By Patsy Hunter

OVINE pulmonary adenocarcinoma (OPA) or Jaaksiekte remains a serious problem in many sheep flocks and while there is no vaccine or cure for this contagious, respiratory disease that causes significant losses in affected flocks, diagnostic tests are being developed.

Recent research from the Moredun Institute, Edinburgh, has seen scientists develop tests to diagnose the early stages OPA in order to enable test-and-cull in affected flocks.

As a result, three tests have been developed to combat this infectious and fatal lung disease, with the best of these at present being transthoracic ultrasound scanning which is proving useful to confirm a diagnosis of OPA in individual sheep or for screening bought-in, or for sale animals. But, even this test is not 100% as the Moredun cannot guarantee that a sheep is free of OPA as it unable to detect the very smallest tumours.

In the past year, whole flock screening of known OPA-affected flocks by Dr Phil Scott of Capital Veterinary Services, has found that on average 1-2% of sheep have OPA without clinical signs at the time of scanning.

Older and thinner sheep were more likely to be positive but it he also found OPA in gimmers and shearling rams and in sheep with good condition scores.

According to the Moredun report, generally the sensitivity and specificity of scanning for OPA is good, however, occasionally lung consolidation from bacterial or parasitic infection gives a false positive scan. The veterinary advice nevertheless would be that if there are very large lung lesions, such as abscesses, to cull anyway as these sheep will never do well.

Very small lesions found by ultrasound could also be false positives therefore the best course of action is to quarantine and re-scan in 3-6 months.

The benefits of whole flock scanning are that it enables OPA-affected sheep to be selected for culling at a time when they are still in good condition and of some value, thereby avoiding later knackery uplift costs. Culling these sheep early should also reduce spread of the virus within the flock.

Scanning is also useful to identify thin sheep that don’t have OPA so that other health issues may be addressed. The effectiveness of using ultrasound scanning as a basis for a test-and-cull policy will be determined over the next few years.

Other projects testing for OPA include cutting edge technology to identify potential biomarkers of the disease in the blood of infected animals. Scientists have examined gene activation in OPA tumours and found a number of cancer-associated markers are produced in increased amounts. The next stage will be to evaluate their potential usefulness for diagnosing OPA. While this work is at an early stage, it is hoped that it will result in a lab test for diagnosing OPA in individual animals in the early stages.

The other test revolves around a diagnostic assay based on nasal swab sampling of thin, cull ewes which has been developed in a pilot study with Biobest Laboratories Ltd. This test shows good potential as a flock test and would be particularly useful as part of a flock health assurance scheme where annual testing was undertaken to provide information on health status which could be used to inform management decisions to improve flock performance and would allow buyers to purchase low risk animals.

The swab test

The Moredun Research Institute and Biobest Laboratories' diagnostic test is based on taking nasal swabs from thin ewes and could be used to screen flocks for OPA.

The study which validated it was funded by the Agriculture and Horticulture Development Board (AHDB), Hybu Cig Cymru (HCC), Northern Ireland Agricultural Research and Development Council (AgriSearch) and Biobest Laboratories, with some support from the Animal and Plant Health Agency (APHA).

OPA (Jaagsiekte) is an infectious lung tumour caused by a virus that affects sheep. There is no treatment or vaccine and once clinical signs appear the disease is invariably fatal.

Affected flocks may lose as many as 20% of stock in the first few years that OPA is seen and the disease may continue to account for the loss of a few per cent of sheep every year for many years thereafter. It appears to be increasingly common throughout the UK and the ability to identify flocks carrying OPA is required, however currently there is no means to do this.

A test that allows producers to determine whether their flock is infected will help in two ways. Firstly, it would identify if the infection is present in flocks and and allow farmers to take control measures to minimise losses.

But, perhaps more importantly, the 70-90% of flocks that are estimated to be free of the disease would be able to buy replacement breeding stock from flocks deemed to have a low risk of being infected with OPA.

In a pilot study, the new test on nasal swabs from thin ewes was evaluated in a small number of flocks and was shown to detect the virus only in flocks that were affected. The virus was not detected in any of the flocks that were free of disease.

As a flock test based on sampling thin ewes, this could be used to identify low-risk flocks and facilitate an OPA assurance scheme.

Further validation of the test is planned, in addition to engagement with stakeholders to determine how a robust and cost-effective assurance scheme can be developed, said Moredun.

How it's spread

* The virus causing OPA can be spread through the air or by contact with respiratory secretions from infected animals and can also be passed to lambs through milk.

* The disease has a variable incubation period and animals infected with JSRV often appear healthy for a considerable length of time (years) and some never develop disease within their commercial lifespan.

* There is no reliable diagnostic test for JSRV infection in individual animals before the clinical signs of OPA appear, as neither antibodies to JSRV nor infected cells are present in reliably detectable amounts in the bloodstream of infected sheep.

* Infected animals may be able to transmit the virus even in the absence of any signs of disease. It is easily introduced into new flocks by the addition of infected animals which appear healthy.