Lymphoid leukosis

Occurrence: Worldwide.
Species affected: Chickens, some exotic birds.
Age affected: Usually in birds of 16 weeks or older.
Causes: Avian leucosis viruses (ALVs) are RNA retroviruses. There are 10 viral subgroups (A-J). 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 and is a major cause of condemnation in adult broiler breeders and layers.

Special note: It is immunosuppressive and was a major cause of condemnation in adult broiler breeders and layers prior to its eradication in major broiler breeds.

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Causes

It is called big liver disease, because the liver is usually enlarged with nodular tumours. Chickens and some exotic birds are affected, but it usually takes 14-30 weeks for disease to occur. It occurs usually in 16-week-old birds or older except for osteoporosis and the J Virus of ALV.  They both occur in broiler- age birds. The J virus is a recombinant of ALVs.

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

Commonly found in broiler breeders around 1994-1999. Nowadays, hardly seen due to severe eradication programs on pedigree level of the top commercial breeds in the world. Local breeds may still harbor the viruses.

Lymphoid leukosis (Big liver disease, lymphomatosis, Visceral lymphoma, J virus)

Mode of transmission

It spreads through the egg (transovarian).  It is a ubiquitous organism in backyard poultry flocks. There is some lateral transmission with congenitally infected chickens.

Clinical signs

Signs include paleness, emaciation, weakness, and inappetance. 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, miliary 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).

Detection

Using ELISA or PCR.