Ovine Pulmanory Adenocarcinoma (OPA)

This heavily under-reported disease is caused by the Jaagsiekte sheep retrovirus (JSRV). The virus is highly contagious, spreading by the aerosol route from sheep to sheep but also through the colostrum and milk to lambs. Rob Howe tells us more.

The virus causes tumours which obliterate the normal lung function, and commonly secrete fluid which then impairs the remaining lung function, especially at the end stage.

It is generally considered a chronic wasting disease with progressive respiratory distress and unfortunately is invariably fatal. Following infection with virus, disease progression takes time, varying from months to years but most commonly the disease becomes obvious at 2-4 years of age.

OPA is actually quite common in the UK, though it often goes under the radar since few sheep deaths are investigated and ewes are commonly culled at the earlier stages of this slowly progressing disease i.e. weight loss stage before overt clinical signs present.

Diagnostic difficulties

Unfortunately there are no blood tests available since sheep do not produce any detectable antibodies or seroconvert to JSRV.

Clinical signs are the main way to watch out for OPA. Consider whether you have had sheep losing weight despite feeding well, unexplained pneumonia deaths or seen the classic fluid from the nostrils.

The classic test is known as the “wheelbarrow test” which involves the head of the sheep being lower down than the chest. In many late cases, abnormal fluid will come out of the nostrils. Post mortem confirmation with visible lesions and histopathology confirm the diagnosis.

Control

There is no cure, no vaccine and no serological antibody test which makes controlling the disease tougher than we would like, but it is achievable and the first step is noticing it when present.

Purchase of OPA-infected but clinically normal sheep is the major risk factor for bringing in OPA into clean flocks. But as ever be aware of boundary fencing as it can be passed ‘over the fence.’

Best practice is to identify and cull out sheep with any sign of the disease as early as possible. Also maintaining sheep in single age groups has been shown to be the most important management factor in reducing clinical disease once on farm.

Some flocks impose a strict policy to try to reduce their incidence of OPA by culling thin sheep, those that breath overly fast or cough. However, culling on the basis of weight loss and respiratory signs will mean a proportion of the sheep culled, will not in fact have OPA.

One alternative that is beginning to be used is vets ultrasound scanners. A quick test over the chest area has been shown to pick up cases earlier. This has the benefit of increasing cull value as well as decreasing transmission to non-infected sheep.

A negative scan cannot however provide a guarantee that the animal is free of JSRV infection nor that it doesn’t have early OPA.

If you are concerned that you may have seen cases or would like to discuss the possibility of scanning your flock, please get in touch.