Comparison of two control systems for cyathostome infections in the horse and further aspects of the epidemiology of these infections.

The small strongylid infections of two groups of three yearling female Shetland ponies and one yearling Shetland tracer pony were studied. One group was set stocked from April to November and was treated monthly with 5 mg kg-1 albendazole from two days before turnout until July. The other group grazed similar pasture until July, was treated with 5 mg kg-1 albendazole and subsequently removed to pasture grazed by sheep from April to July. The tracer ponies were added to both groups in September. The efficacy of both methods was not completely satisfactory probably because of low efficacy of anthelmintic treatment. There were no significant differences between the cyathostome burdens of the two groups. A high proportion of the cyathostome populations of all ponies consisted of inhibited early third stage larvae (L3). The finding of low numbers of immature fifth-stage worms in the tracer ponies indicated that the considerable adult burdens in the permanent ponies originated from infection picked up before the tracer ponies were added. In the group which was removed to sheep pasture after treatment in July it was likely that the majority of the adult worm burden had been ingested as infective larvae before treatment.