Immunohistochemical and serological investigation of experimental Ornithobacterium rhinotracheale infection in chickens.

Immunohistochemical techniques were used to prove that Ornithobacterium rhinotracheale was the causative agent of lesions in the air sacs and lungs in chickens, but only after infection with Newcastle Disease virus (NDV). At first, the bacteria attached to the epithelium of the air sacs. Subsequently, they infiltrated the air sacs, and caused thickening of the air sacs, the formation of oedematous and granulomatous tissue, and accumulation of macrophages. The infection peaked at 5 to 9 days, after which recovery was seen. In the lungs, some areas with bronchially-associated lymphoid tissue were affected. The other organs investigated were shown not to be affected. In the absence of NDV infection, aerosol exposure of chickens to O. rhinotracheale only resulted in minimal and temporary microscopic air sac lesions. No O. rhinotracheale cells or fragments could be detected at any time point later than 2 days post-exposure. In spite of the absence of visible lesions, chickens exposed to O. rhinotracheale without prior NDV infection reacted serologically. The duration and the titre of this immune response was indistinguishable from that obtained in chickens exposed after NDV infection. Thus, infection with O. rhinotracheale appears to be restricted to the respiratory tract, with lesions only evident in birds previously infected with NDV, even though a strong serological response can be established in the absence of prior viral infection.