Endogenous leukosis viral antigen in eggs from meat-type chickens on an avian leukosis virus eradication programme

In an effort to eliminate exogenous avian leukosis virus (ALV) from a susceptible population, albumen of one egg per hen from each of four generations was tested by ELISA for group-specific antigen (gsa) of leukosis/sarcoma viruses. From 1510 to 2099 hens were tested in each generation. Hens were not used as breeders if optical density readings for gsa were 0.4 or greater. Despite this procedure, there was no appeciable change in the occurrence of gsa in eggs from one generation to the next and in the sire population the percentage of hens with levels of 0.4 or greater was 7.1 and 8.7 in the first and fourth generations tested, respectively. There were no consistent differences in antigen levels in any of the eight strains that comprised the two populations. For the most part, antigen detected in eggs resulted from expression of endogenous viral genes since there was a low incidence of exogenous infection. Six of 234 hens of the second generation tested from the sire population were positive for ALV in serum and three of 161 were positive for antibody to subgroup A virus; no antibody to subgroup B was detected. Five of the six hens that tested positive for ALV were from the same strain and had been reared together. With one exception these hens would have been eliminated as breeders based on tests for antigen in eggs. In the fourth generation tested, no virus or subgroup A antibody was found in serum from approximately 250 hens from the two populations.