In the past decade, veterinarians and horse owners have confidently followed preventive deworming practices against equine tapeworms. A double dose of praziquantel has had 95% efficacy against Anoplocephala perfoliata and praziquantel has a reported >99% efficacy against this parasite by 14 days after administration. A recent study evaluated performance of these anthelmintics in the field [Nielsen, MK. Failure of praziquantel and pyrantel pamoate against tapeworms. International Journal for Parasitology: Drugs and Drug Resistance June 2023; doi: 10.1016/j.ijpddr.2023.06.002].
A Thoroughbred farm in Central Kentucky used routine deworming and testing from February – April 2023. Yearlings were spread out over four different barns, and two mare groups were stabled at two other barn facilities. The deworming program was aligned with current recommendations and fecal egg count testing. Treatment with piperazine and fenbendazole at 2-4 months of age was followed by ivermectin and praziquantel at five months. This was then followed by moxidectin and praziquantel in the late fall of 2022. The mares received ivermectin/praziquantel in the spring and moxidectin/praziquantel in the fall; foaling mares were treated with ivermectin within five days of foaling. Horses tested as high shedders also received ivermectin treatment during the summer.
Fecal Egg Count Techniques for Tapeworms in Horses
Nielsen noted that standard fecal egg count techniques for tapeworms are lacking in performance, with egg counts reported at the low end of what may be present. He commented, “Further development and refinement of fecal egg counting techniques for equine tapeworm eggs is warranted before we can develop a framework for appropriate treatment efficacy testing. Having said this, however, it should be acknowledged that several field efficacy trials for both praziquantel and pyrantel pamoate have demonstrated that tapeworm fecal egg counts can provide some indication of anti-cestode efficacy despite the described limitations.” He also noted that serum and saliva ELISAs to detect antibodies against A. perfoliate are not useful to assess treatment efficacy due to the long half-life of the antibodies and continued parasite exposure.
Treatment Failure
The data in this study has identified apparent treatment failure to two different anthelmintic classes with different modes of action: “Praziquantel eliminated tapeworm eggs from three of 17 yearlings, but another five yearlings went from negative to positive status following treatment. Pyrantel pamoate failed to eliminate tapeworm eggs from any of 14 treated tapeworm-positive yearlings. Nine of 84 mares tested positive for tapeworm eggs, and seven were still positive post-praziquantel treatment.”
Antithelmintic Resistance
Besides treatment of tapeworms, the study identified that efficacy of ivermectin has fallen to fecal egg count reduction (FECR) testing of strongyle eggs of 30-70%; for pyrantel, testing data demonstrates strongyle FECRs of 38-75%. This data is comparative to data previously reported in past decades with very high efficacy of these anthelmintics. Regarding tapeworms, Nielsen added, “Resistance could have developed at different paces over the course of several years, if not decades, before it finally was noticed at a stage where resistance had fully developed to both active compounds.”
At this time, no new anti-cestode drugs are marketed in the USA, but there is a licensed formulation of bithionol in Japan that has documented efficacy. A US study has looked at nitazoxanide with favorable results against cestodes. Nielsen also mentioned potential for niclosamide and closantel to show promise as anti-cestode medications.
Final Thoughts on Tapeworm Resistance in Horses
In summary, Nielsen reported, “Given the lack of FECRT standards for equine tapeworms and limitations of available fecal egg counting techniques, the data presented herein do not allow the conclusion of anthelmintic resistance at this stage. Nonetheless, the apparent lack of efficacy of both praziquantel and pyrantel pamoate is deeply concerning and warrants further research.” He emphasized the importance of monitoring anti-cestode activity of praziquantel and pyrantel around the world.
