ULTRASOUND SCREENING on-farm is assisting early diagnosis of Ovine Pulmonary Adenocarcinoma in sheep, according to Moredun research scientists.

OPA is a cancer caused by the Jaagsiekte Sheep Retrovirus (JSRV) which has led to the death of 20% of adult sheep in OPA-affected flocks every year.

The virus causes cells in the lung to develop into tumours which eventually take over a large proportion of the lung so that the sheep is unable to breathe properly.

The OPA tumour cells act as ‘virus factories’ producing new JSRV virus which can infect new areas of the lung or be transmitted to other sheep, by air, milk or colostrum.

Sheep with OPA suffer from breathing problems, weight loss and may produce clear or frothy fluid from their lungs which is highly contagious to other sheep.

However, these clinical signs often present themselves too late, by which point the sheep may only have days or weeks to live. Over the past few years, scientists at Moredun have been looking at ways to diagnose OPA at a pre-clinical stage and have found that whole flock ultrasound screening at an early point has resulted in a steady reduction in the number of cases of OPA seen each year.

Since 2015, Moredun’s lead scientist on OPA, Chris Cousens, has been working with farms carrying out ultrasound screening every six to 12 months, removing any sheep diagnosed with OPA.

“There is a lot of talk in the sheep industry about OPA at the moment and there is something we can do about it now. So far, with our test and cull approach, we have been able to detect tumours when they are much smaller and before a sheep loses confirmation. This has also led to a reduction in transmission of the virus as we believe at the earlier stage the sheep is less contagious.

“Usually more than half the lung can be affected by tumours before you see respiratory problems and by this point the sheep is likely going to die,” he continued. “With this pre-clinical approach, we are seeing cases dropping but it is important to stress to farmers that the screening should be interpreted correctly as some scans will miss small tumours. However, by scanning the whole flock and taking out positives year on year, you will increasingly reduce your risk of contracting the virus.

Ms Cousens went on to explain that his research has found that in some cases tumours can be rapid in growth which counters previous theories that OPA develops slowly over many years.

She also made the point that farmers must scan their whole flock not just those who are losing weight or going for sale, stressing: “This is a long-term business of getting the number of positives in your flock down. One test is a waste of time, as is just testing sale animals.”

Ms Cousens concluded by warning that despite these successes it is important to remember the test’s limitations: “Ultrasound can reliably detect OPA tumours of 2cm or more at the ventral surfaces of the lungs, but it can never provide a guarantee that a sheep is free of OPA as scanning cannot detect the very smallest tumours nor sheep that are infected with JSRV but do not (yet) have tumours.”