Investigations in Yorkshire have discovered a significant number of horses are infected with anthelmintic resistant worms . Particularly concerning was the finding that a five day treatment with fenbendazole did not overcome the resistance. Previous work had suggested that fenbendazole (at a dose rate of 7.5mg/kg for 5 days) was effective against inhibited mucosal cyathostome larvae even when resistant adult cyathostomes were present.
Keith Chandler, and his colleagues, collected faecal worm egg counts (FWECs) from 200 horses which were seen as part of the work of an equine veterinary practice in York. About 25% of samples contained worm eggs. All horses with worm eggs in their faeces (positive FWEC`s) were treated with a five day course of fenbendazole (Panacur Equine Guard). A second sample was examined 10-14 days after the last treatment. Overall 50% of the horses that received the five-day course of fenbendazole still had positive FWECs 10-14 days later, suggesting that resistance to this product is widespread in the animals under study. The eggs found in the faeces after treatment were cultured. All were found to be cyathostomes.
The prevention of cyathostome-associated disease relies on regular anthelmintic dosing to suppress faecal worm egg production and to reduce the build up of infective larvae on the pasture. A common cause of failure in worm control programs is the development of anthelmintic resistance. Fenbendazole resistance was found to be widespread in the south of England in the early 1990`s. More recently, pyrantel resistance has been reported in the UK and elsewhere.
Benzimidazole resistance seems to develop in stages.
* Individual worms are still susceptible to higher doses of the drug. In this case giving a 5-day course of fenbendazole may overcome the resistance.
* Increased level of resistance - the worms are no longer susceptible even to repeated doses, as in the animals in this study.
Mr Chandler concludes that these findings are of concern because, if they are representative of other equine populations, we can assume that fenbendazole based parasite control programs will fail to control pasture contamination on a wide range of equine properties.
He suggests that regular screening for anthelmintic resistance should be considered. "Resistance to fenbendazole is so widespread now that this anthelmintic should be avoided in prophylactic dosing regimes, unless faecal egg count reduction tests prove that the drug is still effective. A five-day course of fenbendazole should be reserved for the treatment of clinically affected animals or those animals at higher risk of developing cyathostomosis."
For more details see: K.J. Chandler, M.C.Collins, S.Love. Efficacy of a five-day course of fenbendazole in benzimidazole- resistant cyathostomes.Veterinary Record (2000) 147, 661-662