Genomic Regions Associated with Resistance to Gastrointestinal Nematode Parasites in Sheep-A Review

Abstract

Gastrointestinal nematodes (GINs) can be a major constraint and global challenge to the sheep industry. These nematodes infect the small intestine and abomasum of grazing sheep, causing symptoms such as weight loss, diarrhea, hypoproteinemia, and anemia, which can lead to death. The use of anthelmintics to treat infected animals has led to GIN resistance, and excessive use of these drugs has resulted in residue traced in food and the environment. Resistance to GINs can be measured using multiple traits, including fecal egg count (FEC), Faffa Malan Chart scores, hematocrit, packed cell volume, eosinophilia, immunoglobulin (Ig), and dagginess scores. Genetic variation among animals exists, and understanding these differences can help identify genomic regions associated with resistance to GINs in sheep. Genes playing important roles in the immune system were identified in several studies in this review, such as the CFI and MUC15 genes. Results from several studies showed overlapping quantitative trait loci (QTLs) associated with multiple traits measuring resistance to GINs, mainly FEC. The discovery of genomic regions, positional candidate genes, and QTLs associated with resistance to GINs can help increase and accelerate genetic gains in sheep breeding programs and reveal the genetic basis and biological mechanisms underlying this trait.

Keywords: candidate genes; gastrointestinal nematodes; genome-wide association study; genomics; quantitative trait loci; sheep.

Figure 1

Genes (in red) and quantitative trait loci (QTLs) (in blue) identified as overlapping among genome-wide association studies and association analyses for each ovine chromosome. The genes and QTLs were associated with different traits measuring resistance to gastrointestinal nematodes. The positions were retrieved from the NCBI and the Sheep Animal QTL Database using the Oar_v3.1 sheep assembly. * Genes were described in the main text based on the positions presented in their reference study, which diverges from the recently updated version of the sheep assembly used to generate this figure.