Lymphoid leukosis is a neoplastic (tumour causing) viral infection of chickens that is found in flocks worldwide. The virus has been eradicated from some SPF (specific pathogen free) flocks. Globally, the frequency of infection has been reduced substantially in the primary breeding stocks of several commercial poultry breeding companies. This control program has led to infection becoming infrequent or absent in commercial flocks. The frequency of avian leukosis tumours, even in heavily infected flocks, is typically low (<4%), and disease is not often apparent in infected flocks. However, mortality up to 1.5% excess mortality per week has been reported in commercial broiler-breeder flocks naturally infected with an avian leukosis virus.
Affected birds show non-specific clinical signs including reduced feed intake, weakness, diarrhoea, dehydration, weight loss, depression and reduced egg production. Palpation often reveals an enlarged bursa of Fabricius and sometimes an enlarged liver. The disease can be immunosuppressant which increases susceptibility to other diseases.
A subclinical disease syndrome characterised by depressed egg production in the absence of tumour formation is economically more important than mortality from lymphoid leukosis.
The incubation period for lymphoid leukosis is 4-6 months and as a consequence the disease is usually seen in broiler flocks.
Lymphoid leukosis is caused by certain members of the leukosis/sarcoma group of avian retroviruses. These viruses are commonly called avian leukosis viruses and belong to subgroups A, B, C, D, E, and J. Subgroups A and B have been most prevalent in western countries, until the emergence of subgroup J.
The virus can be vertically transmitted (passed directly from parent to offspring). Hens with subclinical disease usually shed virus or viral antigen into the albumen of eggs. Chickens infected at hatching shed virus their entire lives. Horizontal transmission (spread from bird to bird) can occur by the faecal-oral route but is of secondary importance to vertical transmission.
There is no treatment for lymphoid leukosis. Lymphoid leukosis appears to be controlled best by reduction and eventual eradication of the causative virus, which are rapidly inactivated at ambient temperature and on exposure to most disinfectants. Prevention is also helped by obtaining chicks from breeder flocks that are free of the virus and rearing birds in isolation with adequate ventilation. Follow good biosecurity and management procedures to prevent stress and control other diseases, and dispose of dead birds by composting, incineration or deep burial.
Some chickens have specific genetic resistance to infection with certain subgroups of avian leukosis virus, although genetic resistance is unlikely to replace the need for reduction or eradication of the virus. Thus far, vaccination for tumour prevention has not been promising. However, recombinant vaccines have been developed that can induce antibodies in breeders to ensure protective maternal antibodies in chicks and these may be useful to assist with eradication programs.