Causes: Avian leucosis viruses are RNA retroviruses. There are 10 viral subgroups (A-J). Type A and J are most important. Vertically transmitted some horizontal transmission at young age in case of intensive contact.

 

Effects: Paleness, emaciation, weakness, inappetance. Feed/gain ratio increased, decreased egg production and increased culls. External tumours may be seen, and the abdomen is enlarged and feathers are sometimes spotted with urates and bile. The disease is immunosuppressive.

 

Detailed causes:

The subgroup virus was commonly found in broiler breeders around 1994-1999. Nowadays, it is hardly seen due to severe eradication programs at pedigree level. Osteopetrosis occurs in broiler-age birds.

 

Lymphoid leukosis virus is an RNA virus in the family of retroviruses. Ten viral subgroups (A-J) are known. Avian leucosis viruses (ALVs) were common around the world, because they are transmitted through the eggs of common commercial breeders.

 

Mode of transmission

It spreads through the egg (transovarian). It is a ubiquitous organism in commercial poultry flocks.

 

There is some lateral transmission with congenitally infected chickens.

 

Special note

It is immunosuppressive and has been a major cause of condemnation in adult broiler breeders and layers.

 

Clinical signs

Signs include paleness, emaciation, weakness and inappetence. The abdomen is enlarged and feathers are sometimes spotted with urates and bile. The feed/gain ratio is decreased. Increased culls and decreased egg production occurs.

 

Postmortem lesions

Peripheral nerves are never involved. Bursae are always enlarged and may contain nodular tumours. Visceral tumours are soft, smooth and glistening. Tumours may be nodular, military or diffuse. Tumours are common in gonads, lungs, liver, spleen, heart, kidney, mesentery and bone marrow.

 

The skin is never affected and skeletal muscles often contain tumours. Eyes are never affected. Bone sometimes contains tumours (especially with J virus). The blood often contains tumours (especially with J virus). Fat tumours are common (Liposarcoma).

 

Diagnosis:

Tumours in adult birds are characteristic for this disease. Liposarcoma or myeloid tumour (J virus) is also characteristic.

 

ELISA tests are available to detect both the antigen (P27) and the antibodies against J virus.

 

Histopathology is confirmatory. It reveals large lymphoblasts in tumours. Lymphoblasts have an ovoid nucleus and a finer, more delicate chromatin network. PCR test is used to select out J virus positive broiler breeder grandparent stock.

 

ELISA test use specific serum to determine the presence of the organism in serum, vaginal swab or egg albumin. PCR test used for J virus.

 

Different diagnosis: E. coli, Aspergillus, MD, tuberculosis, erythroblastosis and myelobastosis.

 

The tumours are grossly distinct from those caused by other ALVs. However, the definitive diagnosis is based on virological isolation or histologic observation of the tumour cells. The myeloid cells can be differentiated from other tumour cell types by a trained histopathologist.

 

Treatment and control:

Prevention

Select birds on breeding level that are leucosis virus negative using serologic methods to prevent spread of the disease.

 

Treatment

None.

 

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