ALVs

Avian leukosis viruses (ALVs) are a group of alpharetroviruses that infect birds, most notably chickens, where they cause a range of diseases collectively termed avian leukosis. Most ALVs are exogenous viruses transmitted between birds, but some resemble endogenous retroviruses integrated in the host genome and typically lack pathogenicity. ALVs belong to the genus Alpharetrovirus in the family Retroviridae. They are enveloped, with a ~8–9 kb single-stranded positive-sense RNA genome that encodes the gag, pol, and env genes, and rely on reverse transcription and integration into the host genome for replication. Subgroups are defined by the viral envelope glycoprotein; the major exogenous subgroups are A, B, C, D, and J, with E and some others representing endogenous or defective forms. Subgroup J (ALV-J) has been especially associated with myeloid leukosis and a variety of solid tumors such as hemangiomas, and it has caused significant economic losses since its description in the late 1980s.

ALVs transmit horizontally via contact with infected secretions and vertically through the egg. Infection can lead to tumors (leukosis), immunosuppression, reduced hatchability, and slower growth, but clinical outcomes depend on the viral subgroup and host genetics. Diagnosis relies on virus isolation or molecular assays, serology, and histopathology. Control focuses on preventing introduction and spread through biosecurity, testing and culling of infected breeders, and maintaining ALV-free breeding stock; vaccines are not widely available or effective against all subgroups. ALVs remain a major concern in poultry health and research due to their impact on production and their use as models for retroviral biology. They are host-restricted to birds and are not known to infect humans.