Pathogenicity of novel APEC serogroups
Emerging serogroups of E. coli could cause problems in younger birds
5 March 2025
2 minute read
By: Colibacillosis, caused by Avian Pathogenic Escherichia coli (APEC), results in high morbidity and mortality in poultry. Currently, 188 O groups are linked with bacterial antigenicity and pathogenicity. APEC serogroups O1, O2, and O78 are often implicated in disease. However, an examination of colibacillosis cases in Georgia has identified several emerging APEC serogroups.
Klao Runcharoon and colleagues, University of Georgia, presented a study during the 2025 International Poultry Scientific Forum which evaluated the pathogenicity of 10 APEC strains, representing nine serogroups. The researchers used the embryo lethality assay (ELA), chick assay, and 3-week-old chicken challenge assay.
In the ELA, each group consisted of 10,12-day-old specific pathogen free (SPF) eggs inoculated with APEC strains (300-500 CFU/0.1 ml) via the allantoic fluid. Negative control and positive control (APEC WT O18) were included. Eggs were candled daily, and deaths recorded for 6 days. Results showed all APEC strains were virulent, with the highest mortality (100%) for O152 and O145, while the lowest mortality (50%) was noted for O88.
In the chick assay challenge, 10 one-day-old chicks per group were inoculated subcutaneously with 100 µL (108 CFU) of APEC strains. Death times and clinical scores were combined for pathogenicity scores. E. coli isolates killing >50%, 10%-50%, and 0%-10% of chicks were classified as virulent, moderately virulent, and avirulent, respectively. A one-way ANOVA comparing pathogenicity scores among APEC serogroups showed O15, O91, and O88 had significantly lower (p < 0.05) pathogenicity scores than the positive control (APEC O18), while O25, O152, O115, and O45 showed numerically higher pathogenicity scores.
In older birds, 3-week-old SPF chickens were challenged with 108 CFU/ml of bacteria via the intratracheal route. Mortality and clinical signs were observed for 5 days, and lesions and bacterial counts were evaluated. APEC O91 was highly virulent, causing 80% mortality 1 day post inoculation, while O115 and O86 were moderately virulent.
These novel APEC serogroups exhibited varying pathogenicity and high virulence in embryos, chicks, and chickens. Virulence varied across models, suggesting that the route of infection and immune status influence disease development. These serogroups could significantly impact poultry health, especially on younger birds, warranting further investigation and new mitigation strategies.
