Outbreaks of fowl pox were observed in five chicken layer flocks assuming a respiratory diphtheritic form without cutaneous lesions and was responsible for severe economic losses due to drop in egg production and increased mortalities. The clinical signs were mainly respiratory distress, rales, and death from suffocation. Lesions were localized on laryngeal opening and in trachea in the form of yellow caseous plugs that occluded the tracheal lumen. The isolated pox virus was characterized as fowl pox by histopathological examination, tissue culture inoculation, host range infection, agar gel precipitation (AGP) and neutralization tests. The histopathological findings revealed proliferative and degenerative altera could be propagated in chicken embryos producing characteristic pock lesions on the chorioallantoic membrane (CAM), as well as in tissue culture producing a cytopathic effect. Plaques were formed on overlaid monolayer chicken embryo fibroblast cells with presence of intracytoplasmic inclusion bodies in tissue culture. The isolated virus was found to be pathogenic to chickens and turkeys but nonpathogenic for pigeons and ducks. Vaccination of chickens with fowl pox vaccine protected partially against intratracheal (I/T) inoculation with isolated pox virus, but good protection was obtained against intraderinal (1/D) and intravenous I/V) inoculations.
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