Respiratory tract lesions from infectious laryngotracheitis virus of low virulence

The pathogenicity of the A4557-5 strain of infectious laryngotracheitis virus for eight-week-old chickens was investigated by aerosol route of infection; chickens were necropsied five days after infection. The virus caused mild catarrhal tracheitis, peribronchial lymphoid infiltration, and focal lymphocytic infiltration in the lung and focal lymphocytic infiltration in the air sacs of some chickens. Chickens infected with this virus developed low levels of humoral antibody and were resistant to intratracheal challenge with the virulent V154 strain. By comparison, aerosol infection with a similar dose of virulent V154 strain caused severe necrotizing laryngotracheitis with marked suppurative bronchopneumonia and airsacculitis.