Vet Emily Gascoigne considers Ovine Pulmonary Adenomatosis (OPA) virus in sheep flocks.

You may have heard this disease called Jaagsiekte, driving sickness or “the wheelbarrow disease” in reference to the huge volumes of fluid which can be produced from the lungs and the difficulty affected sheep display when subject to physical exertion such as being moved with a dog.

This virus is spread via aerosols from infected sheep and results in tumour growth in the lungs of sheep. The tumours infiltrate the cells which produce the fluids which naturally lubricate the lungs resulting in an over production. The tumours and accompanying fluid create a perfect environment for bacterial growth and affected animals either lose condition and fade due to reduced lung function, or succumb rapidly to secondary infections such as Pasteurella. The incubation period of the disease is highly variable but the youngest report to date published is a 12 month old animal. Typically animals are 2-4 years old and can include male and female. There is not necessarily a familial link - close contact with affected sheep is the main risk factor, so sharing a sheep shed or trough space or drench gun with an affected flock mate may be sufficient to cause infection.

There are limited estimates of how common this disease is in the UK but it was identified in 0.9% of animals in an abattoir survey. This is likely to be an underestimate as typically only fit and healthy animals make it to the abattoir, and in the last 6 years I have experienced three confirmed cases within our practice alone. There was no common breed affected. There is no cure for the disease and vaccination is not possible. In the most seriously affected flocks depopulation may need to be considered.

Not all flocks have OPA, but clues that a flock may have OPA include ewes too thin after weaning despite improved nutrition and no obvious evidence for poor thrift, i.e. teeth are in good order. Additionally adult sheep mortality over 2% should be investigated. This may be difficult to assess in small flocks (one dead ewe in a flock of ten is a significant percentage and not necessarily indicative of endemic disease). Subtle clues may include drops in performance, as infected adult sheep are thinner, reducing milk yield and consequently lamb growth rates. Lastly, increased coughing may be heard in flocks (although other diseases can cause this). Finally, on occasion, due to the risk of secondary infection with Pasteurella there may be no evidence of disease but instead animals are found dead.

One of the most challenging things about this condition is the difficulty in diagnosing it and preventing its introduction. At the moment, there is no commercially available blood test and the definitive way of diagnosing OPA is on post-mortem with tissues examined. This makes it very difficult for flocks wishing to avoid introducing this disease.

Veterinary surgeons can ultrasound scan lungs to identify lesions, but owners should be aware this has limitations as it is unlikely to identify small lesions which will subsequently develop into tumours, so may still miss tumours even in young animals screened at purchase. Scanning is being performed nationally in other breeds, but it is essential owners discuss this technique with their own vets before embarking on screening. Flocks should also consider screening of all fallen stock on farm for evidence of lesions. Speak to your vet about any post-mortem services they offer and your wish to screen adult fallen stock for evidence of OPA lesions. When purchasing animals, asking about flocks’ previous history of OPA may help give you confidence about their disease status. Ideally source replacements from flocks which are actively looking and have not found disease.

OPA potentially has serious implications for all UK flocks and there is no reason that rare breed flocks should be any different. The introduction of this disease into a rare breed flock, one with rare bloodlines or unique traits could be devastating for breeding programmes and preservation of genetics. Flock owners are advised to speak to their vets to discuss the implications and monitoring strategies for their own flocks.