Avian leukosis

Avian Leukosis Complex (ALC)

Avian leucosis viruses are grouped together because they share important characteristics. They produce variety of transmissible benign and malignant neoplasm. The most common is lymphoid leucosis.

Etiology
ALC is caused by RNA oncogenic virus type C. Ten years ago, researchers at the institute for animal health at Compton in UK isolated a new type of ALV from meat type breeding chickens in UK. This new virus is named ALV – J. It increases mortality in adult birds for myeloid leucosis (Myelomatosis). This uncommon form of leucosis starts at the bone marrow and affects the granulocytic series of blood cells. ALC occurs in the birds having 3 to 8 months age.

Epidemiology

Vertical transmission i.e. from parent to offspring is common and chick-to-chick transmission is also possible. ALC is transmitted through contaminated saliva, droppings, hatchers or incubators.Epidemiological studies reveal that compared to LL, Erythroblastosis occurs infrequently and sporadically in 5-week-old birds. Myelocytomatosis occurs in adult birds; nearly 1% of the birds are affected. Nephroblastoma – 19% broilers, 3-10% layers.Osteopetrosis occurs much less frequently then LL. ALV- J spreads congenitally through the egg from such infected hens to their chicks and to uninfected birds by contact. Viraemic cock shed virus but they are believed not to transmit ALV to their progeny at mating although they may infect hens via their semen.

Exogenous virus (Horizontal)
Exogenous virus (vertical)

Endogenous
Genetic: Male - Female bird : Viral DNA integrated in the gamete DNA
• Viremia or antigen expressed immune tolerance to endogenous virus lymphoid leucosis very rare
Pathogenesis
Leukosis virus develops anemia, hepatitis, immuno-depression, ataxia, lethargy, and imbalance by meningoencephalomyelitis.
1. Anemia is due to aplastic crisis in the bone marrow in which RBC fails to incorporate iron into hemoglobin.
2. Immunodepression may inovolve atrophy or aplasia of lymphoid organs and hypergammaglobulinemia.
3. Hens that shed virus produced 20- 35 fewer eggs per hen housed, produce smaller eggs at a lower rate and with thinner shells compared to non-shedders.
4. Mortality causes other than neoplasm was 5 -15% higher, fertility was 2.4% lower in shedders than non-shedders.
5. LLV in semen has an effect on semen quality and fertility.

Clinical symptoms

1. Lymphoid leucosis : Liver is grossly affected and markedly enlarged and have granular or nodular surface. Liver exerts pressure on the abdominal organs. Spleen is enlarged and ovary may show cauliflower like growth. Heart and lung will have diffuse nodular growth, emaciation and accumulation of fluid in the abdominal cavity. Feathers are sometimes spotted with urates and bile pigments.
2. Myeloblastosis : Liver, spleen and kidneys are enlarged and have reddish brown granular appearance. There is increased leukocyte count.
3. Erythroblastosis: Liver, spleen and kidneys are enlarged and assume dark red color. Body is yellowish in color, diarrhoea, loss of weight, pale combs and wattles. Profuse hemorrhage from one or more feather follicles.
4. Nephroblastoma: Kidneys become nonfunctional and assume embryonic features.
5. Osteopetrosis has enlargement of the leg and wing bones. The enlarged bones are very hard and brittle. Birds walk with jerky stilted gait. There is exostosis of the bones and thickening of the epiphysis.
6. Myelocytomatosis causes mortality in adult stock of both sexes. Affected birds die with tumorous infiltrations and enlargement of the liver, spleen, kidneys and other organs and commonly there is creamy tumor formation on the ribs, vertebrae and particularly on the inner aspects of the sternum.
The tumors are myelocytomas but occasionally other types of tumors are also seen. Skeletal growth of myelocytes may result in abnormal portuberences of the head and thorax.
Diagnosis

Hematology:
• Lymphoid leukosis : Predominant lymphoblast with large size, eccentric nucleus and spongy chromatin with basophilic cytoplasm.
• Erythroblastosis: blood is watery, light red and clots slowly, stained blood smear reveals variable number of erythroblasts.
• Myeloblastosis: up to 2 million myeloblasts/mm3. Lots of large cell with larger nucleus and basophilic clear cytoplasm.
• Nephroblastoma – Aleukemic cases
• Osteopetrosis: Aleukemic and secondary anemic with active erythropoiesis in the bone marrow.
• Myelocytomatosis: Aleukemic but may be leukemic granulated or non-granulated myeloid cells.
Postmortem lesions

• Lymphoid leukosis: Tumors are present in the liver, spleen and bursa of Fabricious, heart, gonad, bone marrow, mesentry etc.
• Erythroblastosis: petechial hemorrhage in visceral organs. There is thrombosis, infarction, rupture of liver, ecthyma of lungs, hydropericarcium, ascites etc.
• Myeloblastosis: Diffuse tumor in the liver, bone marrow is firm and reddish grey.
• Nephroblastoma: small pinkish grey nodules embedded in the kidney, which is pedunculated.
• Osteopetrosis: diaphysis of tibia, tarsometatarsus and other long bones are involved. Periosteum is thickened and abnormal bone is spongy.
• Myelocytomatosis: Tumors on the costochondral junction of ribs, sternum, skull etc.
Serological tests: AGPT, VNT, FAT, ELISA
Differential diagnosis
Marek’s Disease:
• It is caused by DNA Herpes virus.
• Mostly 2 to 4 months birds mostly affected.
• Both CNS and ANS are affected with ocular lesions.
• Airborne horizontal transmission occurs.
• Bursa of Fabricious is atrophied.
Connective tissue tumors
• Show the presence of granulomas i.e. TB like nodules. It results from trauma, myelocytomas, or leiomyomas.
Renal Tumors
• Large masses and are found only on the kidney.
Control

1. Separation of infected and clean lines in the hatchery and use of separate hatchers or hatching days.
2. Processing clean lines should be brooded and reared apart.
3. Live vaccine challenges delayed where possible and vaccine needle should be changed between groups.
4. Comprehensive testing programs, prevention of re-infection and time are needed to allow eradication programs.
5. No vaccines.