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What is OPA and how can you eliminate it from your flock?

Eradicating OPA from a flock of sheep is not for the faint hearted. It needs determination, but fortunately that is a characteristic of the team at Balnabroich Farm, Strathardle.

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What is OPA and how can you eliminate it from your flock?

Flock owner Sir Michael Nairn and farm manager Alex Smith are both focused on reducing the instance of ovine pulmonary adenocarcinoma (OPA) in their flock to as near nil as possible.

 

Making sure they do not buy the virus in with breeding stock is seen as vital and their call to ram breeders in particular is to do more to screen for the disease.

 

Beyond this, the focus on this Perthshire hill farm is on regular scanning of the flock and culling out any sheep showing lesions.

 

With 600 Scotch Mule ewes and a similar number of Blackfaces to scan at the rate of 70-80 per hour, this is a chunky task for Kirriemuir vet Ed Hill. A partner in Thrums Veterinary Group, he has taken a particular interest in this mission to root out OPA.

 

Using an £8,000 scanner, he has to inspect both sides of each sheep and interpret what to the untrained eye is nothing more than a blurred image on the screen.

 

The telltale signs of a tumour can, however, be detected, despite as often as not the sheep showing no clinical signs of the disease. The policy is to cull any positives.


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The task is physically and mentally demanding but has been made easier thanks to an invention by Balnabroich shepherd Lynsey Marshall.

 

Realising the job could be sped up considerably if the vet did not have to move from one side of the animal to the other, she designed a conversion for an old weigh crate.

 

By mounting a central hub on the base of the crate supported by a stub axle set into concrete, she has created a holding device which can be easily spun through 180 degrees, allowing Mr Hill to scan first one side of the ewe and then the other without him having to move. A novel head yoke meanwhile keeps the sheep steady.

 

Proving the old adage the simplest ideas are often the best, Lynsey’s new scanning crate worked perfectly on its first outing. The cost of scanning varies considerably depending on numbers handled, but can be put at £1-£3 per head.

 

Mr Hill says: “There is a benefit over cost where incidences of OPA run at more than 2 per cent a year. Even where farmers are not scanning whole flocks, I would like to see more screening of batches of cull ewes to see how many of these older animals have lung tumours.

 

"This would give some idea of the level of infection. The only alternative is post-mortem examination.”

What is OPA?

 

Ovine pulmonary adenocarcinoma (OPA, also known as Jaagsiekte) is an infectious and fatal lung disease in sheep. It is caused by a virus which infects cells in the lung making them form tumours.

 

The tumour cells then produce more of the virus which can infect new areas of the lung or other sheep.

 

The disease signs associated with OPA are loss of condition, difficulty breathing and, in about half of cases, production of clear or frothy fluid from the affected lungs appearing as discharge dripping or pouring from the nose.

 

The sheep may survive for many weeks after the signs of disease appear or may die suddenly. In affected flocks, OPA may be the cause of death of 1-20 per cent of the flock in one year.

 

The early stages of OPA are not apparent as the tumours are too small to cause any breathing problems, even though they are able to produce virus which can infect other sheep.

 

(Source: Moredun Research Institute)

The Balnabroich experience

 

OPA was first suspected as a problem at Balnabroich in 2014 with Mr Hills’ colleague, Ian Gill, calling the Moredun Research Institute for advice.

 

In 2015, 330 Blackface ewes were scanned. This has moved on with most of the Blackfaces going through the process once a year since 2015 and the cross ewe stock twice a year since October 2017.

 

The first step in tackling an OPA infection is acknowledging its presence, believes Mr Smith. He is critical of those ram breeders who prefer not to think they have problem. This applies to all breeds and, whenever possible, he buys from flocks which scan before sale.

 

He says: “We bought a Blackface ram in 2015 out of a pen of 15. Within three weeks it was dead as the result of fighting but we had a post mortem carried out anyway and discovered that, despite looking healthy, it had a massive tumour.

 

“There is a good chance others in the pen were infected too, so the disease was effectively spread across the country. This applies to all breeds and my suggestion is tups are scanned before sales.”

 

Sir Michael says: “My view was as soon as we knew we had the disease we had to do something about it. The Moredun Research Institute has been incredibly helpful to us. OPA infected sheep are not always easy to spot but they tend not to perform well and do not cope with stress.

 

"As a result of the Beast from the East, we lost too many ewes. Of the 11 we had tested post-mortem, eight had OPA tumours. It is clear evidence of the problem.”

 

Other measures

 

Scanning and culling is a central part of the OPA strategy at Balnabroich but other measures are under consideration.

 

Mr Smith says: “We breed our own Scotch Mules from our older Blackface ewes but we may stop doing this for a few years and possibly buy in Cheviot Mules. This would break a possible chain of disease transmission in to the cross-bred flock.”

 

There are also plans to stop using block or bucket feeders. The thinking is ill sheep tend to stand beside the blocks as they are not grazing well. Mucus comes out of their nose and lands on the block, possibly infecting the next sheep which come along.

 

Mr Smith said: “The same applies to water troughs in sheds, so maybe we will move to outside lambing. If we could get to less than 2 per cent OPA infection consistently, that would be fantastic.

 

There is still the problem of finding tups but we just need to learn to walk away from pens which have not been tested.”

 

Sir Michael says: “We now have the trend moving in the right direction but there is a lot of work ahead of us.”

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