Susceptibility of three genetic lines of chicks to infection with a nephropathogenic T strain of avian infectious bronchitis virus.

Mortality rates were compared in three genetic lines of specific pathogen-free chicks inoculated with one of two doses of a nephropathogenic strain of avian infectious bronchitis (IB) virus. The mortality rates were influenced primarily by the chick strain, but also by age and dose of virus. Chicks of the inbred S line were highly susceptible. After inoculation with a low dose of virus at 2 and 4 weeks of age, mortality was 90 and 45%, respectively. Chicks of the HWL non-inbred line were also susceptible, with mortality rates after inoculation at 2 and 4 weeks of age of 70 and 25%, respectively. Chicks of the inbred W line were resistant and non-significant mortality of 10% occurred only in 2-week-old chicks inoculated with a high dose of virus. Viral distribution in tissues of susceptible S and resistant W chicks did not differ, and virus was present in the trachea, lung and kidney of chicks from both lines throughout the acute phase (between days 3 and 7) of infection. Viral titres in the trachea and kidney in susceptible S chicks were slightly but not significantly higher than in the other chicks during the acute phase of infection. Histopathological assessment indicated an earlier onset of a regenerative phase in the trachea of W chicks than in S chicks. S chicks, in contrast to W chicks, showed no signs of renal regeneration. Additionally, the kidneys of S chicks differed from those of W chicks in showing more severe nephritis, more tubular necrosis and less heterophil infiltration and lymphocytic response throughout the acute phase of infection. The results indicate that chicken lines may differ greatly in their susceptibility to fatal IB nephritis and that resistance is likely to be under the control of immune responses to viral infection.