An ongoing trial to test the efficacy of a test and cull approach to OPA control is showing positive results.
In the absence of a diagnostic test to categorically determine whether sheep are infected or uninfected with Jaagsiekte sheep retrovirus (JSRV), the virus that causes ovine pulmonary adenocarchinoma (OPA), ultrasound scanning remains the most useful method for detecting the disease before clinical signs appear.
This was the message from Dr Chris Cousens at a recent Moredun sheep health meeting in Peebles when sharing the latest results from the organisation’s ongoing research into the effectiveness of ultrasound scanning to control OPA.
Dr Cousens reiterated that, as the virus is airborne, most transmission between animals is via respiratory routes, but it had also been shown to transfer in colostrum between ewes and lambs.
Despite continued efforts by Moredun to develop a specific OPA vaccine or treatment, which most recently had included work looking into the potential of biomarkers and an antibody based test, it had so far not been able to develop a definitive test or treatment.
This means preventing the spread of infection is the only viable option to control OPA at present.
Dr Cousens explained that Moredun had been working with some 14 flocks across Scotland to investigate whether a scan and cull approach could reduce incidence rates of OPA year-onyear, if animals which show OPA lesions detected by ultrasound were culled from the flock.
Underway for the past five years, this has seen whole-flock scanning take place at each of the farms taking part every six to 12 months during this time.
Results up to now had shown that, generally, flocks were seeing a reduction in OPA positive results year-on-year since they had started the process.
Dr Cousens said: “Ultrasound screening is never going to be able to detect the very smallest OPA tumours, but it can detect the disease before an animal begins to display any clinical signs.
“This means those animals can be identified for culling at a saleable value, but it also goes some way to eliminating the welfare issue of keeping sheep, although not intentionally, on-farm with an incurable disease.
“The overall trend is good but we have noted that it does take time and persistence, particularly in flocks which had a high number of positive cases to start with [more than 5 per cent positive tests at first scan], to reduce prevalence.
“We also do not know how long it is going to take to get all flocks to an overall negative screening result.”
With the popularity and uptake of the scan and cull approach said to be increasing, Dr Cousens said it was also key to understand what ultrasound results meant.
She highlighted that a single negative scanning result on an animal could not serve as a guarantee that OPA was not present, only that ‘no lesions were found’ as a result of the scan.
Lessons had also been learnt about the scanning procedure during the research, as well as OPA itself.
Although previously thought of as a slow developing disease, very regular scanning as part of the research [every three weeks] on one group of sheep had shown it to develop more rapidly, from less than 1cm in diameter to up to 7cm over three months.
Alternative In terms of the next steps for controlling OPA, Dr Cousens said she would like to see accreditation-type schemes in the future based on ultrasound scanning, given the absence of an alternative diagnostic test at present.
Designing a useful scheme of this nature and rolling this out would, however, require further research by Moredun into infection and disease susceptibility and training of more vets to carry it out.
Dr Cousens noted that, because the process was a diagnostic test, only vets were able to legally carry out OPA ultrasound scanning at present, although it was possible that derogation could be applied for from the Royal College of Veterinary Surgeons to widen this to other scanning professionals if vets were unable to meet demand for the test.
Although county-wide eradication was still a long way off, it was thought that progress had been made in terms of improved awareness about OPA and more open discussion in the sheep industry, alongside research progress in understanding how disease spread could be controlled.
A new study, led by Moredun and Edinburgh University has identified genetic changes occurring during the growth of lung cancer in sheep.
The study looked at how lung cells change when infected with Jaagsiekte sheep retrovirus (JSRV) and found that cells related to the immune system and the formation of cancer had altered gene expression, which affects the production of proteins important for cell function.
Dr David Griffiths of Moredun said: “Understanding the genes that are switched on or off during JSRV infection improves understanding Better understanding may lead to new diagnostic tests as to how the virus initiates cancer, which could lead to novel routes for combating the disease.
“Investigating ovine pulmonary adenocarchinoma also represents a valuable tool for studying some forms of human lung cancer, due to similarities in how the cancer cells are activated in the sheep and human diseases.
“Lung cancer is difficult to diagnose in its early stages in both sheep and humans and the discovery of common markers could lead to improved early detection of can