Studies on the epidemiology of Dictyocaulus filaria infection in Blackface sheep on a low-ground Scottish farm.

The excretion of Dictyocaulus filaria larvae in the faeces of Blackface sheep on a heavily stocked lowland farm in south east Scotland, showed considerable individual, week to week and year to year variation. Patent infections were first observed in lambs in May or June but the heaviest and most prevalent infections occurred in the autumn. Infection levels were generally low but infections tended to be prolonged and reinfection occurred following anthelmintic treatment until the late autumn or winter. Only a few very light, short-lived, patent infections were observed in yearlings or adult ewes. Pasture sampling and the use of tracer animals showed that the numbers of infective larvae on the pasture were minimal during the summer months but reached a low peak in the autumn.