Avian Leukosis and Reticuloendotheliosis (Avian Leukosis Virus - ALV Subgroups A-J)

Avian leukosis and reticuloendotheliosis are neoplastic diseases of poultry caused by retroviruses, primarily Avian Leukosis Virus (ALV) and Reticuloendotheliosis Virus (REV). ALV is an alpharetrovirus with subgroups A through J, affecting chickens, while REV is a gammaretrovirus with a broader host range. These viruses induce a variety of tumors, most commonly lymphoid leukosis (LL) in the bursa of Fabricius and visceral organs, but also erythroblastosis, myeloblastosis, and other neoplasms. The diseases are economically significant in commercial broilers, layers, and breeders, causing mortality, reduced egg production, and increased condemnations at processing. ALV-J, in particular, has emerged as a major pathogen in broilers, causing myeloid leukosis and immunosuppression. Reticuloendotheliosis is less common but can cause runting, immunosuppression, and tumors, and has been associated with contaminated vaccines. The diseases are transmitted both vertically (through the egg) and horizontally (by contact), and control relies on strict biosecurity, testing, and eradication programs in breeding flocks.

The primary causative agents are retroviruses: Avian Leukosis Virus (ALV) belongs to the genus Alpharetrovirus, family Retroviridae. ALV is classified into subgroups A through J based on the envelope glycoprotein (gp85) and host range. Subgroups A and B are common in commercial layers and breeders, while subgroup J (ALV-J) is prevalent in broilers. ALV-E is an endogenous virus that is generally non-pathogenic but can interfere with diagnosis. Reticuloendotheliosis Virus (REV) is a gammaretrovirus, with strains such as REV-A, REV-T, and others. REV is antigenically distinct from ALV and can cause a range of tumors, including lymphomas, and immunosuppression. Both viruses are enveloped, with a single-stranded RNA genome that replicates through a DNA intermediate (provirus) integrated into the host genome. The viruses are relatively fragile in the environment, inactivated by heat, lipid solvents, and common disinfectants. ALV-J is characterized by a unique gp85 sequence that allows it to evade immunity induced by other subgroups.

Avian leukosis and reticuloendotheliosis affect primarily chickens, but REV can also infect turkeys, ducks, geese, and other avian species. ALV is most significant in commercial layers and broiler breeders, where vertical transmission from infected hens to progeny is the primary route. Horizontal transmission occurs through contact with infected birds, feces, saliva, and contaminated equipment. ALV-J is particularly problematic in broilers, causing myeloid leukosis and immunosuppression, leading to increased susceptibility to secondary infections. The prevalence of ALV-A and ALV-B has decreased in commercial flocks due to eradication programs, but ALV-J remains a challenge. REV is less common but can cause outbreaks, especially in backyard flocks and in association with contaminated vaccines. Morbidity and mortality vary: in ALV-J infected broiler flocks, mortality can reach 10-20%, with condemnations due to tumors and immunosuppression. In layers, egg production can drop by 10-20% in affected flocks. The diseases are more common in older birds, with tumors typically appearing after 16 weeks of age. Housing systems with high stocking density and poor biosecurity increase the risk of horizontal spread. Wild birds can serve as reservoirs for REV, but ALV is primarily chicken-specific.

ALV and REV infect cells by binding to specific receptors on the host cell surface. ALV-A and B use the tv-a and tv-b receptors, respectively, while ALV-J uses a different receptor (chicken Na+/H+ exchanger type 1). REV uses a different receptor. After entry, the viral RNA is reverse-transcribed into DNA, which integrates into the host genome as a provirus. The provirus can remain latent or be transcribed to produce new viral particles. In susceptible birds, the virus infects cells of the bursa of Fabricius, bone marrow, and other tissues, leading to neoplastic transformation. In ALV-induced lymphoid leukosis, the virus integrates near the c-myc oncogene, causing its overexpression and leading to B-cell lymphoma. Tumors typically arise in the bursa and metastasize to the liver, spleen, and other organs. ALV-J induces myeloid leukosis by activating the c-myc or other oncogenes in myeloid cells. REV can cause lymphoma by activating c-myc or other oncogenes. The viruses also cause immunosuppression by depleting lymphocytes in the bursa and thymus, leading to increased susceptibility to other pathogens. The incubation period is long, often several months, and tumors are usually seen in birds over 16 weeks of age.

