Significance of inhibited development in the epidemiology of Chabertia ovina and Oesophagostomum venulosum

Abstract

Worm counts from ewes, lambs and tracer lambs during a study on the epidemiology of gastro-intestinal nematode infections in sheep at Utrecht State University between 1974 and 1977 substantiated the following descriptions of the epidemiology of Chabertia ovina and Oesophagostomum venulosum infections. Development of inhibitated larvae and recently acquired infection from pasture leads to a spring rise in the numbers of adult worms and developing stages in lactating ewes and some barren yearling sheep. However, in barren ewes and some barren yearling sheep very few adult worms develop. The low residue of overwintered larvae on pasture results in infection of the lambs in spring and early summer. These larvae develop without inhibition. Numbers of infective larvae on pasture remain low until the autumn. This autumn increase leads to substantial worm burdens in lambs and ewes, but a high proportion of these worm burdens, especially in ewes, consists of inhibited larvae. A decrease in pasture larval contamination occurs during winter and the propensity of the larvae for inhibited development diminishes. The lambs lose most of their worms during winter. Experimental infection of worm free sheep shows that it is possible to condition infective larvae, especially of O. venulosum , for inhibition of the development, by storage of the larvae for 5 weeks at 15 or 4°C.