Outbreak of atypical fowlpox in chickens with persistent reticuloendotheliosis viraemia.

During the Autumn in S/Eastern Australia, eight chickens of 7 months of age with an experimentally-induced reticuloendotheliosis virus (REV) viraemia developed cutaneous manifestations similar to fowlpox. Ten hatchmates housed in a separate pen nearby, and known to be free of REV infection, remained clinically normal. Fowlpox virus was isolated from the affected birds and histologically the lesions were characteristic of fowlpox. Unsuccessful application of fowlpox vaccine by wing web 'stab', 2 days after the primary lesions were seen, showed both groups of chickens to have been infected previously by a natural strain of fowlpox virus. The fowlpox virus isolate was found to be virulent for 1-week-old chickens.