Chicken embryo lethality assay for determining the virulence of avian Escherichia coli isolates

Multiple isolates of Escherichia roll from clinical cases of colibacillosis and E. coli from the intestinal tracts of normal broilers at slaughter wer e assayed by the embryo lethality test to determine their virulence. The as say was repeated five times in order to establish reproducibility and deter mine the statistical parameters of the test. This study showed that the ino culation of approximate to 100 colony-forming units in the allantoic cavity of 12-day-old embryos discriminated between virulent and avirulent E. coli isolates. Gross lesions included cranial and skin hemorrhages in addition to encephalomalacia in embryos inoculated with virulent isolates. Abnormali ties were observed by microscopic examination of the heart, brain, and live r in embryos inoculated with virulent isolates. Analysis of data indicated that the length of the test should be 4 days. In the virulent group, day 2 postinoculation had the most significant death patterns. Sample size calcul ations indicated that 11 embryos are sufficient for the assay. On the basis of death rates, isolates considered to be avirulent had an embryo death ra te of <10%, moderately or secondary pathogens had a 10%-29% death rate, and virulent isolates had a death rate of >29%. An important aspect of this as say is the accessibility of good-quality fertile embryonated eggs.