Cases of bovine TB that sometimes appear in a low risk area like ours are unwelcome and worrying, but in a way, not unexpected. This is because the standard test used can never be 100 per cent accurate on all occasions.

In fact, this applies to all tests for disease and vets and doctors will talk in terms of false positives (those that react to the test, but don’t have the disease) and false negatives (those that have the disease but do not react to the test).

The skin test for TB gives very few false positives, meaning any animal that is declared to be a reactor probably has TB. The test is good at detecting TB at a relatively early stage – often before there are any obvious swellings or abscesses in the body. This is why some cows that react can have no obvious signs of disease at post-mortem.

The test is less good at demonstrating that animals don’t have TB and a figure of 1 in 5 false negatives is often stated. To this there is an added complication that how accurate the test is will depend upon how much disease is present on the farm, or in the area.

The difficulties of the test mean that it is not particularly useful for individual animals, but quite effective for herds. Even so, infected cattle can still be missed, meaning cases pop up occasionally. It has to be repeated that TB is mostly carried onto a farm either by cattle or by badgers (the principle wildlife reservoir). We are fortunate in this region that as yet, we have no disease in wildlife (badgers and deer from around reactor farms are tested).

The best protection against infection is currently to not buy cattle from the high risk and edge areas. Measures to prevent cattle and badger contacts such as raised water troughs and doors on feed sheds are also sensible precautions.

There is always an anxious time for the farmers whose herds contract TB . They will hope that their’s is an isolated case and is cleared up rapidly.