TABLE 1.

Lethality for 1-day-old chicks

Organisma	Mean lesion scoreb	P valuec
APEC O1	2.69
Controls
High	2.75	0.54
Medium	0.89	<0.0001
Low	0.17	<0.0001

^aAPEC O1 and the control organisms (one each for high, intermediate, and low pathogenicity) were classified by their virulence towards chicks (53).

^bAverage of lesion scores (ranked from 0 to 3 based upon occurrence of airsacculitis, pericarditis, and perihepatitis) for 12 birds for each organism tested.

^cP value generated using Fisher's exact test (63). There was no statistically significant difference between the lesion scores following inoculation with APEC O1 and the high-pathogenicity group control. However, the lesion scores obtained with APEC O1 did differ significantly from those in birds inoculated with the medium- and low-pathogenicity control organisms. Collectively, these results demonstrate that APEC O1 is classified as a “high-pathogenicity” organism.