Epidemiological studies on gastro-intestinal nematode parasites of sheep: the seasonal number of generations and succession of species.

Monthly post mortem worm counts were carried out from June to September on grazing lambs, in association with faecal egg counts and pasture larval counts. Similar patterns were recorded by all three methods, and egg counts proved to be a reliable reflection of worm burden over this period. A succession of species was recorded, in particular Ostertagia spp and Nematodirus spp in June; Haemonchus contortus in July; Trichostrogylus virinus in August; T axei, T colubriformis and Cooperia curticei in September. A new explanation for this succession is given, based on the overwintering capabilities and freeliving development time of individual species.