The anthelmintic resistance of gastrointestinal nematodes (GINs) poses a significant threat to sheep worldwide, but genomic selection can serve as an alternative to the use of chemical treatment as a solution for parasitic infection. The objective of this study is to conduct genome-wide association studies (GWASs) to identify single nucleotide polymorphisms (SNPs) in Rambouillet (RA) and Dorper × White Dorper (DWD) lambs associated with the biological response to a GIN infection. All lambs were genotyped with a medium-density genomic panel with 40,598 markers used for analysis. Separate GWASs were conducted using fecal egg counts (FECs) from lambs (<1 year of age) that acquired their artificial infections via an oral inoculation of 10,000 Haemonchus contortus larvae (n = 145) or naturally while grazing on pasture (n = 184). A GWAS was also performed for packed cell volume (PCV) in artificially GIN-challenged lambs. A total of 26 SNPs exceeded significance and 21 SNPs were in or within 20 kb of genes such as SCUBE1, GALNT6, IGF1R, CAPZB and PTK2B. The ontology analysis of candidate genes signifies the importance of immune cell development, mucin production and cellular signaling for coagulation and wound healing following epithelial damage in the abomasal gastric pits via H. contortus during GIN infection in lambs. These results add to a growing body of the literature that promotes the use of genomic selection for increased sheep resistance to GINs.

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.