Lesions induced in the respiratory tract of chickens by encapsulated or nonencapsulated variants of Haemophilus paragallinarum.

Lesions induced in chickens by an encapsulated or nonencapsulated strain of Haemophilus paragallinarum were investigated. In terms of lesion severity, major differences in pathogenicity were observed between the encapsulated and nonencapsulated variants. The principal lesion manifested by the encapsulated variant was an acute catarrhal inflammation of the upper respiratory tract, mainly of the nasal cavity and paranasal sinus. Infiltration of a large number of mast cells into the lamina propria of the mucous membrane of the nasal cavity was also characteristic. Numerous organisms were found on the cilia or on the surface of the epithelial cells of the nasal mucosa. Chickens with these histologic lesions had severe clinical signs of coryza, and organisms were recovered in high numbers from the nasal cavity and paranasal sinus. In contrast, chickens that were given the nonencapsulated variant did not have clinical signs of coryza, and the organisms were recovered in low numbers from the inoculated sites; slight histopathologic lesions were observed in the nasal mucosa at postinoculation day 1. Mast cell infiltration in the chickens inoculated with the encapsulated variant indicated that mast cells may be responsible for producing clinical signs of coryza via the activation of pharmacologic mediators. Adherence to and colonization of the encapsulated variant on the nasal mucosa seems to be a first step of the infection.