Evidence of tapeworm treatment failure on a Central Kentucky Thoroughbred farm

Background: Equine anoplocephalid cestodes are tapeworms that occur worldwide in horses and are known to cause pathological manifestations and clinical disease within the digestive system of horses. Objective: While these clinical manifestations and the efficacy of currently available anthelmintics against equine tapeworms have been well documented, a recent study has demonstrated treatment failure of praziquantel and pyrantel pamoate, suggesting resistance; however, this has not been heavily investigated. Study design: This case study examined 24 Thoroughbred yearlings on one Kentucky Thoroughbred horse farm to investigate the elimination of tapeworm eggs after administration of routine anticestodal anthelmintics after confirmation of positive faecal egg counts. Methods: The Mini-FLOTAC faecal egg counting method was used to count anoplocephalid eggs in yearlings treated with pyrantel pamoate paste (13.2 mg base/kg), moxidectin-praziquantel (0.4 and 2.5 mg/kg) and ivermectin-praziquantel (0.2 and 1 mg/kg) over a 6-month period in 2022. Results: Data demonstrated that tapeworm egg shedding persisted post-deworming with all three anthelmintic products. Out of the 24 Thoroughbred enrolled, 11/24 (45.8%) had persistent infections in the month of March and 3/24 (12.5%) had persistent infections in April after deworming with ivermectin-praziquantel. With moxidectin-praziquantel, 2/24 (8.3%) remained egg count positive. For pyrantel pamoate, 3/24 (12.5%) were detected with tapeworm eggs post-treatment. During July and August, selected yearlings had faecal sampling done before and 2 weeks post-deworming with moxidectin-praziquantel. For July 1/6 (16.7%) horses had tapeworm eggs both before and after deworming, and one additional horse was positive in the post-treatment sample only. For the August treatment, 4/14 (28.6%) were egg count positive both before and after deworming, with one additional horse being positive in the post-treatment sample only. Conclusions: This study demonstrated evidence of anthelmintic treatment failure against equine tapeworms by means of regular faecal egg count monitoring. Veterinarians should be aware of this development and implement strategies to monitor anticestodal treatment success.