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SUMMARY
Ovine pulmonary adenomatosis (OPA), also known as ovine pulmonary adenocarcinoma and jaagsiekte, is a contagious tumour of sheep and, exceptionally, of goats. It is a progressive respiratory disease, principally affecting adult animals. The disease occurs in many regions of the world. Two viruses, a herpesvirus and a retrovirus, have been associated with the disease, but only the latter has an aetiological role. The retrovirus of OPA is distinct from the non-oncogenic ovine lentivirus, and classified as a betaretrovirus.
Identification of the agent: The betaretrovirus of OPA cannot yet be propagated in vitro, therefore routine diagnostic methods, such as virus isolation, are not available for diagnosis. Diagnosis relies, at present, on clinical history and examination, as well as on the findings at necropsy and by histopathology. Viral DNA or RNA can be detected in tumour, draining lymph nodes, and peripheral blood mononuclear cells by polymerase chain reaction.
Serological tests: Antibodies to the retrovirus have not been detected in infected sheep and, therefore, serological tests are not available for diagnosis.
Requirements for vaccines and diagnostic biologicals: There are no vaccines or diagnostic biologicals available.
A. INTRODUCTION
Ovine pulmonary adenomatosis (OPA), also known as ovine pulmonary adenocarcinoma, jaagsiekte (Afrikaans = driving sickness) and ovine pulmonary carcinoma (OPC), is a contagious lung tumour of sheep and, to a lesser extent, of goats. It is the most common pulmonary tumour of sheep and occurs in many countries around the world. It is absent from Australia and New Zealand and has been eradicated from Iceland.
A number of different viruses have been linked aetiologically to OPA, including a herpesvirus and lentiviruses that have been propagated from tumour tissue. However, the former does not have an aetiological role in OPA and the latter exhibit characteristics of non-oncogenic lentiviruses. It has been demonstrated, recently, that OPA is caused by a betaretrovirus that cannot yet be cultured in vitro, but the virus has been cloned and sequenced. The term jaagsiekte sheep retrovirus (JSRV) is used in referring to this virus.
B. DIAGNOSTIC TECHNIQUES
At present, diagnosis of OPA relies on clinical and pathological investigations. In flocks in which the disease is suspected, its presence must be, at least once, confirmed by histopathological examination of affected lung tissue. For such an examination, it is imperative to take specimens from several affected sites and, if possible, from more than one animal. This is because secondary bacterial pneumonia, which might be the immediate cause of death, often masks the lesions (both macroscopic and microscopic) of the primary disease. In the absence of specific serological tests, which can be used for the diagnosis of OPA in live animals, disease control relies on regular flock inspections and prompt culling of suspected cases and, in the case of ewes, their offspring.
| 1. | Identification of the agent
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| | Although ovine herpesvirus 1 (OvHV-1) had been isolated exclusively from OPA tumours, epidemiological studies and experimental infections provide no evidence for a role in the aetiology of OPA. Ovine herpesvirus 2 (OvHV-2) is the sheep-associated malignant catarrhal fever herpesvirus and has never been linked to OPA.
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| | The association of retroviruses with OPA has been recognised for several years. Ovine lentiviruses have been isolated on a number of occasions, but these viruses have no aetiological role in OPA (15, 16).
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| | For many years, the inability to culture JSRV and the lack of antibodies to the virus in affected sheep impeded the confirmation of this virus as the aetiological agent. However, molecular biological techniques provided a key advance, namely, the cloning and sequencing of the 7.5 kb JSRV genome following purification of virions from lung washes of naturally affected sheep (26). JSRV has been designated as a betaretrovirus because of its genetic organisation and its structural proteins. Although cloned JSRV genes, used as hybridisation probes, have revealed a range of homologous endogenous sequences in the genome of both healthy and OPA-affected sheep (1, 7, 26), JSRV is clearly exogenous and associated exclusively with OPA (12). JSRV is detected constantly in the lung fluid, tumour, peripheral blood mononuclear cells, and lymphoid tissues of sheep affected by OPA or unaffected in-contact flockmates, and never in sheep from unaffected flocks with no history of the tumour. Recently, a full-length proviral clone of JSRV has been obtained from OPA tumour DNA. JSRV virus particles, prepared from this clone by transient transfection of a cell line, were used for intratracheal inoculation of neonatal lambs. OPA tumour was induced in two of four lambs, thus demonstrating that JSRV is the causal agent of OPA (14).
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| | What contribution, if any, the endogenous viral sequences make to the aetiology of OPA remains unclear, but their expression in the fetus may, by induction of tolerance, account for the apparent lack of immune response of mature animals to exogenous JSRV.
