The effect of ivermectin treatment against inhibited early third stage, late third stage and fourth stage larvae and adult stages of the cyathostomes in Shetland ponies and spontaneous expulsion of these helminths.

A controlled and critical test on the efficacy of ivermectin against larval and adult stages of the cyathostomes was carried out in six yearling castrated male Shetland ponies. The ponies grazed together as one group from 3 May to 4 October 1990, after which they were housed. Three ponies were treated with ivermectin on 29 October while the others served as controls. The shedding of helminths in the faeces was followed in all ponies until necropsy on 14 November. Comparison of worm counts of both groups before and after necropsy showed no evidence for an effect of ivermectin against inhibited early third stage larvae (EL3) and mucosal late third stage (LL3) and fourth stage larvae (L4). However, a high, but not 100%, efficacy was observed against adults and lumenal L4. A remarkable observation was the high incidence of spontaneous expulsion of L4 and adult populations of some species in two of the untreated ponies.