AVIAN RETROVIRUS INFECTIONS

Avian retroviruses cause neoplastic diseases, chronic lym­phomas, or immunosuppressive disease syndromes in birds. They have a broad host range, and are well docu­mented in commercial poultry.

LEUCOSIS/SARCOMA GROUP

Leucosis/sarcoma (L/S) retroviruses are alpharetroviruses. Exogenous L/S retroviruses cause a variety of neoplastic diseases of the haematopoietic system, or mesenchymal tumours. Neoplastic conditions associated with L/S viruses in poultry were named after the tissue or cell type trans­formed. Endogenous L/S retroviruses do not cause disease in their bird hosts, and have been shown to provide protec­tion against exogenous L/S retrovirus infections in chickens.

Lymphoreticular neoplastic conditions have been described in a number of captive and free- ranging wild birds, but a causal relationship between L/S retroviruses and neoplasia has rarely been documented in wild birds⁽⁹⁾. Retrovirus sequences have been amplified in tumour lesions of a European starling (Sturnus vulgaris) presenting as multicentric lymphoma with lesions similar to lym­phoid leucosis in chickens(¹⁰). Gag genes of endogenous and possibly exogenous L/S retroviruses have been dem­onstrated in 26 species of wild galliform birds(¹¹). Domes­tic chickens are reservoirs of L/S retroviruses. The role of wild birds in the epidemiology of species specific L/S ret­roviruses is unknown. Co- speciation of endogenous L/S retroviruses with wild galliform species, such as partridges (Perdix spp.) and ptarmigans (Lagopus spp.), indicates an ancient association with vertical transmission¹-¹¹).

Infection with L/S retroviruses can be acquired congeni­tally via the albumen, or via faecal- oral transmission⁽¹²⁾.

Chickens with L/S retrovirus infection may be anaemic, or there may be an increase in immature forms of specific white or red blood cells in the peripheral blood. Peripheral leucocyte changes have been observed in captive and free- ranging wild birds with lesions similar to those of lym­phoid leucosis in chickens⁽⁹⁾.

In wild birds, lymphoid leucosis lesions are the tumours most frequently reported. These are generally described as soft, smooth, white nodular to diffuse tumours affecting a variety of tissues — typically parenchymal organs, notably the liver, the spleen and the kidneys. Microscopically, lym­phoblastic lymphomas are most frequently reported.

In wild birds, a tentative diagnosis can be made based on the presence of neoplastic lesions, and the cell types involved in these tumours(⁹). A definitive diagnosis of L/S retrovirus infection can only be made by virological inves­tigation. Besides virus isolation or antigen detection by ELISA, detection of retroviral nucleic acids can be achieved by PCR-based methods. Serological methods to detect antibodies are of little practical use for wild bird species.

There are no regulation, management or control plans for L/S retrovirus infections in European free-ranging wild birds. There are no indications that L/S retroviruses are zoonotic.

The impact of L/S retrovirus infection on European free-ranging wild birds is unknown and likely to be minimal⁽⁹⁾.

Reticuloendotheliosis group

The reticuloendotheliosis (RE) retroviruses are gammaret­roviruses causing acute reticular cell neoplasia, chronic lymphomas and a runting immunosuppressive disease syn­drome in commercial poultry, notably turkeys.

RE retroviruses are common in commercial poultry worldwide. Prevalence is typically low, and their role as reservoirs of RE retroviruses is debated¹-⁹). Other avian species with RE retrovirus infection or tumours possibly associated with RE retrovirus infection include wild Pal­earctic species present in Europe, i.e. grey partridge (Perdix perdix), ring-necked pheasant (Phasianus colchicus) and rock pigeon (Columba livia)(⁹).

RE retroviruses are transmitted both vertically in poultry with persistent viraemia, and horizontally via a direct or indirect route. Direct contact with infectious virus in faeces, nasal and ocular secretions and contaminated litter is believed to be the main route of transmission in poultry. Mechanical transmission by mosquitoes is documented.

Infection is acquired via the albumen in poultry infected congenitally. These develop a tolerant infection with per­sistent viraemia and no antibody response. In these birds, rates of vertical transmission are high, as are rates of immu­nosuppression and tumour development. In older poultry, infection is acquired via a faecal-oral or respiratory route. Viraemia is transient, and an antibody response develops. Non-infectious virus can remain detectable for weeks after disappearance of infectious RE retroviruses. These birds generally do not develop clinical disease or tumours.

RE retroviruses infect cells of the lymphoreticular system. RE retroviruses are detected in plasma 3 to 6 weeks post-infection, and can be isolated from internal organs in diseased poultry.

Acute reticular cell neoplasia is observed only in newly hatched poultry, which do not show major clinical signs, although mortality can be high. Older chickens develop runting immunosuppressive disease syndrome with pale extremities and mucous membranes.

Only chronic lymphomas and associated lesions have been described in wild birds. Most lymphomas involve parenchymal organs or develop along peripheral nerves. Tumours are typically smooth enlargements of the spleen, liver, heart, thymus or bursa of Fabricius, or nodules in visceral organs. Microscopically, chronic lymphomas asso­ciated with RE retrovirus infection involve uniform blastic lymphoreticular cells.

In wild birds, a tentative diagnosis can be made based on the presence of macro- and microscopic lesions. A definitive diagnosis can only be made by demonstrating the presence of virus, viral antigen, viral nucleic acid or virus-specific antibody. Most of the antibody tests are not suitable for wild bird testing. There are no regulation, management or control plans for RE retrovirus infection in European free-ranging wild birds. There are no indica­tions that L/S retroviruses are zoonotic. The impact of RE retrovirus infections on wild bird populations is unknown.