Intrinsic factors include genetic susceptibility, as some chicken lines are more resistant to ALV infection. Age is a factor, with older birds more likely to develop tumors. Immunosuppression from other infections (e.g., IBD, MDV) can exacerbate the disease. High production stress in layers and broiler breeders may increase susceptibility. Extrinsic factors include poor biosecurity, which allows horizontal transmission; high stocking density and poor ventilation increase viral load. Contaminated vaccines, especially those for Marek's disease, have been implicated in REV outbreaks. Vertical transmission from infected breeders is a major risk factor. Feed contamination with mycotoxins can further immunosuppress birds. Environmental stress, such as heat or cold, can also predispose to disease.

Clinical signs are often non-specific and may include depression, reduced feed intake, weight loss, and paleness. In layers, there is a drop in egg production and an increase in abnormal eggs. Birds may develop tumors that are palpable in the abdomen, especially in the liver and bursa. In ALV-J infected broilers, there may be an increased incidence of myeloid leukosis, characterized by bone marrow involvement and sometimes visible tumors on the sternum or ribs. Neurological signs can occur if tumors compress nerves, leading to paralysis or torticollis. In REV infection, runting and immunosuppression are common, with poor feathering and increased mortality. Some birds may show no clinical signs but shed the virus. The disease is often chronic, with tumors developing over months.

Differential diagnoses include Marek's disease (MD), which causes T-cell lymphomas, often with nerve involvement and skin lesions; MD can be differentiated by the presence of tumors in nerves, skin, and gonads, and by histopathology showing T-cell markers. Reticuloendotheliosis (REV) can cause similar tumors but is less common and can be differentiated by PCR and serology. Other neoplasms such as erythroblastosis, myeloblastosis, and hemangiomas can be caused by ALV. Bacterial infections like colibacillosis can cause similar gross lesions but are differentiated by culture and histopathology. Nutritional deficiencies, such as vitamin E/selenium deficiency, can cause similar immunosuppression but lack tumors. Mycotoxins can cause immunosuppression and poor performance but no tumors. Other viral diseases like infectious bursal disease (IBD) cause bursal atrophy but not tumors. Avian influenza and Newcastle disease can cause systemic signs but are acute and have respiratory signs.

The diagnostic approach begins with a thorough flock history, including age, breed, vaccination status, and clinical signs. A complete necropsy is essential, with examination of the bursa, liver, spleen, and other organs for tumors. Gross lesions suggestive of leukosis include enlarged liver with miliary tumors, enlarged bursa, and tumors in other viscera. Histopathology of affected tissues is crucial to differentiate tumor types. Serology using ELISA can detect antibodies to ALV and REV, but may not distinguish between subgroups. Virus isolation in cell culture (e.g., chicken embryo fibroblasts) can be performed, but is time-consuming. PCR and RT-PCR are highly sensitive and specific for detecting viral nucleic acid and can differentiate subgroups. For ALV, group-specific antigen (p27) ELISA can detect viral antigen in serum or egg albumen. For REV, PCR is the preferred method. Immunohistochemistry can be used to detect viral antigens in tissues. The diagnostic algorithm should include molecular testing to confirm the presence of ALV or REV and to rule out other causes.

Serology: ELISA for ALV antibodies (subgroup-specific) and REV antibodies. For ALV, the group-specific antigen (p27) ELISA is used to detect viral antigen in serum, egg albumen, or cloacal swabs. HI titers are not typically used for ALV. Molecular diagnostics: RT-PCR and real-time PCR for ALV and REV, with subgroup-specific primers for ALV-A, B, J, etc. Virus isolation: ALV can be isolated in chicken embryo fibroblasts or chicken kidney cells; REV can be isolated in chicken embryo fibroblasts. Histopathology: Lymphoid leukosis shows B-cell lymphoma with bursal involvement; myeloid leukosis shows myeloblasts in bone marrow and liver. Blood chemistry: May show elevated liver enzymes (AST, LDH) in cases of liver tumors. CBC may show anemia or leukocytosis. Coccidiosis lesion scoring is not applicable. Mycotoxin feed assays may be performed to rule out immunosuppression.

Radiography: May show hepatomegaly, splenomegaly, or bone lesions in cases of myeloid leukosis. Ultrasonography: Can be used to detect abdominal tumors in live birds, but is not commonly used in poultry practice. Gross necropsy photography is essential for documentation. Imaging is not a primary diagnostic tool for these diseases.

