Observations on the effects of the h52 and h120 vaccine strains of infectious bronchitis virus in the domestic fowl

The pathogenicity of H120 and H52 infectious bronchitis vaccine viruses was compared in one-day-old and 12-weeks-old chickens. Both viruses produced moderate respiratory symptoms. Both were also nephrotropic but whereas H52 was accompanied by gross and microscopic kidney changes, which probably accounted for the mortality which occurred, H120 produced no change in the kidney and no mortality. Both viruses caused gross and microscopic changes in the bursa of Fabricius.