Leukosis in fowl

Leukosis is a viral disease in chickens, caused by the avian leukosis virus (ALV). The virus can be transmitted in two ways. On the one hand, it can pass vertically through the egg, from hen to chick. On the other, it can also spread horizontally between birds, mainly via the faecal-oral route: chickens take up the virus through faecal particles, contaminated feed, drinking water, equipment or a contaminated environment.

Unlike Marek's disease, the leukosis virus is less resistant in the environment and survives less long outside the host. Its spread therefore depends less on long-term contaminated dust or dander, and more on infected parent birds passing the virus to young stock. Chicks and young birds in the first weeks and months of life are especially susceptible to infection.

A typical feature of leukosis is that the disease develops very slowly. Birds are often infected at a young age but usually only show symptoms after a long incubation period. In practice, clinical signs frequently appear from around 14 to 24 weeks of age, though they can still emerge later in life. Moreover, not every infected bird actually develops disease. Some chickens remain symptom-free for life while still being infected or keeping the virus present in the population. As a result, leukosis can remain unnoticed in a flock for a long time.

The most common form in chickens is lymphoid leukosis. Here, tumours arise from certain immune cells, mainly in the liver, spleen, bursa of Fabricius and other internal organs. Unlike Marek's disease, leukosis usually does not affect the peripheral nerves, so signs of paralysis are less typical.

The symptoms of leukosis are often non-specific and can occur with many other conditions as well. Affected birds frequently show reduced appetite, weight loss, listlessness and general weakness. Diarrhoea, a drop or complete stop in egg laying, and a distended or swollen abdomen may also occur. As the disease progresses, birds usually continue to deteriorate. In some cases, sudden death is the first obvious sign.

Because leukosis attacks the lymphoid tissue, the birds' general resistance can also decline. Infected chickens may therefore be more susceptible to secondary infections, or recover less well from other health problems.

There is no treatment for leukosis and no commercially available vaccine. Control therefore rests mainly on prevention and selection. Because the virus can be passed through the egg, breeding on with birds that test positive for leukosis is not advised.

Detecting and excluding infected breeding stock is the most important way to limit the spread of the virus within a flock or breeding line.