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| | a) | Nucleic acid recognition methods
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| | | Sequencing of JSRV and endogenous sequences in the sheep genome has led to the development of polymerase chain reaction (PCR) tests that specifically detect JSRV (1, 12). Using this sensitive procedure, JSRV has been detected in peripheral blood mononuclear cells of unaffected in-contact sheep from flocks with OPA, as well as experimentally infected lambs (5, 9). This finding holds promise that it may be possible to develop a molecular test for identifying infected animals during the preclinical stages, thus offering hope that this will provide a method to diagnose OPA.
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| | b) | Animal inoculation
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| | | OPA can be transmitted only with material that contains JSRV, such as tumour homogenates and, more significantly, concentrated cell-free lung fluid from natural cases of OPA. The infectious fraction of the fluid contains JSRV virions and reverse transcriptase (RT) activity. Following the experimental inoculation of adult sheep, clinical disease develops only after several months or years. Similar inoculation of newborn lambs results in a reduced incubation period of 3-6 weeks, and many of the transformed epithelial cells contain intracytoplasmic retrovirus particles (21, 25). At this time there is no practical animal incoculation method for the diagnosis of OPA.
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| | c) | Virus isolation
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Some cell cultures prepared from the tumours occurring in young lambs can support virus replication for a short period (10, 22). Following intratracheal inoculation of concentrated supernatant fluids of such cultures into three lambs within 24 hours of birth, clear histological evidence of OPA was detected in one lamb 6 months later (19). These are preliminary studies and at this time there is no established method to isolate the virus
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| | d) | Clinical signs and pathology
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| | | As there is no reliable laboratory method for the diagnosis of OPA at this time, clinical signs and post mortem lesions are the primary method for the diagnosis of the disease. As OPA has a long incubation period, clinical disease is encountered most commonly in sheep over 2 years of age, with a peak occurrence at the age of 3-4 years. In exceptional cases, the disease occurs in animals as young as 2-3 months of age. The cardinal signs are those of a progressive respiratory embarrassment, particularly after exercise; the severity of the signs reflects the extent of tumour development in the lungs. Accumulation of fluid within the respiratory tract is a prominent feature of OPA, giving rise to moist râles that are readily detected by auscultation. Raising the hindquarters and lowering the head of affected sheep may cause frothy mucoid fluid to run from the nostrils. Coughing and inappetance are not common but, once clinical signs are evident, weight loss is progressive and the disease is terminal within weeks or months. Death is often precipitated by a superimposed bacterial pneumonia, particularly that due to Mannheimia (formerly Pasteurella) haemolytica. In clinically affected animals, a peripheral lymphopenia characterised by a reduction in CD4+ T lymphocytes and a corresponding neutrophilia may assist clinical diagnosis, but the changes are not pathognomonic and are not detected during early experimental infection (17, 23).
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| | | In some countries, another form of OPA (atypical OPA) occurs, which generally presents as an incidental finding at necropsy or the abattoir (3, 4).
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| | e) | Necropsy
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| | | OPA lesions are in most cases confined to the lungs, although intra- and extrathoracic metastasis to lymph nodes and other tissues can occur. In typical cases, affected lungs are considerably enlarged and heavier than normal due to extensive nodular and coalescing firm grey lesions affecting much of the pulmonary tissue. Usually lesions are present in both lungs, although the extent on either side does vary. Tumours are solid, grey or light purple with a shiny translucent sheen and often separated from the adjacent normal lung by a narrow emphysematous zone. The presence of frothy white fluid in the respiratory passages is a prominent feature and is obvious even in lesions as small as a few millimetres. In advanced cases, this fluid flows out of the trachea when it is cut or pendant.
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| | | Pleurisy may be evident over the surface of the tumour and in some cases abscesses are present in the adenomatous tissue.
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| | | In atypical OPA, tumours comprise solitary or aggregated hard white nodules that have a dry cut surface and show clear demarcation from surrounding tissues. The presence of excess fluid is not a prominent feature.
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| | | Adult sheep, which on post-mortem examination appear to have died from acute pasteurellosis, should have their lungs examined carefully, as lesions of OPA may be masked by coexisting bronchopneumonia, verminous pneumonia, chronic progressive pneumonia (maedi-visna) or combinations of these. Samples should be taken at necropsy for PCR for retrovirus genome examination. Storage of these samples at -80°C is recommended, prior to RNA extraction and testing.
