Review of Genetic Parameters for Disease Resistance in Sheep in New Zealand and Australia

Genetic factors, mainly heritability estimates, have been reviewed briefly for all major disease traits affecting sheep production in New Zealand and Australia. The traits reported included resistances to nematode parasites, liver fluke, flystrike and fleece-rot, lice, mycotoxic diseases (facial eczema, ryegrass staggers, heat stress caused by ergovaline, and infertility caused by zearalenone), mastitis, foot rot and pneumonia. Selection lines have been under study for at least six of these traits. In general, all the traits are characterised by heritabilities of sufficient size that selection progress can be made, if it is cost-effective to carry out the scoring and apply selection pressure. Given the difficulty of scoring many of the diseases under field conditions, the advent of DNA marker technology could provide a large boost in the near future to reducing incidence of these diseases by breeding. The quantitative trait locus studies carried out will provide pointers to candidate genes controlling the expression of disease resistance traits. It is likely to be beneficial to investigate the underlying genes controlling each resistance trait, with a view to developing additional possible forms of disease control.