Factors influencing the severity of Escherichia coli and avian adenovirus group II infections in chickens.

Chickens from lines selectively bred for either a high (HH) or low (LL) antibody response to sheep erythrocytes were challenged intravenously with avian adenovirus group II (AA). Spleen size was determined 6 days later. In some experiments the responses of chickens to AA and Escherichia coli infections were compared. The level of corticosterone in the feed (15 mg/kg) which resulted in the lowest incidence of pericarditis in response to E. coli resulted in the greatest incidence of large spleens in response to AA infection. Incidence of enlarged spleens in response to AA infection was increased in fasted chickens and reduced in socialized LL-line chickens. Among ignored chickens harshly treated for 2 weeks before challenge, LL-line chickens had a higher incidence of enlarged spleens than HH-line chickens. Socialized HH-line chickens subjected to social stress 1 day before challenge had more severely affected spleens than socialized LL-line chickens. The HL cross was more severely affected by AA than the LH cross but was less severely affected by E. coli. Antibody responsiveness to sheep erythrocytes did not affect the severity of AA infection. Factors that increased the severity of AA infection seemed to result in a decreased severity of E. coli infection.