Sarcomas and myelocytomas induced by a retrovirus related to myeloblastosis-associated virus type 1 in White Leghorn egg layer chickens

An outbreak of subcutaneous sarcomas in commercial White Leghorn egg layers was observed in the northeastern United States during late 2004. Subcutaneous tumors were confined to three flocks distributed in two locations and belonging to the same company. The tumors were first observed grossly by farm personnel at approximately 7 wk of age and persisted throughout the economic life of the flocks. Most of the tumors observed during the growing period were present on the facial region or around the head, wings, and legs. There was no gross evidence of bursal or visceral involvement. Microscopically, most tumors were undifferentiated sarcomas and myxomas. There was no microscopic evidence of Marek's disease or lymphoid leukosis. Reticuloendotheliosis virus proviral DNA was not detected by polymerase chain reaction either in tumors or in cell cultures. Egg production and mortality rates were within normal limits in the affected flocks and many of the chickens exhibiting tumors seemed healthy otherwise, albeit approximately one-half of the daily mortality exhibited tumors. Avian myeloblastosis-associated virus type 1 (MAV-1) was isolated from tumors, plasma, and serum. Upon initial virus neutralization, the viruses isolated seemed at least partially related antigenically to avian leukosis virus (ALV) subgroups A and B but not to subgroup J (ALV-J). Sequencing of the variable and hypervariable regions of gp85 in the envelope gene revealed that the viruses involved are closely related to MAV-1. Attempts to reproduce subcutaneous sarcomas with MAV-1 isolated from White Leghorn chickens in the case herein reported produced exclusively myelocytomas indistinguishable histologically from those induced by ALV-J in meat type chickens.