Questions over reliability of TB culling tests

Chris Benfield reports on worrying figures about the number of animals slaughtered following positive bovine TB tests where post-mortem tests have raised doubts.

Only about one in three of the farm animals slaughtered in order to contain TB in cattle over the past few years has been confirmed as infected after death, a Yorkshire Post inquiry has discovered.

In most cases, there has been no post-mortem examination, despite scientific advice to the Department of Environment Food and Rural Affairs to prove the reliability of its tests.

The checks that have been made have produced nearly as many inconclusive results as confirmations.

Defra says failure to confirm bovine TB post-mortem does not mean it was not present. It is not easy to detect in its early stages.

But the figures raise a question mark over Government assurances that their tests are more or less beyond doubt and will fuel the discontent of farmers like Ken Jackson of Stubbs Walden, near Doncaster, who is still fighting for the life of his best-ever British Blonde bull, which was condemned on the basis of a test result he doubted.

Bovine TB is rare in Yorkshire. Mr Jackson believes one of his heifers was infected when he took her to a market and brought her back unsold. In a routine inspection of his farm in March this year, the heifer was the only animal to fail the skin test. Six, including the bull, Boxster, then failed the blood test – very narrowly, in the case of Boxster. But no confirmatory lesions were then found in five more animals sent for slaughter.

The family wanted a second opinion on the bull but could not get one without Defra approval. Then they discovered Defra's laboratory tests had relied on blood from more than one sample, because of problems with the samples. Defra says that made no material difference but the Jacksons are challenging the order to kill their bull on procedural grounds. Defra has refused to give in because it says it must eliminate all animals which test positive once, to be on the safe side.

The National Farmers Union supported Mr Jackson through an argument about the technicalities of test procedure in his particular case but eventually gave up in the face of Defra intransigence, leaving him to take it to court at his own expense.

The NFU broadly agrees there is no alternative to the Defra policy. But some farmers are questioning the whole direction of it. They say the debate has got stuck on whether or not culling badgers will help the bTB problem when there are other big questions.

We asked Defra for the numbers summed up in the chart after a report on Ken Jackson's problems brought calls from other farmers who believed they had lost valuable animals for no good reason.

The figures for the first six months of 2010 show that out of 16,850 condemned cattle, only 6,127 (36 per cent) were confirmed as infected, 3,466 (21 percent) were inconclusive cases and 6,994 (42 per cent) were "not sampled". Results were still awaited in 263 cases when the figures were compiled.

Over the whole of 2009, 2,004 out of 36,529 (33.5 per cent) were confirmed cases, 8,234 were unconfirmed, 16,001 were not sampled and 20 cases were unresolved. In previous years, more cattle were tested but that only added to the tally of inconclusive results. The numbers confirmed were still no better than around one in three.

An NFU source told the Yorkshire Post: "Nobody says all the others are false positives but anecdotal evidence suggests something between 10 and 15 per cent might be. At one point, Defra was claiming 99.9 per cent accuracy and there is a hell of a difference."

There are two tests approved by the European Commission for bTB. The first is the so-called "skin test", which involves measuring the swelling caused by a subcutaneous injection of sterilised TB bacteria. If the animal has been made sensitive by previous exposure to the bacteria, this might show in its reaction. It works well enough to be useful for routine screening of whole herds for any sign of exposure. But it can miss individual cases and it quite easily gets false positives.

A positive skin test in a herd leads to blood tests – samples taken by syringe to inspect in the laboratory, for traces of immune system reaction to TB. It gives fewer false negatives but even more false positives. A report for Defra in 2007, by the Independent Scientific Group on Cattle TB, recommended using both tests but said field results for the blood test should be monitored. Performance could vary according to climate, type of cattle or strain of TB.

Defra pronouncements on the tests in the past have amounted to an assurance that the results are as near 99 per cent reliable as makes no difference. It won an important test case on this basis. But the figures from actual UK experience show that, in effect, nobody has any idea how good the Defra system is.

After providing its spectacularly inconclusive figures, Defra told us: "In the early stages of the disease it is not always possible to observe lesions during post-mortem examination and, due to the fastidious nature of this organism, it is very difficult to isolate it from tissue samples without lesions. The primary purpose of post-mortem inspections and culture is not to establish the presence or absence of disease, but rather to identify the severity and strain of infection."

We then asked: "If confirmation is difficult, how do we know the tests are as reliable as we are told they are?" Defra said: "They have been evaluated in properly designed field trials." But critics dispute this and Yorkshire Post inquiries have failed to elicit a fuller explanation.

Defra appears to have set aside any doubts while it waits for the development of tests which could differentiate between TB-infected and TB-vaccinated cattle. This would theoretically make it possible to vaccinate and also keep exporting live animals. At the moment, vaccination muddies the waters by making it hard to distinguish between vaccinated animals and those which have been exposed to TB. We could get dispensation to vaccinate but it would mean losing the right to export live cattle to the rest of the EU.

However, there is an argument that losing the live export business would cost less than trying to contain the problem without vaccines – currently 100m a year for testing and culling cattle and compensating farmers.

David Torgerson, a specialist in human health economics at York University, and his brother Paul, a veterinary professor in Zurich, published a paper last year titled Public Health and Bovine Tuberculosis: What's all the fuss about?

It said the case for trying to eliminate bTB was based on out-of-date fears of transmission to humans. The risk was more or less eliminated by universal pasteurisation of milk and the government is wasting 100m a year which would be better spent on worse public health threats.

Paul Torgerson told the Yorkshire Post: "As far as I can see, Defra has never done a full cost-benefit analysis of its policy. And there is now a big vested interest in the system. Vets, for example, get quite a lot of income from it."

The regional NFU has set up two meetings to feed into a government consultation on TB policy – at The George, Piercebridge, County Durham, DL2 3SW, on Thurs. Nov. 11; and at The Huntsman Inn, Greenfield Road, Holmfirth, HD9 3XF on Thurs. Nov. 18. Both start at 7.30 pm. More from Adam Bedford on 01904 451550.

CW 6/11/10