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| | f) | Histopathology
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| | | Histologically, the lesions are characterised by proliferation of mainly type II pneumocytes, a secretory epithelial cell in the pulmonary alveoli. Nonciliated (Clara) and epithelial cells of the terminal bronchioli may be involved. The cuboidal or columnar tumour cells replace the normal thin alveolar cells and sometimes form papilliform growths that project into the alveoli. Intrabronchiolar proliferation may be present. In advanced cases, extensive fibrosis may develop and, occasionally, nodules of loose connective tissue in a mucopolysaccharide substance may be present.
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| | | A prominent feature is the accumulation of large numbers of alveolar macrophages in the alveoli adjacent to the neoplastic lesions.
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| | | Where maedi-visna is concurrent, perivascular, peribronchiolar and interstitial lymphoid infilrates may be prominent.
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| | | The histological appearance of atypical OPA is essentially the same as classical OPA, but with an exaggerated inflammatory response (mostly lymphocytes and plasma cells) and fibrosis (3, 4).
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| | | For more detailed accounts of the clinical, post-mortem and histopathological aspects of OPA, the reader is referred elsewhere (4, 18, 20, 24).
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| | | There appears to be a synergistic interaction between OPA and maedi-visna. Lateral transmission of maedi-visna virus appears to be enhanced in sheep affected by OPA (2, 6).
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| 2. | Serological tests
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| | At present, there are no laboratory tests to support a clinical diagnosis of OPA in the live animal. JSRV has been associated exclusively with both typical and atypical forms of OPA, but antibodies to this virus have not been detected in the sera of affected sheep, even with highly sensitive assays such as immunoblotting (11).
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C. REQUIREMENTS FOR VACCINES AND DIAGNOSTIC BIOLOGICALS
There are no vaccines or diagnostic biologicals available at the present time.
REFERENCES
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| 2. | Dawson M., Venables C. & Jenkins C.E. (1985). Experimental infection of a natural case of sheep pulmonary adenomatosis with maedi-visna virus. Vet. Rec. 116, 588-589.
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| 3. | De Las Heras M., Calafat J.J., Jaime J.M., Garcia De Jalon J.A., Ferrer L.M., Garcia Goti M. & Minguijon E. (1992). Sheep pulmonary adenomatosis (jaagsiekte) in slaughtered sheep: variation in pathological characteristics. Medicina Veterinaria, 9 (Suppl.), 52-53.
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| 4. | Garcia-Goti M., Cousens C., Cortabarria N., Extramiana A.B., Minguijon E., Ortin A., Sharp J.M., De las Heras M. & Gonzalez L. (2000). Sheep pulmonary adenomatosis: different pathological forms of the tumour are associated with jaagsiekte retrovirus. J. Comp. Pathol., 122, 55-65.
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| 5. | Gonzalez L., Garcia-Goti M., Cousens C., Dewar P., Cortabarria N., Extramiana B., Ortin A., De las Heras M. & Sharp J.M. (2001). Jaagsiekte sheep retrovirus can be detected in the peripheral blood during the preclinical period of sheep pulmonary adenomatosis. J. Gen. Virol., 82, 1355-1358.
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| 6. | Gonzalez L., Juste R.A., Cuervo L.A., Idigoras I. & Saez De Ocariz C. (1993). Pathological and epidemiological aspects of the coexistence of maedi-visna and sheep pulmonary adenomatosis. Res. Vet. Sci., 54, 140-146.
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| 7. | Hecht S.J., Carlson J.O. & De Martini J.C. (1994). Analysis of a type D retroviral capsid gene expressed in ovine pulmonary carcinoma and present in both affected and unaffected sheep genomes. Virology, 202, 480-484.
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| 8. | Herring A.J., Sharp J.M., Scott F.M.M. & Angus K.W. (1983). Further evidence for a retrovirus as the aetiological agent of sheep pulmonary adenomatosis (jaagsiekte). Vet. Microbiol., 8, 237-249.
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| 9. | Holland M.J., Palmarini M., Garcia-Goti M., Gonzalez L., de las Heras M. & Sharp J.M. (1999). Jaagsiekte retrovirus establishes a pantropic infection of lymphoid cells of sheep with naturally and experimentally acquired pulmonary adenomatosis. J. Virol., 73, 4004-4008.
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| 10. | Jassim F.A. (1988). Identification and characterisation of transformed cells in jaagsiekte, a contagious lung tumour of sheep. PhD thesis. University of Edinburgh, UK.
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| 11. | Ortin A., Minguijon E., Dewar P., Garcia M., Ferrer L.M., Palmarini M., Gonzalez L., Sharp J.M. & De Las Heras M. (1997). Lack of a specific immune response against a recombinant capsid protein of Jaagsiekte sheep retrovirus in sheep and goats naturally affected by enzootic nasal tumour or sheep pulmonary adenomatosis. Vet. Immunol. Immunopathol., 61, 229-237.
