Aetiology
It is caused by leucosis sarcoma virus which belong to the family Retroviridae. These viruses are pluripotential in the causation of the pathological type of tumor. Some may produce lymphoid leucosis, others myeloid leucosis and some erythroid leucosis. Lymphoid leucosis is the most prevalent of the three manifestations of the leucosis. The virus is ubiquitous in most commercial flocks and their presence is not necessarily resulting in disease.
Host
It is a disease of domestic fowl. The disease is rare before 25 weeks of age and usually occurs after 40 weeks of age.
Transmission
Avian leucosis is transmitted vertically from parent to progeny and horizontally by direct or indirect contact.
Clinical signs
Incubation period from infection to development and death is long and variable about 4 months or more. The birds may be anorexic, weak and emaciated. Diarrhea may occur and wattles may peel. In lymphoid leucosis, enlarged liver may be palpable.
Post-mortem lesions
There are 3 post-mortem forms of Avian leucosis. They are erythroid, lymphoid, myeloid and lymphoid leucosis.
Lymphoid leucosis(big liver disease)
It is the most prevalent manifestation caused by avian leucosis sarcoma virus. It is usually characterized by enlargement of the liver by infiltrating lymphoblast and the pattern of involvement is diffused but occasionally it can be nodular. Other organs like the spleen, kidney, heart and ovaries are also involved. Nodular tumors can be found in the bursa of fabricius in nearly all cases.
Erythroid leucosis(erythroblastosis)
It is a rare and sporadic disease. There is anaemia which is associated with large number of immature red blood cell in the blood. The liver and spleen are moderately enlarged and are cherry red in colour. The disease originate from the bone marrow and leukaemia is present.
Myeloid leucosis
This involves an extravascular proliferation of cells of the granulocyte series with a sporadic disease. It may occur as a myeloblastosis or a myelocytomatosis . the liver and spleen are diffusely and greatly enlarged and the liver has a granular morocco death appearance. The tumours of myelocytomatosis are nodular and discrete and thus a yellowish white colour and affect a wide range of organs including the liver, spleen and have a predilection of the visceral surface of flat bones such as ribs. Solid tumor caused by avian leucosis sarcoma virus include fibrosarcoma, endothelioma, chondroma, haemangioma, nephroblastoma and hepatocarcinoma.
Diagnosis
It is based on combination of clinical signs, post-mortem lesions, age at infection and histology. Cytological examination of may grumwald giemsa stained impression smears of fresh tumor tissues is a useful aid in diagnosis.
Control
No treatment or vaccine is available. Control must be based on a high standard of vaccine to reduce infection from the environment and using birds from a disease free sources and genetically resistant stocks.