Gross necropsy lesions: In lymphoid leukosis, the bursa is enlarged and may contain nodular tumors; the liver is enlarged with diffuse or nodular tumors; spleen and other organs may also be affected. In myeloid leukosis, the bone marrow is pale or gray, and tumors may be present on the sternum, ribs, or other bones. Histopathology: Lymphoid leukosis shows a uniform population of large lymphoblasts with basophilic cytoplasm and round nuclei, often with mitotic figures. Tumors arise in the bursa and metastasize to other organs. Myeloid leukosis shows myeloblasts with eosinophilic granules in the cytoplasm. REV-induced lymphomas are similar to lymphoid leukosis but may involve T-cells. Immunohistochemistry can differentiate B-cell (ALV) from T-cell (MD) lymphomas.

There is no specific treatment for avian leukosis or reticuloendotheliosis. Antiviral drugs are not approved for poultry. Management focuses on supportive care, including providing optimal nutrition, reducing stress, and controlling secondary infections. Antibiotics may be used to treat secondary bacterial infections, but they do not affect the virus. In severe outbreaks, depopulation may be necessary to prevent spread. For breeding flocks, eradication programs involve testing and culling of positive birds. Vaccination is not available for ALV or REV. Biosecurity measures, including strict hygiene, all-in/all-out management, and control of vertical transmission, are essential. In commercial layers, if the disease is present, egg production may be reduced, and affected flocks may be kept until the end of the laying cycle, but with reduced profitability.

The prognosis is poor for individual birds with tumors, as the disease is progressive and fatal. For flocks, the prognosis depends on the prevalence and severity. In broiler flocks, ALV-J can cause significant mortality and condemnations, leading to economic losses. In layers, egg production may be reduced by 10-20%, and the flock may not recover to full production. In breeding flocks, eradication is necessary to prevent vertical transmission. The long-term prognosis for the flock is guarded, and depopulation may be recommended in severe cases. With good biosecurity and management, the disease can be controlled, but not eliminated.

After an outbreak, it is important to monitor the flock for recurrence. This includes regular clinical examinations, necropsy of any dead birds, and serological testing for ALV and REV. For breeding flocks, a testing and culling program should be implemented to eliminate positive birds. The environment should be thoroughly cleaned and disinfected, and litter should be removed. Biosecurity protocols should be reviewed and strengthened. For replacement flocks, ensure they are sourced from ALV/REV-free breeders. Monitor egg production and mortality rates. In broiler flocks, monitor processing condemnations for tumors. Regular testing of sentinel birds may be useful.

Pearls: Always examine the bursa of Fabricius in birds over 16 weeks of age for tumors; an enlarged bursa with nodular tumors is highly suggestive of lymphoid leukosis. ALV-J often causes myeloid leukosis, which may present with tumors on the sternum or ribs. REV can cause runting and immunosuppression, and may be associated with contaminated vaccines. Pitfalls: Do not confuse ALV with Marek's disease; MD typically causes nerve enlargement and T-cell lymphomas, while ALV causes B-cell lymphomas in the bursa. Do not rely solely on serology, as antibodies may not be present in immunotolerant birds. Always confirm with PCR or virus isolation. Avoid using live vaccines that may be contaminated with REV. Do not overlook the possibility of vertical transmission in breeding flocks.

There are no specific antiviral drugs for ALV or REV. Supportive therapy includes vitamins and electrolytes to reduce stress. Antibiotics may be used to control secondary bacterial infections, e.g., amoxicillin at 10-20 mg/kg body weight orally every 12 hours, or oxytetracycline at 20-30 mg/kg, or tylosin at 20-25 mg/kg, or tilmicosin at 10-20 mg/kg, or enrofloxacin (where legal) at 10 mg/kg, or florfenicol at 20-30 mg/kg. These are typically administered in drinking water for 3-5 days. Anticoccidials are not indicated unless coccidiosis is present. Vitamins A, D3, E, C, and K may be supplemented in water or feed. Vaccination is not available for ALV/REV, but vaccination against other immunosuppressive diseases (e.g., IBD, MD) is important to reduce secondary infections. Withdrawal times must be observed for antibiotics.

Landmark studies have characterized the molecular biology of ALV and REV, including the identification of ALV-J in the 1990s. Eradication programs in commercial breeders have successfully reduced ALV-A and B, but ALV-J remains a challenge. Research has shown that ALV-J causes immunosuppression, leading to increased susceptibility to other diseases. Studies have demonstrated the importance of vertical transmission in the epidemiology of ALV. For REV, outbreaks have been linked to contaminated vaccines, highlighting the need for vaccine safety. Consensus guidelines from AAAP and WOAH recommend strict biosecurity, testing, and culling for control. Recent research focuses on developing vaccines and antiviral strategies, but none are commercially available. Meta-analyses have shown that ALV-J infection in broilers leads to significant economic losses due to mortality and condemnations.