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| 12. | Palmarini M., Cousens C., Dalziel R.G., Bai J., Stedman K, Demartini J.C. & Sharp J.M. (1996). The exogenous form of Jaagsiekte retrovirus (JSRV) is specifically associated with a contagious lung cancer of sheep. J. Virol., 70,1618-1623.
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| 13. | Palmarini M., Fan H. & Sharp J.M. (1997). Sheep Pulmonary Adenomatosis: a unique model of a retrovirus-associated cancer. Trends Microbiol., 5, 478-483
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| 14. | Palmarini M., Sharp J.M., De las Heras M. & Fan H.Y. (1999). Jaagsiekte sheep retrovirus is necessary and sufficient to induce a contagious lung cancer in sheep. J. Virol., 73, 6964-6972.
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| 15. | Perk K., De Villiers E.M., Dawson M., Herring H.J., Sharp J.M. & De Martini J.C. (1985). Comparison by Western blotting of the retroviruses associated with sheep adenomatosis (jaagsiekte). In: Slow Virus Diseases in Sheep, Goats and Cattle. Sharp J.M. & Hoff-Jorgensen R., eds. CEC Report EUR 8076 EN, Luxembourg, 349-351 |
| 16. | Querat G., Barban V., Sauze N., Vigne R., Payne A.L., York D.F., De Villiers E.M. & Verwoerd D.W. (1987). Characteristics of a novel lentivirus derived from South African sheep with pulmonary adenomatosis (jaagsiekte). Virology, 158, 158-167.
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| 17. | Rosadio R. & Sharp J.M. (1992). Leucocyte frequency alterations in sheep with naturally and experimentally infected lung cancer. Medicina Veterinaria, 9 (Suppl.), 49-51.
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| 18. | Rosadio R.H., Sharp J.M., Lairmore M.D., Dahlberg J.E. & De Martini J.C. (1988). Lesions and retroviruses associated with naturally occurring ovine pulmonary carcinoma (sheep pulmonary adenomatosis). Vet. Pathol., 25, 58-66.
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| 19. | Sharp J.M. & Angus K.W. (1990). Sheep pulmonary adenomatosis: studies in its aetiology. In: Maedi-Visna and Related Diseases. Developments in Veterinary Virology. Petursson G. & Hoff-Jorgensen R., eds. Martinus Nijhoff, The Hague, The Netherlands, 157-175.
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| 20 | Sharp J.M. & Angus K.W. (1990). Sheep pulmonary adenomatosis: clinical, pathological and epidemiological aspects. In: Maedi-Visna and Related Diseases. Developments in Veterinary Virology. Petursson G. & Hoff-Jorgensen R., eds. Martinus Nijhoff, The Hague, The Netherlands, 177-185.
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| 21. | Sharp J.M., Angus K.W., Gray E.W. & Scott F.M.M. (1983). Rapid transmission of sheep pulmonary adenomatosis (jaagsiekte) in young lambs. Arch. Virol., 78, 89-95.
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| 22. | Sharp J.M., Herring A.J., Angus K.W., Scott F.M.M. & Jassim F.A. (1985). Isolation and in vitro propagation of a retrovirus from sheep pulmonary adenomatosis. In: Slow Virus Diseases in Sheep, Goats and Cattle. Sharp J.M. & Hoff-Jorgensen R., eds. CEC Report EUR 8076 EN, Luxembourg, 345-348
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| 23. | Summers C., Neill W., Dewar P., Gonzalez L., van der Molen R., Norval M. & Sharp J.M. (2002). Systemic immune responses following infection with jaagsiekte sheep retrovirus and in the terminal stages of ovine pulmonary adenocarcinoma. J. Gen. Virol., 83, 1753-1757.
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| 24. | Verwoerd D.W., Tustin R.C. & Payne A. (1985). Jaagsiekte: An infectious pulmonary adenomatosis of sheep. In: Comparative pathobiology of Viral Diseases, Vol. II. Olsen R.G., Krakowa S. & Blakeslee J.R., eds. CRC Press, Florida, USA, 53-75.
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| 25. | Verwoerd D.W., Williamson A. & De Villiers E.M. (1980). Aetiology of jaagsiekte: transmission by means of cellular fractions and evidence for the involvement of a retrovirus. Onderstepoort J. Vet. Res., 47, 275-280.
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| 26. | York D.F., Vigne R., Verwoerd D.W. & Querat G. (1992). Nucleotide sequence of the jaagsiekte retrovirus, an exogenous and endogenous type D and B retrovirus of sheep and goats. J. Virol., 66, 4930-4939.
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