Quantitative assessment and comparison of susceptibility to colibacillosis in pure lines of broiler breeders and their commercial offspring.

Colibacillosis, caused by avian pathogenic Escherichia coli (APEC), is a disease of major economic importance to the broiler industry. This study aimed to investigate genetic variation in susceptibility to colibacillosis by comparing four pure broiler breeder lines and their commercial four-way cross offspring. Three consecutive experiments were performed assessing mortality, growth retardation and mean lesion scores (MLS) after E. coli challenges. In the first experiment, birds were challenged intratracheally with a high dose of E. coli at 8 days of age. All pure lines showed significantly higher mortality rates (54.2-80.6 %) compared to the commercial line (20.9 %). The second experiment tested the most and least susceptible pure lines from Experiment 1, alongside the commercial line. Low, medium, and high E. coli doses were inoculated at the same inoculation day as Experiment 1. A clear dose-dependent effect on mortality and growth was observed across all lines, confirming that disease severity is linked to challenge dose. The third experiment compared the most susceptible pure line and the commercial line using a dual-infection model. Prior to the E. coli-inoculation, an infectious bronchitis virus (IBV) vaccine strain was administered. The results were in line with the first two experiments, the commercial line performed better on all parameters compared to the tested pure line. No significant differences in susceptibility were found between males and females in any experiment. The commercial broilers, a four-way cross of the tested pure lines, demonstrated superior resistance to colibacillosis compared to the pure lines in all experiments. Among the pure lines, genetic differences in susceptibility to colibacillosis were observed. These differences indicate that genetic selection for resistance to colibacillosis might be feasible.