Epidemiological and experimental evidence for immunodeficiency affecting avian infectious bronchitis

We evaluated the effects of viral immunodeficiency on the outcome of infectious bronchitis virus (IBV) infection in chickens as a hypothetical cause for failure of adequate protection in vaccinated chickens. Initially, we investigated IBV isolations from cases of respiratory disease in association with the presence of thymic and/or bursal atrophy in 322 submissions during 1997 to 2002. Arkansas (Ark)-type IBV was most frequently isolated in spite of extensive ArkDPI vaccination in the broiler industry. The number of IBV isolations was consistently higher in broilers aged 27 to 43 days, coinciding with lymphocytic depletion of the bursa and/or thymus, providing circumstantial evidence that immunodeficiency and IBV incidence may be linked. S1 gene sequence analyses, antigenic characterizations, and challenge of susceptible chickens demonstrated that the field IBV isolates tested were closely related to vaccine strains and had low pathogenicity for chickens. We experimentally evaluated the effects of immunodeficiency caused by co-infection with chicken anaemia virus and infectious bursal disease virus on the outcome of IBV infection. Clinical signs and histological lesions were more persistent in immunodeficient chickens. Local specific IgA production was delayed and lower levels were achieved in immunodeficient chickens. At the same time, IBV RNA concentrations in tracheas and lachrymal fluids were higher and more persistent in immunodeficient chickens. Collectively, these results indicate that viral immunodeficiency most probably plays a relevant role in the epidemiology and outcome of IBV infection. Nous avons évalué les effets de l'immunodéficience virale sur le résultat de l'infection par le virus de la bronchite infectieuse aviaire (IBV) chez des poulets comme une cause hypothétique d'échec de protection adéquate chez des poulets vaccinés. Initialement, nous avons investigué les isolements d'IBV de cas de maladie respiratoire en association avec la présence d'atrophie bursale et/ou thymique à partir de 322 soumissions durant les années 1997-2002. L'IBV de type Arkansas (Ark) a été le plus fréquemment isolé en dépit de la vaccination ArkDPI à grande échelle des poulets de chair en élevage industriel. Le nombre d'isolement d'IBV a été nettement supérieur chez les poulets de chair âgés de 27 à 43 jours, coïncidant avec la déplétion lymphocytaire de la bourse de Fabricius et/ou du thymus, fournissant une évidence circonstanciée que l'immunodéficience et l'i