Submitted to: Avian Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/1995
Publication Date: N/A
Citation: N/A Interpretive Summary: An unusual increased rate of cull birds and mortality due to lymphoid tumors occurred in two broiler breeder flocks. Field diagnosis based on gross and microscopic examinations of tissues from diseased chickens revealed lesions similar to those of lymphoid leukosis (LL), a virus-induced lymphoid tumor of chickens. Thus, the case was tentatively diagnosed as LL. However, further laboratory investigations including virological, serological and molecular assays revealed that the condition was in fact an LL-like disease induced by a virus unrelated to that causing LL, reticuloendotheliosis virus (REV). Attempts to determine the source of REV infection indicated that a commercial fowl pox (FP) vaccine used to immunize flocks at 7 days of age was contaminated with REV. The data represent the first report of REV contamination of a live-virus poultry vaccine produced in the United States. Based on results in this report the emanufacturer removed the vaccine from the market. Also, the data clearly show the need for a standard protocol for testing live-virus vaccines of poultry for contamination with REV.
Technical Abstract: Gross and microscopic examinations of affected tissues from chickens of two commercial broiler breeder flocks aged 27 and 31 weeks revealed lesions similar to those of lymphoid leukosis (LL). Reticuloendotheliosis virus (REV), but not avian leukosis virus (ALV) was isolated from blood of affected chickens. Further, DNA extracted from tumors tested positive for REV, but not for ALV or Marek's disease virus by polymerase chain reaction (PCR) test. Attempts to determine the source of REV infection included testing a fowlpox (FP) vaccine used to immunize affected flocks at 7 days of age. Chicken-embryo fibroblasts inoculated with 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 at hatch; 2 of 8 (25%) of inoculated chickens developed lymphoma by 34 weeks of age. Antigenic characterization of REV isolated from commercial broiler breeder chickens and from FP vaccine, using monoclonal antibodies, revealed that both isolates belong to subtype 3 of REV. The data represent the first report of an outbreak of REV-induced LL-like disease in commercial chickens. The data also indicate that the source of REV infection is an REV-contaminated FP vaccine.