Avian Leukosis

Avian Leukosis is also known as Avian sarcoma, Big liver disease, visceral lymphoid leukosis and Lymphomatosis.

The Avian Leukosis sarcoma viruses cause erythroid, lymphoid and myeloid leukosis, a variety of other tumours, such as fibrosarcomas, hemangiomas, nephroblastoma and bone disorder osetopetrosis.

• Lymphoid leukosis- common in layers.
• Myeloid leucosis and erythroid leukosis- common in meat type birds (Broilers).

Lymphoid leukosis is a common neoplasm caused by ALV subgroup A and B. ALV-J has weak ability to cause lymphoid leukosis in layers.

Etiology

• Avian Leukosis is caused by an Alpharetrovius  of family Retroviridae.
• The viral structural protein p27 is major group specific antigen encoded by gag gene. Which is common to all viruses of leukosis or Sarcoma group.
• ALSV in chickens are classified into six subgroups (A, B, C, D, E & F) on the basis of differences in their viral envelope glycoprotein gp 85.
• Most of the viruses causing avian leukosis are exogenous. Endogenous ALGVs, which mostly belong to subgroup E and occur commonly in chickens and other avian species are of little or no pathogenicity.

Host affected

• Chicken are natural host for the virus, the virus also isolated from pheasants, partridge and quail.
• Adult bird greater than 16 weeks of week old are commonly affected.

Transmission

• Vertical transmission- most common (from hen to progeny through the egg)
• Horizontal transmission through faeces of infected chicks to susceptible host
• Congenitally infected chicks develop immunological tolerance to the virus and fail to develop neutralizing antibodies and permanently shed the shed organism.

Pathogenesis

• ALV infection in the absence of neoplastic disease can adversely affect egg production traits in layers, including age at first egg production, number, size, fertility and hatchability.
• Broilers: suppress the growth rate

Clinical manifestation

• The incubation period is variable and may be as long as months or years
• Unthriftiness, inappetence, weakness, diarrhoea, dehydration and emaciation
• Pale comb
• Enlarged abdomen is due to tumour growth in liver
• Infected chicken is depressed before death
• Reduced egg production occur in subclinical infection

Necropsy finding

• Diffuse enlargement of liver and nodular tumour is common in liver, spleen and bursa and occasionally found in kidney, gonads and messentry.
• Histopathological examination- uniform infiltration large lymphoblast cells in liver, spleen and bursa.

Sample collection

Buffy coat, egg white, paired sera sample, liver, spleen and bursa of fabricious.

Diagnosis

• Isolation of virus in cell culture
• Histopathological examination
• Cytological examination of May-Grunwald-Giemsa stained impression smear of fresh tumour
• Detection of antigen by RT-PCR
• Detection of antibodies by ELISA

Control

• Eradication of  avian leukosis virus from primary breeding flocks
• Breeder flock is evaluated by shedding of virus in  albumin of egg by ELISA
• Eggs from shedder are discarded
• Selection of genetically resistance line for breeding
• There is no commercial vaccine

Erythroid leukosis (Erythroblastosis)

• Erythroid leukosis (Erythroblastosis) is caused by ALV- J subgroup
• Meat type bird (broilers) are commonly affected
• Adult chickens are mostly affected
• Anemia with immature RBCs in the blood
• Disease originate from bone marrow
• In Malignant  form- the cells remain within the blood vessels throughout the course of the disease
• Erythrostasis in sinusoids of liver, spleen and bone marrow imparting cherry red colour
• Liver and spleen is moderately enlarged

 Myeloid leukosis (Myeloblastosis and Myelocytomatosis)

•  Myeloid leukosis (Myeloblastosis and Myelocytomatosis) is caused by ALV- J subgroup
• It involves extravascular and intravascular proliferation of myeloid (granulocytic series
• Myeloblastosis – liver  and spleen are diffusely enlarged, the liver  frequently as a granular, morocco leather appearance
• Myelocytomatosis- tumour is nodular or diffuse have a creamy white colour. The tumour growth occur in liver, spleen, kidney, flat bones of rib, skull, sternum and pelvis

Osteopetrosis

• Osteopetrosis is a sporadic disease that occurs primarily in males
• The shafts of the long bones are thickened, often resulting in a stilted gait
• Affected birds may be anemic and also have lesions as that of lymphoid leukosis.