Abstract
Globally, sheep are important contributors of food and fiber to society, and have historically been an important component of United States agriculture. The US sheep industry has the opportunity for growth but is currently being limited by some long-standing and emerging challenges, including the gastrointestinal nematode (GIN) Haemonchus contortus. Sheep infected with GIN are traditionally treated with one of three classes of anthelmintics approved for use in the U.S., however resistance of H. contortus to these deworming products is expanding rapidly, creating potentially lethal animal health consequences. Genetic selection of sheep to reduce GIN susceptibility is a necessary strategy for sustainability and expansion of the sheep industry especially in more temperate or tropical climates. The first project described in this dissertation focuses on the predictability of sire fecal egg count (FEC) estimated breeding values (EBVs), on lamb biological response in a parasite species-specific artificial challenge in Rambouillet. Results show the EBV of the lamb’s sire is predictive of lamb FEC, ultimately providing validation that EBVs from the National Sheep Improvement Program can be used to reduce susceptibility to H. contortus, in this breed. The second project utilized a similar artificial parasite treatment model and shows that FEC EBVs are also predictive of Dorper lamb FEC following inoculation with H. contortus , but that Dorpers have greater susceptibility to GIN than Barbados Blackbelly composites. From a subset of parasitized lambs, RNA sequencing of abomasal tissue revealed breed differences in gene expression that provides insight into the varied immune responses of low and high EBV Dorpers as well as Barbados Blackbelly composites. In the third chapter, a genome-wide association study was conducted with both the Rambouillet and Dorper lambs from the previous studies, plus naturally parasitized lambs from these breeds. In total 26 single nucleotide polymorphisms were identified as associated with lamb FEC, providing further insight into the genetic underpinnings of GIN susceptibility and outlines favorable genotypes for marker assisted selection. Collectively, this research helps describe the genetic and physiologic response of sheep to H. contortus and lays the foundation for improving resistance in GIN-susceptible breeds.