Investigating Host Resistance to Avian Pathogenic Escherichia coli Using Colonization Levels in a Chicken Advanced Intercross Line

Avian pathogenic Escherichia coli (APEC) is an extraintestinal Gram-negative bacillus that can cause severe respiratory and systemic colibacillosis in poultry.  Infections with APEC can lead to airsacculitis, perihepatitis, pericarditis, septicemia and mortality, and cost the poultry industry millions of dollars each year.  Identifying quantitative trait loci (QTLs) in chickens (Gallus gallus) associated with resistance to APEC colonization through genome wide association (GWAS) will provide potential markers for genomic selection and target regions for further investigation.  This study utilized the F₂₄ generation of an advanced intercross line (AIL) generated as a fine-mapping population from outbred broiler breeder and inbred Fayoumi lines.  Across three replicates, 324 14-day old AIL chicks and 46 chicks from parental lines (broiler and Fayoumi) were challenged with APEC O1:K1:H7 (APEC O1) via intra-airsac injection.  The highly virulent APEC O1 strain is known to have variable pathogenicity in broilers.  After 1 day of exposure, blood and tissue samples (right lung, left lung, spleen, and liver) were collected from each individual.  Ten-fold serial dilutions from tissue homogenates were plated for bacterial enumeration.  Bacterial loads (log₁₀(CFU/g)) ranged from 0-11.8 between samples, with significantly lower mean log₁₀(CFU/g) in blood in comparison to the tissues.  Bayesian GWAS will be performed using genotypes collected on the Affymetrix Axiom 600K Genome-wide Chicken Genotyping array with these tissue colonization levels as quantitative phenotypes.  Potential QTLs associated with lower APEC levels will provide a first step towards genomic improvement of commercial bird resistance to colibacillosis.  Support: USDA-NIFA-AFRI US-UK Collaborative grant, Hatch project #5424.