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Immunopathogenesis of infection in SPF chicks and commercial broiler chickens of a variant infectious bronchitis virus of economic importance.

The immunopathogenesis of an economically important variant of infectious bronchitis virus (IBV) has been studied in day-old specific pathogen-free and 6-week-old broiler chickens. Experimental infection caused very mild respiratory disease and no mortalities. Diarrhoea was seen in the broilers. Virus distribution and titre in respiratory, urinary and enteric tissues were assessed. In SPF chicks, maximum virus isolations were from oesophagus, followed by trachea, kidney, lung and rectum. In contrast, in the broilers, maximum isolations were from ileum, followed by caecal tonsils, rectum, bursa of Fabricius and harderian gland, but virus was recovered less frequently from trachea, lung and kidney. In both groups virus replication was demonstrated in the epithelium of the trachea, kidneys and tissues of the lower gut by immunofluorescence. Immunostaining for CD4 and CD8 T-cell subsets in trachea, lung and kidney showed earlier and increased recruitment of CD8 cells over CD4 cells. From the kidneys of SPF chicks, no virus was isolated on days 10 or 14 p.i., but was recovered to high titre on day 21. Immunostaining showed the presence of IgM deposits in the glomeruli on those days when virus could not be recovered. In the broilers, gross muscle changes were produced which were less severe than reported in the field and without histopathological changes or increased creatine kinase levels in serum. There was no evidence of virus-induced immunosuppression. The spread of infection and rate of seroconversion to this IBV were similar to other IBV strains. The possible pathogenesis of the muscle changes is discussed.