Effect of infectious bursal disease virus on infections produced by Escherichia coli of high and low virulence in chickens.

The effect of infectious bursal disease virus (IBDV) on the infections caused by Escherichia coli strains of high (Expt 1) and low (Expt 2) virulence was examined in specific-pathogen-free chickens. The chickens were inoculated orally with IBDV at 1 day of age and via the air sac with E. coli at 1 week of age. In the groups given 1 x 10(5) cfu of E.coli of high virulence (Expt 1), mortality of IBDV-inoculated group (90%) was significantly higher than that in the non-IBDV-inoculated group (40%). The septicaemic lesions (splenic necrosis with fibrinous exudation) in the IBDV-inoculated-group were of significantly greater severity than those in the non-IBDV-inoculated group. The lymphocytic depletion in the bursa of Fabricius was most severe in the group inoculated with both IBDV and E. coli, then in descending order, in the group inoculated with IBDV alone and with E. coli alone. Lymphocytic depletion of the thymus was caused mainly by E. coli infection while IBDV induced mild lymphocytic depletion of the thymus. In Expt 2. the groups given 1 x 10(9) cfu of E. coli of low virulence revealed mortality of 50% when inoculated with IBDV and 10% when non-IBDV-inoculated. This study suggests that IBDV may increase the chickens' susceptibility to septicaemic infections produced by E. coli strains of high and low virulence and that IBDV and E. coli may induce additively marked lymphocytic depletion in the bursa of Fabricius and thymus.