Genetic resistance to a gastro-intestinal parasite in young sheep is weakly effective during peripartum: evidence from a divergent selection experiment

Gastrointestinal nematode (GIN) infections are a major hindrance to the development of sustainable grazing systems as they cause significant production losses. The current GIN control strategy (anthelmintics) is unsustainable due to the emergence of drug resistant parasites. Although genetic selection for resistance to GIN is a promising alternative, its effectiveness across different physiological stages remains unclear. To look into this issue, we studied the responses to GIN artificial infection in genetically resistant and susceptible female meat sheep at four different stages: during growth (n = 91), during the periparturient period of 1st lambing (n = 48 out of the 91), in 1st third of 2nd pregnancy (n = 81 out of the 91), and around 2nd lambing (n = 55 out of the 91). Females were from the 2nd generation of a divergent selection experiment on GIN fecal egg count (FEC) measured at 6 months of age (between-line difference > 3 genetic standard deviations). The parasitic challenges consisted of a single-dose of 10,000 third-stage Hæmonchus contortus larvae, except during 1st peripartum where a trickle infection was applied (1,000 larvae per week during 9 weeks). Each infection was ended by drenching. A large and consistent between-line difference was observed in FEC and haematocrit loss during growth, but also later during early pregnancy. However, the difference was largely reduced during the peripartum period, regardless of the infection protocol. Overall, our results suggest that selection based on experimental infection of young sheep during growth promotes adult resistance, but is temporarily limited during periparturient relaxation of immunity.