Avian leukosis/sarcoma virus infections - status and socio-economic impact revisited

Avian leukosis/sarcoma viruses (AL/SV) infect large segments of modern poultry industry, hence are also present in commercial chickens and eggs that widely expose humans on a consistent basis. Horizontal/contact transmission pattern maintains a rate of vertical/congenital transmission of exogenous (infectious) virus from one generation to next, giving rise to avian leukosis virus (ALV) or group specific antigen (gsAg) shedding chickens (shedders); thus, a permanent source of infection at the farm. Genetically transmitted endogenous (majorly non-infectious sequences) virus may or may not be expressed; and believed to be not shed or shed at low levels. Final outcome of AL/SV infections depends on interaction of exogenous viruses with that of endogenous viruses or their sequences in a chicken host.

AL/SV infections include a variety of transmissible benign and malignant neoplastic conditions, nonneoplastic conditions, and subclinical infections. Avian leukosis, a list C disease, is the most common disease seen in field flocks; recently though, myeloid leukosis has become more prevalent. Economic losses due to ALV tumor mortality and reduced productivity due to subclinical infections are estimated to be millions of US dollars each year. Shedder hens are characteristically lower, than gsAg non-shedding hens, in fitness traits, reproductive traits and performance traits.

Diagnostic procedures consist of both direct and indirect biological, molecular and serological assays. Direct assay of viral major gsAg or p27 by an enzyme linked immunosorbent assay (ELISA) is the most common biological assay to determine presence of AL/SV, but p27 is shared by both exogenous and endogenous viruses, hence can not be used to differentiate between the two groups of viruses. Additionally, most of the biological assays are cumbersome and time- consuming; and even the most sensitive biological assay may not identify all the infected dams. Molecular assays are costly and unsuitable for field conditions; and serological assays only help identification of viral envelope subgroups. Therefore, an urgent need is felt for development of an alternate antigen for convenient, precise and early diagnosis of AL/SV infections in poultry flocks with objective to reduce incidence of ALV infection in general. Current programs for control and eradication of AL/SV infections in chicken breeder flocks are based on selective breeding and elimination of dams that test positive for the virus, breaking vertical transmission of the virus from dam to progeny and early detection and removal of virus shedding birds to reduce spread of congenital and contact infections in other birds.

Recent reports of public health significance of AL/SV infections with regard to consumption of chicken origin products, contamination of chicken origin live virus vaccines of humans and animals and emergence of new viruses from “parent” AL/SV have been alarming and need immediate attention.