Cause

Lymphoid leucosis (LL) is caused by a retro (leuco) virus.

Transmission

The LL virus appears to be transmitted through the eggs. However, horizontal transmission at a young age may occur. Infected breeders can be detected by testing hatching eggs and cloacal swabs for the presence of the virus. In this manner eradication of the lymphoid leukosis will be possible.

Clinical signs

Visceral tumors are the main feature of lymphoid leukosis. They can be found in the liver, spleen, kidneys and bursa of birds that are in general, older than 25 weeks. In affected layer flocks a lower egg production can be observed.

Osteopetrosis is lymphoid leukosis of the bones of the legs and wings which become enlarged ( but incidence is quite rare). Affected birds have bowed and thickened legs. Lymphoid leukosis can also occur as blood leukosis. However, such erythroid and/or myeloid leukemias are also quite rare.

Affected birds are listless, pale and are wasting away. Because of the tumours, LL may be confused with Marek's disease. In LL, the nervous system is never involved (no paralysis). LL generally causes birds to weaken, lose weight and eventually die. Histopathological examination is essential for a proper diagnosis.

"J" Virus

More recenly, within the known subgroup of exogenous LL viruses (A, B, C. and D), a new subgroup denominated "J" has emerged. The new "J" virus shows tropism for cells of the myelomonocytic series, causing tumors and which are identified tropism for meat-type birds. The tumors caused by this virus are frequently located on the suface of bones such as the junction of the ribs, sternum, pelvis, mandible and skull. Tumors may also be found in visceral organs.

Treatment and Control

No treatment is known. The best control method is the laboratory detection of infected breeders. Breeding leukosis-free offspring from leukosis-free breeders can eventually lead to the eradication of the disease.