Nematode parasitism in sheep in North-East England: The epidemiology of Ostertagia species

This investigation was undertaken to determine the epidemiology of a single parasite genus, Ostertagia, in natural parasite populations in sheep raised under intensive conditions in North-East England.Over the 2 years of study, striking differences were found in the overall level of parasitism which were related to different climatic conditions, specifically rainfall. Despite the low source of infection derived from the ewes grazing on clean pasture in 1973 the overall level of parasitism in the lamb crop was high, this was attributed to the ideal climatic conditions for parasite translation during Spring and Summer. Ostertagia spp. was found to survive well over Winter and to initiate early infections in the young lambs in the following year. Despite the two sources of egg contamination, from the ewes and lambs, unusually dry conditions were experienced from March to June which resulted in much lower levels of infection in the second year of investigation.Regulation of the parasite population within the lambs was clearly demonstrated. Inhibition of Ostertagia was recorded somewhat earlier in the season than had been hitherto reported and was considered to be induced by a multiplicity of factors.