Dose-dependent differential resistance of inbred chicken lines to avian pathogenic Escherichia coli challenge.

Avian pathogenic E. coli (APEC) cause severe respiratory and systemic disease. To address the genetic and immunological basis of resistance, inbred chicken lines were used to establish a model of differential resistance to APEC, using strain O1 of serotype O1:K1:H7. Inbred lines 7 ₂ , 15I and C.B12 and the outbred line Novogen Brown were inoculated via the airsac with a high dose (10 ⁷ colony-forming units, CFU) or low dose (10 ⁵ CFU) of APEC O1. Clinical signs, colibacillosis lesion score and bacterial colonization of tissues after high dose challenge were significantly higher in line 15I and C.B12 birds. The majority of the 15I and C.B12 birds succumbed to the infection by 14 h post-infection, whilst none of the line 7 ₂ and the Novogen Brown birds developed clinical signs. No difference was observed after low dose challenge. In a repeat study, inbred lines 7 ₂ and 15I were inoculated with low, intermediate or high doses of APEC O1 ranging from 10 ⁵ to 10 ⁷ CFU. The colonization of lung was highest in line 15I after high dose challenge and birds developed clinical signs; however, colonization of blood and spleen, clinical signs and lesion score were not different between lines. No difference was observed after intermediate or low dose challenge. Ex vivo, the phagocytic and bactericidal activity of lung leukocytes from line 7 ₂ and 15I birds did not differ. Our data suggest that although differential resistance of inbred lines 7 ₂ , 15I and C.B12 to APEC O1 challenge is apparent, it is dependent on the infectious dose. Research Highlights Lines 15I and C.B12 are more susceptible than line 7 ₂ to a high dose of APEC O1. Differential resistance is dose-dependent in lines 15I and 7 ₂ . Phagocytic and bactericidal activity is similar and dose independent.