Detection of exogenous and endogenous avian leukosis virus in commercial chicken eggs using reverse transcription and polymerase chain reaction assay.

Avian leukosis retroviruses (ALV) cause lymphomas and other cancers in chickens. Previous studies have used enzyme-linked immunosorbent assays (ELISA) and indirect immunofluorescence assays (IFA) to detect ALV p27 group-specific antigens (GSA) in commercial chicken eggs. In the poultry industry eradication programme against exogenous ALV, ELISA assays are used to identify chickens infected with the virus. The inability of ELISA and IFA assays to discriminate between ALV GSA of endogenous or exogenous origin, and actual virus, have limited rigorous assessments of viral transmission dynamics. Here, we report the use of a newly developed reverse transcriptase-polymerase chain reaction (RT-PCR) assay, with direct sequencing of the RT-PCR product, to show endogenous and exogenous ALV in albumen from unfertilized chicken eggs. We found that 95% of 20 eggs from ALV-exposed commercial chickens and 14.2% of 240 egg samples from 20 randomly chosen New Orleans retail stores were ALV-positive by RT-PCR. In comparison, only 2.5% of the same egg samples from the retail stores were positive by ELISA. Corresponding direct sequencing of randomly chosen RT-PCR products showed that four of six egg samples contained endogenous ALV, while two of the six samples were positive for exogenous subgroup A ALV. The finding of endogenous subgroup E ALV in unfertilized chicken eggs emphasizes that the transmission of endogenous ALV is common and should be considered in the implementation of ALV eradication programmes by the poultry industry.