Jaagsiekte sheep retrovirus
| Jaagsiekte sheep retrovirus | |
|---|---|
| Virus classification | |
| Group: | Group VI (ssRNA-RT) |
| Order: | Unassigned |
| Family: | Retroviridae |
| Subfamily: | Orthoretrovirinae |
| Genus: | Betaretrovirus |
| Species: | Jaagsiekte sheep retrovirus |
Jaagsiekte sheep retrovirus (JSRV) is a betaretrovirus which is the causative agent of a contagious lung cancer in sheep, called ovine pulmonary adenocarcinoma.
JSRV is the virus that is the cause of the contagious lung tumors in sheep called ovine pulmonary adenocarcinoma (OPA). The disease has also been called “jaagsiekte”, after the Afrikaans words for “chase” (jaag) and “sickness” (siekte), to describe the respiratory distress observed in an animal out of breath from being chased, indicating the breathing difficulty experienced by infected sheep. Transmission of virus is through aerosol spread between sheep.
The exogenous infectious form of JSRV has an endogenous counterpart which is present in the genomes of all sheep and goats. The sheep genome has around 27 copies of endogenous retroviruses (enJSRVs) that are closely related to JSRV. Endogenous JSRV has several roles in the evolution of the domestic sheep as they are able to block the JSRV replication cycle and play a critical role in sheep conceptus development and placental morphogenesis.
Although OPA resembles human lung cancer, human lung cancer is not known to be caused by betaretroviruses. Even though a possibility of a viral cause has been eliminated in bronchoalveolar cancer, understanding the molecular mechanisms leading to the transformation of lung epithelia by JSRV may be of interest in the context of therapeutic approaches in human lung cancers in general and bronchoalveolar adenocarcinoma (BAC) in particular.
JSRV belongs to the family Retroviridae, to the subfamily Orthoretrovirinae and the genus Betaretrovirus.
JSRV is transmitted by the respiratory route and may also infect lymphocytes and myeloid cells, in addition to the lung epithelia. Expression of the JSRV Envelope protein activates signalling cascades that promote cellular proliferation and malignant transformation of the cells. Initially, the tumour cells grow along the alveolar walls in a pattern reminiscent of human BAC, but subsequently become more invasive and metastasize to the local lymph nodes. Larger tumours may be necrotic and fibromatous at their centre. As the tumour grows, fluid production in the lung increases and this is likely to promote virus spread to other sheep. Only when the tumour reaches a size large enough to compromise lung function, do clinical signs appear. Critically, the majority of infected animals in endemic areas never show outward signs of infection, but they may be shedding virus, thus promoting inadvertent introduction of the disease into previously unaffected flocks and new geographical areas.
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