Warning of multiple resistance to worm control drugs

Parasite controlResearchers have found signs of impending multi-drug resistance in strongyles on British thoroughbred stud farms.

The findings of the study, published recently in the International Journal of Parasitology,involved scientists in Edinburgh, Bristol and Liverpool, who looked at the effectiveness of anthelmintics against worms in horses on 16 stud farms.

To assess their effectiveness 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 to 17 days after treatment with one of the four available broad-spectrum deworming drugs – ivermectin, moxidectin, pyrantel or fenbendazole.

The latest edition of Equine Science Update reports that moxidectin was effective in all tests, resulting in mean faecal egg count reduction in the range of 99.8 to 100 percent.

Ivermectin showed reduced less effective (FECR 85.7 to 100 percent) in one group of yearlings. Five groups of yearlings showed the reduced effectiveness of pyrantel (FECR ranging from 0-13 percent). However, pyrantel was still effective when administered to mares.

Fenbendazole always had low effectiveness (FECR 0.4 to 42 percent).

A different pattern of effectiveness was found against the large roundworm Parascaris equorum. Tests on four farms showed reduced effectiveness of ivermectin (FECR 25.5 to 91.2 percent). However, fenbendazole gave acceptable results, at 97.5 to 99.9 percent.

The research team also performed faecal egg counts at about two-week intervals after 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, 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,” they concluded.

“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.”

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.


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