An epidemic of avian influenza (AI) (H9N2) occurred in broiler chicken farms in Iran during 1998–01. Mortality between 20% and 60% was commonly observed on the affected farms. Mixed infections of the influenza virus with other respiratory pathogens, particularly infectious bronchitis virus and Mycoplasma gallisepticum, were thought to be responsible for such high mortality, which resulted in great economic losses. Clinical signs included swelling of the periorbital tissues and sinuses, typical respiratory discharge, and severe respiratory distress. Gross lesions included extensive hyperemia of the respiratory system followed by exudation and cast formation in the tracheal biforcation extending into the secondary bronchi. Light microscopy lesions were characterized by severe necrotizing tracheatis. Serological examination using H9N2 AI viral antigen produced inconsistent results. Ultrastructural findings showed typical viral replication through budding processes on cell membranes of the tracheal epithelium.
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