Animal Health defends TB test

The Animal Health and Veterinary Laboratories Agency has defended its TB testing regime after a bull which had been condemned to slaughter proved clear of the disease in a retest.

The bull, Hallmark Boxster became the subject of a High Court battle in April 2010. Its owner, Ken Jackson of Walden Stubbs, South Yorkshire, argued veterinary workers had bungled the test by mixing test samples in the field.

A High Court judge ruled in favour of Mr Jackson, ordering a retest and the bull has now been given the all-clear.

But animal health officials have insisted that the original test met guidelines.

“We’re confident that the blood test was conducted correctly the first time around,” an Animal Health official said.

“The test is incredibly effective, with a 97% success rate, but there’s always a very small possibility of a false result. The test is so accurate that if an animal tests positive we must look to remove it to stop this devastating disease spreading,” the official said.

In a bid to answer queries about its TB testing procedure the agency has released a Q&A.

TB Test Q&A

Why would two tests give different results?

Research shows that the tuberculin skin and interferon-gamma tests can identify different populations of infected cattle.  The interferon-gamma test can identify infected animals at an earlier stage in the disease than the skin test, as well as infected cattle that simply fail to react to the skin test.  An animal failing either diagnostic test should be considered to be infected with the bacteria which causes bovine TB and the legislation requires that such animals be slaughtered.

Do you have confidence that Boxster is now free of disease?

No test can give absolute assurance of freedom from TB infection. However Boxster has now passed the full series of tests that were required to re-establish the animals’ TB status. Therefore he can be regarded as officially TB-free.

Why did DEFRA not agree to a re-test when the Jacksons first requested one?

It is Defra’s policy only to re-test a TB test reactor animal if there is evidence that a test has not been carried out properly and if the failure to follow field instructions would be likely to affect the validity of the test result, based on our expert scientific advice.  We were of the view that there was no such evidence.  The position was considered by the High Court, which took a different view.  Defra accepted the Court judgement and has worked with the owners to complete the testing requirements for this animal.

Why did Boxster fail the initial gamma test, yet has passed subsequent ones?

The judgement in the High Court set aside the original gamma test result due to irregularities in the sampling process.  The bull has therefore had only one valid  gamma test which he has now passed.

Why has so much time and money been spent on this case?

Bovine TB (bTB) is having a devastating effect on many farm businesses, so we must have strict measures to control it. In parts of the country like Yorkshire, where bTB is rare, it is vitally important to reduce the risk of new, intractable bTB hotspots becoming established. Therefore it is our duty to ensure that, so far as possible, any infection is eradicated.

How much has DEFRA had to pay to the Jacksons in legal costs?

The department has agreed with the Jacksons’ solicitors a contribution towards their legal costs and an interim payment has already been made. The remainder will be paid within the next 10 days.

Why was the interferon-gamma test used in the Jacksons’ herd?

Because the herd is located in a low TB incidence area and bovine Tb had been confirmed in an animal from it.  The GB policy is to use the interferon-gamma test in such herds, in combination with the skin test, to ensure as far as possible that no cases of infection are missed, and so to reduce the risk of TB getting a firm foothold in new parts of the country. 

Is the interferon-gamma blood test effective?

The interferon-gamma test has been approved by the World Organisation for Animal Health (OIE) and in EU legislation as an ancillary test to supplement the skin test in cattle. The interferon-gamma test is more sensitive than the skin test and identifies a slightly different population of infected animals. So maximum testing sensitivity is achieved when the two tests are used in combination on the same animals, and those animals reacting to either test are removed.

All research and field evidence shows that the interferon-gamma test is effective at disclosing infected animals missed by the skin test. The interferon-gamma test has been fully validated and is used across the world.