Nephropathogenic avian infectious bronchitis viruses

Abstract

Avian infectious bronchitis (IB) is a major cause of economic loss to the poultry industry. The disease, which has been identified in many areas of the world, primarily affects the respiratory tract of – usually young – domestic fowl. However, in addition to their common affinity for the respiratory tract, IB virus strains differ in their tropism for such other target organs as the female reproductive system or the alimentary tract and kidneys. The names now normally given to the disease are ‘infectious bronchitis nephritis’, ‘IB nephritis’ and ‘NIB’. These reflect its inflammatory nature and the organ primarily affected. The pathogenicity of IB virus strains varies and the severity of a disease outbreak is also influenced by the age of the birds and by environmental/management factors. In young broilers, which appear to be the type of stock most affected, the main symptoms of nephritis IB are generally the sudden occurrence of mild respiratory signs which tend to decline within a week. This is followed by a rapid rise in mortality causing the loss of 5–25% of the birds over the ensuing 10 days or so. The birds tend to die abruptly. Gross lesions are mainly confined to the kidneys. The kidney parenchyma of affected chickens is pale, swollen and mottled; tubules and urethras are distended with uric acid crystals. In pullets IB nephritis occurs principally in modern hybrid laying stocks shortly after the onset of egg production when, for several weeks, mortality is typically around 0.4% per week. Post mortem lesions resemble those found in broilers but usually visceral urate deposits are also present. Diagnosis of nephritis IB needs to include detection of IB viral antigen using immunofluorescence on frozen kidney sections and/or viral isolation and identification of IBV following standard techniques. Vaccination with the commonly used IB vaccines such as the H120 strain affords only limited protection against nephritis IB. Attention is drawn to the need for further research.