrated complete or near complete clearance of tapeworm eggs from fecal samples in treated horses, so these findings are strongly suggesting emergence of multidrug resistant equine tapeworms. Our findings have been documented in two different peer-reviewed publications, and local veterinarians have made similar observations in other equine operations in the area. This is very concerning given that there are no treatment alternatives currently available with documented efficacy against tapeworms.
These findings emphasize the importance of routine anthelmintic efficacy testing. The veterinarians discovered this lack of treatment efficacy because of having systematic testing protocols in place on these farms. Since praziquantel and pyrantel products are very commonly used around the world, and routine efficacy testing is still rarely implemented, it is likely that a similar, yet undetected, development could be occurring elsewhere. One aspect that separates the tapeworms from all the nematode gastrointestinal parasites is that horses do not develop immunity to the tapeworms. Thus, any age group can harbor these parasites and the occurrence and infection intensity does not decline with age. We, therefore, strongly recommend the implementation of routine anthelmintic treatment efficacy testing procedures in all horse operations.
It is remarkable that these findings were made using routine standard fecal egg counting techniques that were not optimized for tapeworm detection. While it is well described that these techniques can substantially underdiagnose tapeworm infection in horses, it should be noted that they, nonetheless, were able to detect this lack of treatment efficacy because of the tendency to underestimate and not overestimate tapeworm presence. Provided that the evaluator is experienced in identifying tapeworm eggs, the finding of tapeworm eggs post-deworming is suggestive of treatment failure. While work is currently underway to develop refined testing protocols for measuring anthelmintic treatment efficacy against equine tapeworms, the current recommendation is to make sure to count and record the number of equine tapeworm eggs present both pre- and post-deworming.
