In vivo typing of Escherichia coli obtained from laying chickens with the E. coli peritonitis syndrome

Fourteen genetically different chicken Escherichia coli strains were biotyped in hens to examine if any E. coli strain at high dose can induce the E. coli peritonitis syndrome (EPS). Moreover, biotyping was performed in embryos and the median lethal dose (LD50) of three strains was determined in hens. Nine strains were obtained from bone marrow and one strain from cecum of hens that had died from EPS. One strain originated from the inflamed pericardium of a broiler and three strains from the cloaca of specified pathogen free (SPF) broiler breeders. Strains were inoculated intratracheally in separate groups of 32 productive SPF White Leghorn (WL) hens in a dose of 7.8-9.2 log10 colony forming units (cfu) per hen and in the allantoic cavity of separate groups of 20 SPF WL embryos incubated during 14 days in a dose of 4.2-4.6 log10 cfu per embryo. The embryo test was replicated. Bone marrow and pericardium strains induced EPS, the other strains did not. Based on mortality in hens, EPS inducing strains could be classified as very virulent (59-100% mortality), moderately virulent (38% mortality) and low virulent (6% mortality). The LD50 for productive SPF WL hens of three very virulent strains ranged from <2.7 to 5.3 log10 cfu. Virulent and avirulent strains killed 60-95% and 0-30% of embryos, respectively. The embryo lethality test, which showed good reproducibility, did not discriminate within virulent strains, but can nevertheless be considered as a useful alternative for biotyping E. coli in productive hens.