T-cell receptor and immunoglobulin gene polymorphisms and resistance to Haemonchus contortus in sheep

The use of genetically resistant sheep to control nematode parasitism

Research in the last 20 years has firmly established that it is possible to exploit genetic variation in resistance to the nematode parasites of sheep by selection. Selected sheep are more resistant to infection and commercial application of this research is under way in both Australia and in New Zealand. Not all the consequences of these breeding programs have been established, particularly long-term consequences for productivity and the interaction with other control measures, but the technology is available in the public domain with no commercial restrictions. Faecal worm egg count remains the most effective way of selecting sheep although many alternatives, such as DNA markers, host antibody and parasite antigen assays are being developed for use as selection criteria. Strategic nutritional supplementation is an immediate candidate for inclusion in worm control programs and although nematode vaccines remain elusive, it is likely that some will be field-tested in the next few years. For both of these approaches, nutritional and immunological, it is critical that the response of selected genotypes are assessed under commercial conditions and as components of worm control programs. There is evidence to support the possibility that selected sheep will be more responsive to vaccination while the long-term interaction between the effect of strategic nutritional supplements and resistant genotypes needs to be investigated.

Prospects for development of genetic markers for resistance to gastrointestinal parasite infection in sheep

Selection of sheep for resistance to internal parasites represents a viable option for future parasite control. Many phenotypic measures are available for determining the level of infection in individual sheep, although no phenotypic markers are available which allow prediction of an individual's resistance status. Genetic markers are therefore the best way to incorporate parasite resistance into selection programmes. With the recent development of genetic maps, several experiments are underway to search for markers linked to parasite-resistance genes in sheep. It can be predicted confidently that markers associated with resistance will be discovered within 12 months. Markers useful as selection criteria will be available within 5 years, although considerable quantitative genetic analysis needs to be done to find the best way to utilise marker information in selection programmes. In future, methods for differential DNA analysis or mRNA expression will lead to isolation of the genes involved.