Al Rawashdeh, M. S. M. (2017). Immunohistochemical and clinical changes in the respiratory tract of chickens, naturally infected with the fowlpox virus.It is clear that currently the question of study of the pathological changes in the respiratory tract of chickens due to the impact of the fowlpox virus remains relevant, as the pathogenesis of nutritional deficiency, the presence of mycotoxins or the papilloma virus are characterized by the same clinical manifestations. We analyzed 88 chickens, naturally infected with the fowlpox virus, using clinical and immunohistochemical methods of investigation. Among all species of birds, we studied only chickens, data on which were collected for five years from veterinary clinics. The investigation revealed lesions basically characterized by the presence of changes in the typical structure of the chicken's respiratory tract. As a result, we found clear criteria for the pathological process in the respiratory tract of chickens, which are typical for fowlpox. Thus, we discovered respiratory tract obstruction, as well as many intracytoplasmic pale eosinophilic inclusions in hyperplastic cells. We found an accumulation of mononuclear cells consisting mainly of macrophages, lymphocytes, plasma and mononuclear cells inside the mucous and muscle membranes. Bronchial lumens were blocked by necrotic and desquamated epithelial cells, red blood cells, bacterial colonies and amorphous eosinophilic material. We found accumulations of lymphocytes and macrophages in the parenchyma of the lungs. The hyperplastic epithelial cells reacted immunohistochemically with antibodies against the fowlpox virus in the respiratory tract. Immunoreaction occurred mainly in the cytoplasm of infected cells, inclusions, and necrotic and desquamated cells. The study proved immunohistochemical methods of investigation can be a useful additional tool for establishing a final diagnosis, especially in acute and subacute phases of the disease. The following respiratory signs were observed in severe cases of fowlpox: damage to the lungs in 33 cases (46.5%), parabronchium -20 (28.2%), parabronchial connective tissue -8 (11.3%), and mucous membrane of the larynx and trachea -10 (14.1%). In mild and moderate cases of fowlpox, the following respiratory signs were observed: hyperemia and thickening of the mucous membrane of the trachea -14 (82.4%), as well as hyperemia in the nasal conchae and paranasal sinuses -3 (17.7%). In the future, it will be necessary to conduct deeper studies to detect pathological manifestations of this disease, not only in the respiratory tract, but also in other organs and systems of chickens.