Pyrantel pamoate resistance in horses receiving daily administration of pyrantel tartrate

Case Descriptions—16 horses treated daily with pyrantel tartrate (2.64 mg/kg [1.2 mg/lb], PO) as part of a prophylactic anthelmintic program.

Clinical Findings—Fecal worm egg counts (FWECs) were obtained on all 16 horses. Mean FWEC was 478 eggs/g (epg; range, 0 to 4,075 epg). Three of the 16 horses were responsible for 85% of the total fecal egg output for the herd on the day of sampling. Six horses had FWECs < 200 epg. Three horses that had arrived within 4 months of the sampling date had FWECs < 100 epg.

Treatment and Outcome—An FWEC reduction test was initiated the day after FWECs were obtained; all horses with FWECs > 100 epg (9 horses) were treated with pyrantel pamoate (6.6 mg/kg [3 mg/lb], PO), and 14 days later, the FWEC was repeated. During the 14-day period, all horses received pyrantel tartrate (2.64 mg/kg, PO) daily. Fecal worm egg count reduction was calculated for each horse. Mean FWEC reduction for the group was 28.5% (range, increase of 21% in FWECs 14 days after treatment to a decrease of 100% in FWEC 14 days after treatment).

Clinical Relevance—Farms should be monitored for cyathostomes resistant to pyrantel pamoate prior to use of pyrantel tartrate. Fecal worm egg counts should be monitored routinely in horses before and after treatment to ensure efficacy of cyathostome control measures.