AN OUTBREAK OF LYMPHOMAS IN COMMERCIAL BROILER BREEDER CHICKENS VACCINATED WITH A FOWLPOX VACCINE CONTAMINATED WITH RETICULOENDOTHELIOSIS VIRUS

Gross and microscopic examinations of affected tissues from chickens o f two commercial broiler breeder flocks aged 27 and 31 weeks revealed lesions of visceral lymphomas with bursal involvement in some chickens . Reticuloendotheliosis virus (REV), but not avian leukosis virus (ALV ), was isolated from blood of affected chickens. Furthermore, DNA extr acted from tumours tested positive for REV, but not for ALV or Marek's disease virus by polymerase chain reaction (PCR) test. Attempts to de termine the source of REV infection included testing a commercial fowl pox (FP) vaccine used to immunize flocks at 7 days of age. Chicken-emb ryo fibroblasts inoculated with the FP vaccine tested positive for REV by PCR and immunofluorescent tests. REV was also isolated from plasma of pathogen-free chickens experimentally inoculated with FP vaccine a t hatch; two of eight (25%) inoculated chickens developed lymphomas by 34 weeks of age. Antigenic characterization of REV isolated from comm ercial broiler breeder chickens and from FP vaccine, using monoclonal antibodies, revealed that both isolates belong to subtype 3 of REV. Th e data represent the first report of an outbreak of REV-induced lympho mas in commercial chickens. The data also indicate that the source of REV infection is an REV-contaminated commercial FP vaccine.