Warning of multiple anthelmintic resistance

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Research published recently in the International Journal of Parasitology warns of  impending multi-drug resistance in strongyles on British Thoroughbred stud farms.

The study, involving scientists in Edinburgh, Bristol and Liverpool, looked at the efficacy of  anthelmintics against worms  in horses on 16 stud farms.

To assess the efficacy against strongyles, faecal worm egg count reduction (FECR) tests were carried out on samples from 429 horses. Faecal  egg counts were performed before and 14 -17 days after treatment with one of the four available broad-spectrum anthelmintics: ivermectin; moxidectin; pyrantel or fenbendazole.

Moxidectin was efficacious in all tests, resulting in mean faecal egg count reduction in the range of 99.8-100%. Ivermectin showed reduced efficacy (FECR 85.7% -100%) in one group of yearlings. Five groups of yearlings showed reduced efficacy of pyrantel (FECR ranging from 0-13%). However, pyrantel was still efficacious when administered to mares. Fenbendazole always had low efficacy (FECR  0.4 – 42%).

A different pattern of efficacy was found against the large roundworm Parascaris equorum. Tests  carried out on four farms showed reduced efficacy of ivermectin (FECR  25.5% – 91.2%). However fenbendazole had acceptable efficacy (FECR 97.5% – 99.9%).

The research team also performed faecal egg counts at about 2 week intervals after anthelmintic treatment for up to 12 weeks to determine the strongyle egg reappearance period for moxidectin, ivermectin and pyrantel. They found that the egg reappearance period for all three anthelmintics was shorter than had been observed previously. This is considered to be an early indicator of developing resistance.

“Overall,” they conclude, “our results indicate that ivermectin and moxidectin administration provided acceptable efficacy at 14 days; however, egg reappearance period results suggest that these products are working less effectively than measured previously. As shortened egg reappearance period is believed to be an early indicator of resistance, this highlights the issue of impending multi-drug resistance in strongyles on stud farms.”


For  more details see:

Anthelmintic efficacy on UK Thoroughbred stud farms.
Relf VE, Lester HE, Morgan ER, Hodgkinson JE, Matthews JB.
Int J Parasitol. 2014 Apr 15. pii: S0020-7519(14)00078-2.
doi: 10.1016/j.ijpara.2014.03.006.