Response of Rambouillet Lambs to an Artificial Gastrointestinal Nematode Infection

SIMPLE SUMMARY: Meat, milk, and fiber produced from sheep are vitally important resources to society around the world. One of the major challenges to sheep health and performance are parasites, specifically gastrointestinal nematodes. This study was designed to identify the variable responses of a popular breed in the United States, Rambouillet, when inoculated with the gastrointestinal nematode Haemonchus contortus. Identification of Rambouillet sheep that have greater natural resistance to H. contortus is the first step in breeding sheep that can withstand this parasite. Genetic selection for this trait is critical because H. contortus is an extremely prolific parasite that can build up anthelmintic resistance, which leads to reduced efficacy of dewormers. Our results indicate that fecal egg counts, the primary tool for estimating worm population inside the sheep, can have wide variation amongst Rambouillet lambs that were artificially challenged with a known amount of H. contortus. We also provide evidence that this difference in fecal egg count is reflective of their sire’s estimated breeding value for this trait. While we describe the response of Rambouillet lambs to H. contortus, more research is needed to fully elucidate the mechanism behind resistance and susceptibility of Rambouillet sheep to gastrointestinal nematodes. ABSTRACT: Gastrointestinal nematodes (GIN) threaten the productivity and health of sheep worldwide, prompting the need for genetic selection to reduce GIN susceptibility. Fecal egg count (FEC), packed-cell volume (PCV), and various production traits were examined in parasitized Rambouillet sheep and compared to sire FEC estimated breeding value (EBV). Rambouillet lambs (n = 77) were inoculated with 10,000 H. contortus L3 larvae. Subsequently, FEC, PCV, and body weight (BW) were captured at seven-day intervals for six weeks. Lambs were sired by one of two rams with post-weaning FEC EBV of −9% or +9%. Mean FEC differed (p = 0.0132) with lambs from the lower EBV sire (“Sire L”) being reduced, versus those from the higher EBV sire (“Sire H”), being 2135 ± 211 vs. 2912 ± 207 eggs per gram, respectively. Males and females did not differ for FEC, but females exhibited a higher mean PCV than males, (33.74 vs. 29.65%, p < 0.0001). Lambs were shorn ~120 d post artificial infection and wool measurements were captured. A negative correlation between FEC and grease fleece weight was observed. Our results describe the response of Rambouillet lambs to artificial H. contortus infection and suggest FEC EBV can reduce susceptibility to GIN in this breed.