Studies of the eggs of Trichostrongylus Colubriformis and Haemonchus Contortus (Nematoda: Trichostrongylidae) : with particular reference to desiccation survival.

Economic loss attributed to helminthosis in sheep due to death, reduced productivity and control costs has been shown to be substantial and is of major importance to the industry. Owing to their cosmopolitan distribution and the losses associated with both Haemonchus contortus and Trichostrongylus colubriformis, extensive studies have been carried out throughout the World for over sixty years on these two parasites. Unlike bacterial or viral infections these parasites do not multiply within the host. Every parasite present in a host individual must have been separately acquired by the ingestion of an infective larva. Both fl, contortus and l, colubriformis have a direct life cycle where the sexually mature females produce eggs which pass out in the faeces of the host to give rise to a series of three non—parasitic, freeliving larval stages on the pasture. Reproductive potential of the female is enormous which leads to huge numbers of eggs being deposited in faeces and thus establishing the external environment as the site where the major portion of the biomass of the parasitic population is found. The pasture is also of importance in assisting the parasite to colonize new hosts as it acts as a medium for dispersal and exchange of parasite populations between sheep