Avian leukosis (Sarcoma group of retroviruses)
This is a complex of viral diseases caused by an avian retrovirus with various manifestations such as lymphoid leukosis, myeloblastosis, erythroblastosis, osteopetrosis, myxosarcomas, fibrosarcomas and other tumors. The causative viruses are rapidly inactivated at ambient temperature and on exposure to most disinfectants. The ALVs are divided into 10 subgroups based on antigenic characteristics of their envelope glycoproteins, host range, molecular characteristics, and other criteria (subgroups A through J). The viruses are also classified as being either exogenous or endogenous. Exogenous ALVs have the ability to infect birds by horizontal and congenital means and are not permanently incorporated into the host genome. Infection involves insertion of viral genetic material into the genome of the target cell, resulting in production of virus copies or cell transformation. Endogenous ALVs are permanently incorporated into the host cellular genome, do not produce virus copies, and are transmitted to progeny as the infected cells are passed to the offspring, i.e. genetic transmission. Subgroups A and B are the common exogenous field viruses associated with disease in layers. Subgroups C and D are exogenous and are rarely reported in the field in chickens. Subgroup E includes the endogenous ALV present in practically all chickens. Subgroups F, G, and H are endogenous and found in the pheasant and subgroup I is endogenous and is found in quail. The J subtype ALV has only recently been identified in heavy breeders and found to be widespread.
In lymphoid leukosis the incubation period is about 4-6 months. Egg layers are generally more susceptible to lymphoid leukosis, vertical transmission is most important by infection of the egg white in infected breeders (who are long-term carriers), lateral transmission is poor but infection may occur by the fecal-oral route, especially in young birds.
The lymphoid leukosis causing ALVs (subgroups A and B) can be transmitted both congenitally and horizontally. These exogenous viruses transform B-lymphocytes by integrating their reverse transcripted proviral DNA into the host chromosome and then activate the cellular onc gene, resulting in the proliferation of the transformed cells. The J subtype ALV virus causes myeloid leukosis or myelocytomatosis/ myeloblastosis only in broilers and behaves similar to A or B subgroups except that the primary target cell is the myelomonocytic series (bone marrow), with low tropism for B-lymphocytes. It has caused substantial losses in heavy breeders in many areas of the world. The ALV-J was first isolated from heavy breeders and characterized in the United Kingdom in 1989. The J subtype virus appears to be genetically unstable, suggesting that the potential for the appearance of new variations within the J subtype and new subtypes are possible. The ALVs do not cause disease in man or other mammals.
Vertical transmission occurs by congenital infection from antibody-negative females, and horizontally the infection passes by needle-passage via bleeding or vaccination needles; in limited cases of young chicks it may be through ingestion of infected feed and water. The prolonged incubation period may be extended to 2-6 months. Congenitally infected birds tend to remain antibody negative, still shed the virus and develop tumors.
Symptoms and lesions: Many birds in the flock are asymptomatic. Morbidity is low, though there is high mortality of affected birds and lingering for a prolonged period. There is drop in egg production. Due to involvement of immune system, there may be increased susceptibility to other infections. The affected birds show persistent low mortality and survivals show depression, loss of weight, enlargement of abdomen, liver or bursa,
Post-mortem lesions include enlargement of spleen, kidneys or liver often with tumorous foci, chalky white tumors in the bone marrow, particularly of the sternum, ribs and sacral joint. Histopathological examination of tumors may reveal well- differentiated myelocytes. Two cell types may be found in the same tumor.
Diagnosis: Detection of antigen in the albumen is done by PCR however, majority of the birds with egg antigen are antibody negative. ELISA is used to identify antibody positive birds.
Prevention and control: Antigen detection in the albumen is the basis for eradication. It was observed that 80% of the shedders produce infected chicks.
Prevention or reduction of cross-infection in hatchery and farm should be carefully planned. If the infection is traced out on a farm, the best policy would be to depopulate the farm in totality. Critical hatchery practices include separation of infected and uninfected lines, handling of clean lines before infected lines, preferably on separate hatch days in separate machines and general execution of biosecurity measures are the procedures adopted for control.