Field isolates of fowlpox virus contaminated with reticuloendotheliosis virus

The polymerase chain reaction (PCR) method was used to examine samples from field cases of fowlpox for the presence of reticuloendotheliosis virus (REV). The S-strain fowlpox vaccine, known to be contaminated with REV, served as a positive control. Fowlpox virus was grown from field samples and vaccines by inoculation of embryonated hen eggs by the chorioallantoic membrane (CAM) route. DNA was extracted from the CAM lesions and examined for REV proviral sequences using primers specific for the long terminal repeats of REV. Amplicons of the expected length were detected in all the 45 field samples from poultry and in the S strain vaccine. Two other vaccines and two isolates from wild birds contained no detectable REV sequences. The PCR products from the vaccine and one field isolate were sequenced and were identical. These products showed 81 to 87.5% homology with the published sequences for the long terminal repeats of REV. It was not determined whether the REV proviral DNA was integrated with cellular DNA, fowlpox DNA or both. Inoculation of day-old chickens with the S-strain vaccine resulted not only in the production of fowlpox lesions but also feathering defects and proventriculitis. This suggests that the REV present in the vaccine is replication competent. Problems being encountered with protection from fowlpox following vaccination in Australia might be attributed to simultaneous challenge with fowlpox virus and REV.