Avian Leukosis Complex (ALC):

Synonym: Avian sarcoma, Visceral lymphoid leukosis, Big liver disease, Lymphomatosis

• It is an infectious cancerous condition of mature birds involving hemopoietic and lymphoid tissues like liver, bursa, spleen, gonads, kidney, bones, etc.
• It is characterized by an abnormal and unlimited growth of cells in body which usually causes bird’s death.
• There is marked enlargement of liver, so-called big liver disease.

Etiology:

• Avian leukosis/sarcoma virus of genus; Retrovirus, Family: Oncovirinae
• Enveloped, ss RNA
• Overall diameter of particle is 80-120 µm.
• Five sub-subgroups; A, B, C, D and E

Epidemiology:

• Incidence is usually highest at about sexual maturity; in breeder and laying hens
• Incidence of clinical disease is usually low.
• Disease is widely prevalent in commercial chickens
• Chickens and pheasants are the natural hosts of virus.
• De Boer reported mortality of 2.18% in white layers in Netherlands.
• Congenitally infected chicken remains viremic for life.

Transmission:

• Main route of transmission is vertically through infected eggs to progeny.
• Virus is also excreted in saliva and feces.
• Contaminated equipments also acts as contagion.
• Indirect transmission through close contact with infected pen-mates
• Contaminated equipments, incubators and hatching device may transmit the virus.

Pathogenesis:

• Incubation period for disease is 4 months or more.
• Virus gains entry through yolk/egg
• After this, virus multiplies initially in bursa, bone, connective tissue
• Cells gets transformed into cancerous cells and spread into different organs; usually from Bursa.
• Transformed cells are lymphoblasts. This cell proliferates and gives rise to nodular growth into bursa; visible from about 14 weeks of age.
• These neoplastic cells migrate to visceral organs and produce tumors in liver, kidney, lungs, ovary, ribs, sternum, etc.

Clinical Findings:

• Symptoms are usually non-specific.
• Birds may be weak and emaciated.
• Loss of appetite.
• Comb may be pale, shriveled and sometimes cyanotic.
• Diarrhea, wattles may be pale.
• Abdomen is usually enlarged.
• On palpation, nodular tumors of liver can be detected.
• Feathers are sometimes spotted with urates and bile pigments.

PM Findings:

• Liver is greatly enlarged.
• Spleen, Bursa of Fabricius and ovary are also usually enlarged. Bone marrow is often tumorous.
• Tumors are also present in kidney, lung, gonad, heart, bone marrow, and mesentery.
• Tumors are soft, smooth and glistening.
• In nodular form, lymphoid tumors may vary from 0.5 mm to 5 cm in diameter, usually spherical and may occur singly or in large numbers.
• Granular form of tumor consists of numerous small nodules less than 2 mm in diameter and uniformly distributed throughout liver.
• In diffuse form, liver is enlarged uniformly, slightly greyish in color and usually friable.
• Tumors are also found at costochondral junction of ribs, sternum, mandible.

Diagnosis:

• Based on history and clinical findings
• Based on PM findings
• Isolation and identification of virus
• CFT, FAT, SNT, AGPT, ELISA

Differential Diagnosis:

• Disease should be differentiated from Marek’s disease.
• Marek’s is usually seen in 6 weeks or older birds.
• Nervous symptoms are also present in Marek’s disease.
• Mortality is usually high.
• Nerve swelling are also evident in Marek’s disease which is absent in ALC.

Treatment:

• There is no effective treatment.

Control Measures:

• Affected birds should be culled and disposed off properly.
• Birds should be purchased from breeders where parent stock is free of virus.
• Development of genetic resistance bird.
• No vaccine is available for disease.