The absence of visible lesions in TB reactors does not indicate that the animals are not infected with the disease, Agriculture Minister Andrew Muir has said.

In response to a question from North Antrim MLA Timothy Gaston, Minister Muir disclosed data which shows that less than half of skin test reactors typically have lesions at slaughter.

For example, the most recent figures for 2024 show that of the 20,510 skin test reactors that were slaughtered in NI, 9,632 had visible lesions.

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Minister Muir said the presence of visible lesions is an indicator of “disease progression” rather than “disease confirmation”.

“The progression to visible TB lesions may take months or years to develop, and given the specificity of the test, most reactor animals with no visible lesions will actually be infected,” he said.

Minister Muir said the skin test has “a very high specificity” of 99.98%, which suggests only one in 5,000 animals that show a positive result will not be truly infected with TB.

He said specificity with the interferon gamma blood test is much lower at 96.55%, so there will be one false positive for every 30 animals that show a positive test result.

There are also less visible lesions seen at slaughter, with the latest figures showing only 28% of the 1,379 blood test positives in NI last year had visible lesions.

“This may be because animals are detected at an early stage of infection, or because slaughterhouse inspection has a low sensitivity. However, in a small proportion of cases this may be because the animals are false positives,” Minister Muir said.