Estimating the genetic correlation between resistance and tolerance in sheep

Resistance and tolerance are the two main strategies of host’s immune response against infections. Resistance is the ability to resist pathogen by, for example, preventing the pathogen from entering the body or controlling the replication of the pathogen within the host. Tolerance is the ability to minimize the damage caused by pathogen. Breeding for resistance is now widespread in the livestock industries. However, there is uncertainty about whether it is better to breed for resistance or tolerance. Though the genetics of resistance to infection has been widely investigated, the genetics of tolerance to infection and its relationship with resistance remains poorly understood. From a commercial flock of Scottish Blackface sheep 962 pedigreed lambs were studied. Lambs were born outside and were continuously exposed to natural mixed nematode infection by grazing on pasture. We applied a reaction norm model to quantify the change in lambs’ bodyweight towards changes in their faecal nematode egg count. We observed significant genetic variation for tolerance as the slope of the reaction norms, indicating the possibility of improving tolerance by selective breeding. We also applied a bivariate model to study the genetic correlation between resistance (measured as increased IgA and decreased FEC) and tolerance. A negative genetic correlation was observed between tolerance and resistance, indicating that animals that are genetically more resistant are less tolerant. These findings indicate that unless both traits are carefully included in breeding programs, breeding for increased resistance may decrease tolerance.