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Investigation of an increase of solid tumors in chickens vaccinated against Marek's disease.

SUMMARY In study of a reported increase in solid tumors (fibrosarcomas, hemangioendotheliomas, and nephroblastomas) in 9-week-old chickens vaccinated against Marek's disease, an increase in solid tumors was confirmed, though probably not a result of any oncogenic activity of the vaccine. A more likely explanation, though purely speculative, seems to be physical transmission of avian leukosis virus among newly hatched chickens during the vaccination procedure. The chickens in this particular commercial operation were found to have a high incidence of infection with avian leukosis virus. The studies confirmed the effectiveness of Marek's vaccine in reducing the incidence of Marek's disease.