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Lymphoid Leukosis

Avian Leukosis

Lymphoid leukosis (LL) is a tumor-producing (neoplastic) viral infection caused by avian retroviruses, belonging to the Alpharetrovirus genus, in the Retroviridae family. It is predominately seen in mature birds and is characterized by gradual onset of persistent low mortality. 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

Clinical signs that develop---if any, vary widely for they differ depending on the location of the tumor, overall health status of the bird, stage of growth of the tumor, and organ(s) affected. Usually, once signs due develop (often when chickens are between 4 to 8 months of age or later), tumor growth is already in the advanced stage of growth which results in a rapid decline in health. Affected birds usually will die within a few weeks of first observing signs of illness.

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.


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


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