OPA infection rife in Scottish sheep flocks

The contagious lung tumour disease which killed Dolly the transgenic sheep could be present in 38% of Scottish flocks, but most farmers still don’t know what it is or how to identify the symptoms.

Ovine Pulmonary Adenomatosis (OPA), or Jaagsiekte Disease is now more common than either Johnes or Maedi Visna, but scientists are not even close to finding a reliable blood test or establishing an accreditation scheme. And according to a lead researcher, the most popular control method is for farmers to cross their fingers and hope their flock doesn’t become infected.

Chris Cousens of the Moredun Research Institute told farmers at a Scotsheep seminar that OPA had been endemic in the UK for a long time and was always fatal, but in a recent survey of 125 Scottish sheep farmers, 83% said they didn’t know if they had ever had a case of the disease and 12% confirmed their flocks had been infected. Part of the problem is that it is easily confused with bacterial lung infections.

“We know that infection is widespread and common in Scotland and we need to boost awareness,” said Dr Cousens. “An infected sheep loses weight even if it is eating well. It has difficulty in keeping up with the rest of the flock when it is being moved, and you can see it puffing even when it’s not exercising.

A clear sign is the production of lung fluid which can pour out of the nose when the animal’s head is down. In a “wheelbarrow” test, when you hold up the back legs, as much as half a litre of fluid can pour out. But not all of those with the disease produce lung fluid.”

Dr Cousens said that farmers could lose as much as 20-25% of a flock to OPA, followed by steady losses of 1-5% in ensuing years. She said one farmer had been sending diseased sheep to the survey for 20 years.

Biosecurity is the best defence against the disease until a better blood test can be found. Dr Cousens explained that sheep do not make antibodies against the disease, which is how other diseases can be detected in blood samples. Until an effective test can be devised she advised culling any lung fluid producers, the offspring of ewes with OPA and thin or old sheep.

However, she revealed that “healthy” carriers of the disease could transmit the virus to other sheep, making it difficult to pinpoint the source of the disease. The lung fluid is a strong source of virus which can spread the disease across the flock. It can also be spread by aerosol, so sheep in confined spaces are at a higher risk, and it can be transmitted by the ewe to the foetus in utero or in her milk. The incubation period can be up to four years.

Dr Cousens added: “We recommend maintaining closed flocks, decontaminating shared transport and double fencing to prevent contact with neighbouring flocks.”