Hopes are growing that scientists may yet be able to develop a diagnostic test for a silent killer of sheep.
Ovine pulmonary adenocarcinoma (OPA) or jaagsiekte sheep retrovirus (JSRV) is a respiratory disease that is thought to be widespread in flocks across the world.
The disease is known as a silent killer because farmers do not want to speak about it as there is currently no cure for it. OPA infects the lungs of sheep and triggers the growth of large cancerous tumours which eventually kill them. Death usually occurs within a few weeks of clinical signs of the disease appearing. These are generally characterised by nasal discharges and sheep having difficulty breathing as the tumours trigger the over production of fluid in the lungs.
Recent research work by scientists at Edinburgh’s Moredun Research Institute has raised the hopes of a diagnostic test being developed to identify the presence of OPA before clinical signs appear.
Principal scientist David Griffiths said that work had used an advanced screening method to identify antibodies to OPA in a small proportion of naturally infected sheep. This is the first time that any immune response in sheep to the virus has been identified.
His research team is now studying whether these antibodies could potentially be used to develop a diagnostic test.
Moredun has also developed new methods for growing the virus in its labs which has improved the ability of researchers to study the interaction between the virus and sheep lung tissue, advancing their understanding of how disease develops.
Mr Griffiths said a new project is also about to start at Moredun to determine how the sheep lung responds to the infection as well as identify the higher levels of proteins produced by the sheep as a result.
These proteins will the be evaluated for their effectiveness as diagnostic markers for OPA. Mr Griffiths said the virus was easily spread between sheep through the nasal discharge or through the air, by inhalation of the infectious virus from these secretions.
It can, however, take several years from the introduction of the virus until clinical signs are seen. This helps in the spread of the disease through the purchase of infected, but apparently health sheep.
OPA is mainly found in adult sheep, but occasionally infects lambs. Losses to OPA in infected flocks are generally 1-3% annually, but in some can be as high as 10%.
Mr Griffiths said the overall impact of OPA on the national sheep flock was unknown as it is not a notifiable disease. “Estimates of prevalence are based on case reports and it is likely that this disease is greatly under-reported,” he added.
“One reason for this is that sheep with OPA are more susceptible to other infections, such as pasteurellosis, and this is frequently recorded as an immediate cause of death.
“In a recent survey of 125 flocks in Scotland, conducted by Moredun and colleagues at SRUC and BioSS, we found that up to a third of flocks may carry the OPA virus, although very few reported ever having noticed the disease in their sheep. Whilst this survey was limited to Scotland, it is clear that the disease is found in sheep right across the UK. In some flocks more than half the sheep may be infected with OPA.”
Moredun has developed a test which can identify if a flock is infected. But that is unable to detect infection in individual sheep.
Mr Griffiths appealed to farmers with infected sheep to contact him. Their anonymity is guaranteed.
He and researcher Chris Cousens can be contacted on 0131 445 5111.
The Moredun’s work on OPA is funded by the Scottish Government and Scottish Funding Council.