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Lymphois Leukosis

Avian Leukosis, Big Liver Disease

Lymphois leukosis (LL) is a tumor-producing (neoplastic) viral infection which is the most common form of avian leukosis. LL is slow to develop in affected birds, and is caused by a collection of Avian Retroviruses, belonging to the Alpharetrovirus genus, in the Retroviridae family. The virus causes the production of tumors in various organs in the chicken's body, including the liver, spleen, reproductive tract, and kidney.

Clinical Signs of Lymphois Leukosis

Clinical signs that develop---if any, vary widely for they differ depending on the location of where tumors develop, overall health status of the bird, stage of growth of the tumor(s), and organ(s) affected. Usually, once signs develop (often when chickens are between 4 to 8 months of age or later), tumor growth is already advanced, soon leading to a rapid decline in the bird's health. These affected birds often die within a few weeks of when clinical signs of illness are first spotted by flock owners.

When the reproductive tract is affected, sometimes abnormal eggs are observed in infected hens. If several hens in the flock are infected, egg production may be reduced. Tumor growth on the liver can cause greenish diarrhea. When other organs are affected, abdominal enlargement is often observed, often with generalized signs of sickness (loss of appetite, weakness, pale and shriveled comb, dehydration).

Marek's disease is a similar tumor-causing viral disease that is also very common in chickens. Marek's disease differs however in that it primarily affects immature chickens, and is associated with the production of tumors in the nerves---rather than the organs, such as with LL.


There is no treatment for LL, and there is currently no vaccine for tumor prevention available. However there are recombinant vaccines that have been developed that can induce antibodies in birds intended to be used for breeding purposes. Control of the this disease in your flock is dependent on not using infected adult birds for breeding purposes.


LL is transmitted horizontally (from bird to bird by direct or indirect contact) and vertically (from infected hens to their offspring through the egg). Most chicks are infected by close contact with infected chickens, which shed the virus in their feces, saliva, scales, and flakes of skin. The virus does not survive for very long in the environment, as it has a relatively short life span outside of a host.

Clinical Signs

Loss of appetite
Weight loss
Pale, shriveled comb
Abnormal eggs
Reduced egg production
Pale skin
Enlarged abdomen


  • History
  • Virus isolation
  • Post mortem
  • Histopathology


Supportive care



Purchase chickens and eggs from reputable breeders, that are lymphoid-leukosis-free

Scientific References

Good Overviews

Age Range

The disease occurs any time after 16 weeks of age, however most birds are infected at sexual maturity..

Risk Factors

  • Purchasing chickens from unknown breeders or breeders that are not claimed to be lymphoid-leukosis free