How a sheep farmer cut OPA disease cases by two-thirds

A combination of culling, ultrasound scanning and tighter biosecurity has cut cases of ovine pulmonary adenocarcinoma (OPA) by two-thirds on a Monmouthshire sheep farm.

From a high of 11.6%, prevalence in the flock of 550 Texel cross Lleyns and Aberfields at Court Farm, Llanthony, near Abergavenny, is now at 4%.

OPA was diagnosed in the flock in 2021 after several ewes showed clinical signs of the disease. They were thin, breathing heavily and had excessive nasal discharge.

See also: How a Welsh farm reduced ewe mortality by tackling OPA

There had also been a 30% reduction in the flock’s pregnancy scanning rate, from 170% before the disease was detected, to 140%.

A post-mortem on one of the animals identified extensive OPA lesions – the first time the disease had been diagnosed in the flock.

Bryony Gittins, who farms in partnership with her father, Colin Passmore, embarked on a Farming Connect focus site project to tackle the disease.

Thoracic ultrasound scanning was used to establish the true incidence of OPA within the flock and to limit the risk of it being reintroduced after culling affected ewes.

© Debbie James

Scan and cull policy

In February 2021, ultrasound scanning of all ewes more than two years old detected a 3.7% incidence among 545 animals. Six months later, 457 were scanned and the incidence level had increased to 11.6%.

Mrs Gittins has since adopted a scan and cull policy. Any animals that test positive are culled, or, if they are pregnant, they are marked and lamb separately from the main flock.

They are then sold with the progeny as soon as the lambs have hit slaughter weights of 41-45kg.

A further scan in February 2022 showed the infection rate had fallen to 4%. Twenty-three animals were identified with OPA; of those, four had clinical signs and 19 lesions on their lungs.

Biosecurity

Action has also been taken to improve flock biosecurity. Measures include investing in improvements to boundary fences to prevent nose-to-nose contact between the sheep at Court Farm and those grazing neighbouring land.

Pregnant ewes are being kept at grass a month later than usual, until the end of January. This is because there is greater likelihood of OPA spreading when animals are housed.

Housing also increases the risk of ewes with underlying OPA succumbing to secondary illnesses such as pneumonia.

This risk arises from the stress of switching from a grass-based diet in an open-air environment to being inside and eating silage.

“When the sheep are outdoors, they have a lot more space and they are not sharing a water tank because they have access to a stream,” Mrs Gittins explains.

She had been giving trace element boluses to the flock once a year, as well as offering powdered minerals. Now, she gives them boluses twice a year to remove any risk of OPA being spread through the feeding of minerals.

Body condition scoring

Body condition scoring is also being carried out more frequently. Thin sheep are culled: even without an OPA diagnosis, a thin sheep will be less productive, Mrs Gittins points out.

Her next step is to invest in software to record lambs born from ewes with OPA. These ewes will pass the disease to their lambs as they are in such close proximity.

The lambs will be sold fat or as stores, rather than being kept as flock replacements.

“Lambs don’t show clinical signs of OPA until they are two years or older and they don’t pass it on, so there is no issue with selling them as stores,” says Mrs Gittins.

Options for ongoing control

She is working towards a position where the infection rate is stable, and she can rebuild numbers.

Another option would be to cull the entire flock and to restock with “clean” animals. But there is no guarantee those would not be infected with OPA or other diseases.

Many flocks may be harbouring OPA without their owners knowing it. This means the scale of OPA in the UK sheep flock could be much higher than is being reported.

“It is likely there are greater numbers of flocks with iceberg diseases that farmers are just not aware of,” says Mrs Gittins. “Without looking for these and without careful monitoring you just don’t know they are there.

“Building a good relationship with the farm vet can help with diagnosis.”

Culling the whole flock would be costly: at a cull value of £90 a ewe, the sale of 550 animals would raise £49,500, compared with the £93,500 cost of buying in replacements at £170 a head.

Bryony Gittins © Debbie James

However, some of the £44,000 deficit would be recouped: Mrs Gittins calculates that increasing the scanning rate from 145% to 170% would produce 138 additional lambs. These could generate an income of about £9,660/year.

By retaining the flock, there is also an ongoing annual lung-scanning cost of about £2 a ewe, and a higher culling rate.

But Mrs Gittins says she is not willing to take the risk of buying in unless she can be sure that she won’t find herself in a similar position with a new flock.

Restocking remains an option for the future. “I will always keep it at the back of my mind. If there is a flock dispersal where I could get a similar breed from a single farm I might think ‘let’s go with this’.”

Working with Farming Connect on this project has been a valuable experience, she adds.

“It has enabled me to gather knowledge and the information to make my decisions going forward on the future of this flock at Court Farm.”