Rigorous measures needed for control of OPA

Ovine pulmonary adenocarcinoma (OPA) is an infective lung tumour caused by the Jaagsiekte virus. It can be difficult to diagnose and has no treatment, making it a challenge to control once it takes hold within a flock.

The virus is mostly spread by secretions from the nose and mouth but can also be passed from ewes to lambs via milk and colostrum. It can survive in buildings for up to two months, so sheep can pick it up from their environment, especially feed troughs.

It usually takes between three to four years for an infected animal to show symptoms, by which time they are likely to have spread the disease to several others within the flock, making prevention vital.

sheep

Signs of OPA

OPA is usually introduced to a flock by an infected animal, either when buying in replacement ewes, or a new ram. It initially spreads slowly, so farmers may notice a gradual increase in ewe deaths or decreased scanning rates. 

Poor body condition scores (BCS), coughing and liquid coming from the nose and mouth are all possible signs, however these non-specific symptoms could be caused by other diseases and it is important to confirm the cause.     

There are no blood tests for OPA, and it is usually confirmed by post-mortem. It is also possible, however, to scan chests to detect the tumours.

Controlling OPA

Once OPA has been diagnosed, control is multifaceted. Initial control is, unfortunately, through targeted culling.

Culling decisions can be based on BCS, with any ewe over 4 years with a low BCS culled, or a vet can come and chest scan to identify developing tumours.

In addition to culling, there are preventative steps that can be taken to reduce spread of the virus within a flock.

Careful management during lambing when ewes are housed is essential. Separating thin ewes from the rest of the flock can reduce spread, as can thorough disinfection of troughs and the environment. Raising troughs higher from the ground can also help reduce the amount of secretions that build up.  

Often, ewes with OPA are killed by a secondary infection, such as bacterial pneumonia, rather than the tumour itself. Vaccinating against Pasteurella can therefore extend the life of the ewes.

Early detection

AHDB estimate that OPA is present in up to 5.6% of UK flocks, however in reality this could be higher. Often the cause behind an unexpected ewe death is not diagnosed.

With no blood test or accreditation scheme for OPA, it is impossible to guarantee that it is not going to be introduced to a flock when bringing in new stock. 

It is vital to keep thorough flock records, including mortalities, BCS and scanning percentage rates. This can help identify gradual changes, such as increases in mortality rates. If the data shows that something is not quite right, carrying out post-mortems on cull ewes can be a cost-effective way of checking for underlying issues.

Some farms opt for cull ewe screening, which gives the opportunity to test for diseases like Johne‚Äôs, maedi visna (MV), CLA, borders disease and OPA, which all have non-specific symptoms making them difficult to diagnose.  

A recent case of OPA on farm

I recently managed a significant outbreak which demonstrated the impact of this tricky to manage disease. The farm has 800 north county mules, grazed on permanent pasture on the South Downs.  

The farmer initially contacted Westpoint Farm Vets with concerns about a high mortality rate during winter and lambing time.

  • Between December 2018 and May 2019, they lost 80 ewes; 12.5% of the flock
  • Ewes had been in good condition at tupping, but by lambing 2019 they were generally thin despite being fed a generous concentrate ration. Some had a BCS at 1.5, well below their target score of 3
  • In December 2018, the flock scanned at 130%; 180% or above is the usual target for mules

As the ewes had poor body condition and the problems had occurred over winter, our first step was to rule out liver fluke. Once the blood and dung samples confirmed liver fluke was not the issue, we investigated other options.

Some of the ewes had snotty noses and there was a noticeable cough within the flock, which implied either bacterial pneumonia, MV or OPA. As there were no specific symptoms, we decided to euthanise a particularly poor looking ewe and do a post-mortem.

We found large areas of grey discolouration over her lungs, which had the appearance of an OPA tumour, and this was later confirmed by tissue testing.

After discussion with the farmer, we decided to cull based on BCS and age. Approximately 300 ewes were culled in 2019. Thin ewes were also separated at lambing to reduce spread of the virus.

The control measures have proven to be successful, as the flock achieved a scanning percentage of 171% in December 2019 and a much-reduced ewe mortality rate, a massive improvement compared to the previous year. However, we cannot be complacent, as it takes years of robust control measures to fully eradicate OPA from a flock.

Written by Emily Francis BVM&s MRCVS BSc. Article first published in NSA Sheep Farmer.