Lymphoid leukosis is a neoplastic (tumour causing) viral infection of chickens. The infection is found worldwide and is known to exist in virtually all chicken flocks except for some SPF (specific pathogen free) flocks from which it has been eradicated. Globally, the frequency of infection has been reduced substantially in the primary breeding stocks of several commercial poultry breeding companies. In recent years this control program has expanded, and infection has become infrequent or absent in certain commercial flocks. The frequency of avian leukosis tumours, even in heavily infected flocks, is typically low (<4%), and disease is often not apparent in infected flocks. However, mortality can be high in affected birds, and 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 cause damage to the immune system which increases susceptibility to other diseases.
A subclinical disease syndrome characterised by depressed egg production in the absence of tumour formation is more important economically than mortality from lymphoid leukosis.
The incubation period for lymphoid leukosis is 4-6 months. Therefore, the disease is not 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. The causative viruses are rapidly inactivated at ambient temperature and on exposure to most disinfectants.
There is no treatment for lymphoid leukosis. Lymphoid leukosis appears to be controlled best by reduction and eventual eradication of the causative virus. Obtain chicks from breeder flocks free of the virus. Rear 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.
This unique presentation shows the fundamental structure and anatomy of the chicken
Now released in the App Store. Developed by the Poultry CRC. Have fun while learning about what makes a good egg.
Download for free in Australia
These lists contain organisations and individuals associated with the Australian poultry industry