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Future perspectives for the implementation of genetic markers for parasite resistance in sheep.

Scope of this review Gastrointestinal parasites, in particular nematodes of the order Strongylida cause significant losses of production and deaths in sheep and goats worldwide (Over et al., 1992; Woolaston & Baker, 1996; Perry & Randolph, 1999; Nieuwhof & Bishop, 2005; Sackett et al., 2006). The strongylid genera which are important include Haemonchus, Trichostrongylus, Nematodirus, Teladorsagia, Cooperia, Bunostomum and Oesophagostomum. These are related genera by both morphological and molecular criteria, but cause a diversity of pathologies in the host due to different tissue specificities, modes of feeding and varying propensity to undergo hypobiosis in the host at the L4 or L3 stages. Other livestock species are also infected with strongylid parasites in particular production systems and these sometimes are important economically, for example Mecistocirrus digitatus, Ostertagia ostertagi and Haemonchus placei in cattle, Haemonchus contortus in alpacas and Oesophagostomum dentatum in pigs. The development of productive lines of animals with resistance to these parasites is therefore an important objective for graziers, government organisations and research bodies throughout the world. This review will focus on these parasites and their sheep host, with occasional examples from other host-parasite systems. The information should be generally applicable to other situations, especially those involving helminth parasitism in ruminant mammals other than sheep. Because of the importance of strongylid parasites in human medicine (Ancylostoma ceylanicum, Ancylostoma caninum and Necator americanus), there have been various rodent models developed to study strongylid-mammal parasite interactions. The results of studies with these models may also be applicable to work with livestock species, so some information from these models, concentrating in particular on the Heligmosomoides polygyrus model in Mus musculus will be included here.