Genetic markers to gastrointestinal nematode resistance in sheep: a review

Nematode parasites are the major animal health constraint in sheep production on pasture and cause serious economic losses. Because of failure of anthelmintic drenches, a major research effort has been underway to examine alternatives to chemical control. One of them is selecting sheep which are genetically resistant to parasitic nematodes. However, this last is not widely practiced because of the difficulty of measuring parasite resistance which mostly relies on indirect criteria such as number of nematode eggs passed in the sheep faeces (FEC) packed cell volume (PCV) or enhanced number of eosinophils in peripheral blood. Despite the well known host immune reaction it has been impossible to standardize any immunological parameter and use it as an indicator of parasitic infection. The aim of finding some genetic markers associated with resistance/susceptibility to nematodes is to make diagnostic work easier and conduct an earlier selection of desirable genotypes. However, searching for reliable genetic markers is rather difficult due to different sheep’s manifestation of resistance to either the adult or larval stages of the same parasite species and against the same parasitic stage and various manifestations of the immune responses and antigens against parasites. This review summarizes findings reported in the literature relating to genetic markers to gastrointestinal nematodes resistance in sheep.