Genetics of fasciolosis in small ruminants

Is there a single gene for resistance to F. gigantica segregating in ITT sheep and will it be of sufficient magnitude to offer a further option for control of the disease? The consequent questionis whether there is an equivalent gene in the cattle of buffalo genome.' Which method, if any, is most effective for detecting a resistance gene of large magnitude?' Are there also genes that confer high levels of resistance to the bloodsucking nematode parasite Haemonchus in the ITT sheep, which also seriously constrains the production of sheep and goats in the tropics?' In the absence of any single major genes, are there other candidate genes which, together,confer high levels of resistance? Where are these located and how can they be detected?The discovery, albeit based on limited data, of evidence for a single gene of resistance to F. gigantica in Indonesian thin tail (ITT) sheep (Roberts et al. 1997) stimulated significant interest in investigating the genetics of resistance to F. gigantica in smaller livestock that could be experimented on more easily. This interest was further increased by the availability of new genetic and statistical technologies that enhanced the possibility of detecting genes of major effect. While many animals may still be needed, the prospect of detecting major genes for a wide range of disease and production characteristics is an enticing prospect. In this chapter the results of a major ACIAR project are summarised. The project sought to capitalise on the wide variation in production and disease traits between merino and ITT sheep. Crosses of ITT and merino offer an excellent opportunity for resistance and other genes to segregate. The practical questions that both stimulated these studies and emerge from their results include the following. 'The quest for genetic approaches for the control of disease in livestock continued throughout the twentieth century. Emphasis was first on selection of resistant breeds, but with technological advances in genetics, there was an increased focus on quantitative approaches within breeds. Most recently, molecular techniques were used to identify resistance in individuals that could be used for selective breeding (Gray et al. 1995). This pattern has been followed for resistance to liver fluke but from a base of very limited knowledge on how livestock develop resistance to infection, and if there is any genetic component to that resistance. The problems associated with investigating these problems in tropical large ruminants are especially severe, given the lack of basic information on the epidemiology of disease, and relative lack of expertise and facilities to undertake large-scale genetic experiments.