Avian Leukosis Complex

Avian Leukosis Complex

Article originally written by Anusha Gautam B.V.Sc &AH IAAS,Paklihawa


Avian Leukosis Complex is a neoplastic disease of birds, especially chickens. The components of the haemopoietic system that may undergo neoplastic change include the lymphopoietic (lymphocytic) system, the erythropoietic (red cell) system, and the myelopoietic (myelocytic) system.

This disease is characterized by tumor formation in bursa of fabricius with metastasis to other tissues, blood forming cells, lymphoid tissue, bone, spleen liver, kidney and connective tissue.


The disease is caused by certain members of leukosis group of avian retrovirus (ss RNA virus)


Both vertical transmission and horizontal transmission is possible.

In vertical transmission, Avian leukosis virus (ALV) is passed from infected hens to their offspring. Although only a small minority of chicks is usually infected in this way, the route is important in transmitting the infection from one generation to the next, and in providing a source of contact infection to other chicks.

In horizontal transmission, ALV spreads from bird to bird by direct contact, or indirectly by exposure to virus in the environment. This mode is responsible for the high incidence

of infection in flocks.

Sources of virus from infected birds include faeces, saliva and desquamated skin. 

Routes of infection are oculonasal, oral, respiratory and skin.

Similarly, cock may act only as virus carriers and source of contact or venereal infection to other birds, hence females are more susceptible.


There are three forms of avian leuckosis:

  1. Lymphoid leukosis
  2. Erythroid leukosis
  3. Myeloid leukosis

Lymphoid leukosis: This occurs in birds from approximately four months of age. A useful diagnostic feature (although not always present) is gross nodular tumorous involvement of the bursa of Fabricius. Lymphoid leukosis is usually caused by ALV, but sometimes by REV.

Erythroid leukosis: This can occur in young birds, from approximately five weeks, and in adults. A smooth-appearing moderate enlargement of the liver and spleen, of cherry-red colour, occurs; haemorrhage from liver rupture may be present. In the marrow, the marrow, the erythropoietic sinuses are filled with erythroblasts.

Myeloid leukosis: This occurs mainly in adult birds but sometimes in birds as young as five weeks. In the myeloblastosis form, enlarged liver, spleen and other organs can appear grossly similar to changes in lymphoid leukosis. In the myelocytomatosis form, these organs are often enlarged, but the frequent presence of cream-coloured myelocytomas on the skeleton, particularly on the inner aspect of the sternum, and sometimes on the skull, appears pathognomonic. Both forms are often marked by leukaemia, and bone marrow becomes replaced by neoplastic myeloid cells.


  •   Infection occurs horizontally or vertically
  • Insertion of the viral oncogene into the genome of the target cell
  • Induces neoplasm in lymphoid system
  • Neoplastic cells circulate in vascular system
  • Metastatic foci in visceral organs



  • Presenting signs of the leukoses are mostly non-specific such as:

inappetence, weakness, emaciation, diarrhoea, pale wattles, decreased production

  • Enlargement of abdomen, liver or bursa
  • Greenish diarrhoea in terminal stages

Incubation Period

It occurs in chickens from approximately four months of age.

Gross findings

  • Significant enlargement of the liver(hepatomegaly) with miliary, diffuse or nodular tumour foci
  • Nodular tumours in the bursa of fabricius, so bursa are always enlarged.
  • Tumorous enlargement of the spleen(spleenomegaly) and other organs such as abdomen and kidney.
  • External tumors may be seen
  • Tumors are smooth, soft and glistening.

Fig: Diffuse nodular lesion in liver, spleen, intestine and heart

Fig: focal tumor lesion in heart

       Microscopic Findings

  • Microscopically, the lesions consist of coalescing foci of uniformly immature lymphoid cells(lymphoblasts).
    • The tumour cells are B-cells, expressing immunoglobulin M (IgM) and other B-cell markers, which originate in the bursa and metastasise to other organs.
    • Intrafolicular infiltration of lymphoblast in the bursa
    • Tumor cell looks similar to Marek’s disease

Fig: Lymphocytic infiltration in the liver due to lymphoid leukosis, H&E
  • Histologically, growth of single type lymphoblast cells with marked pyroninophilia is observed, i.e. Cells stained with methyl green pyronin stain which have bright red     cytoplasm.


  • Symptoms and PM lesions
    • Virus detection tests, including virus isolation and PCR
    •  ELISA kits for detection of antibodies to avian leukosis
    • Histological staining

Diffrential Diagnosis

  • Marek’s Disease

Differences between ALC and MD


  • Young J.A.T. (1998). – Avian leukosis vims-receptor interactions. Avian Pathology.
  • Hauptli D., Bruckner L. & Ottiger H.P. (1997). – Use of reverse transcriptase polymerase chain reaction for detection of vaccine contamination by avian leukosis virus. J. virol. Meth.
  • Venugopal K. (1999). – Avian leukosis vims subgroup J: a rapidly evolving group of oncogenic retroviruses. Res. vet.

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