Genetic resistance to gastrointestinal parasites in sheep

Abstract

Gastrointestinal nematode (GIN) infection is the most common disease affecting sheep production systems throughout theworld, causing significant productive and economic losses. The control of these parasites was traditionally based on the use ofanthelmintic drugs. However, continuous, extensive, and indiscriminate use of these drugs has led to the emergence and spreadof strains of parasites resistant to the major chemical compounds used. This situation has led to considering alternative strategiesfor worm control. One of them is the selection of individuals for greater resistance to GINs. A number of phenotypic traits havebeen reported in the bibliography, including parasitological, biochemical, hematological, and immunological traits, but fecal eggcount (FEC) is considered the primary and most practical measure of resistance. Genomic studies have reported polymorphismsassociated with GIN resistance traits on almost all sheep chromosomes. Among other loci, FEC has been associated with the majorhistocompatibility complex (MHC) through quantitative trait loci (QTL) analysis and genome-wide association studies (GWAS).This review covers the principal aspects reported in the literature on several parameters considered to evaluate the resistancestatus of sheep to nematodes, mainly Haemonchus contortus, the correlation between resistance markers with economicallyimportant production traits, and the main genomic regions identified as relevant in determining the phenotype for resistance orsusceptibility to GINs.