Question
To ask the Secretary of State for Environment, Food and Rural Affairs what percentage of cows receiving positive (a) gamma interferon blood tests and (b) skin tests for bovine tuberculosis were subsequently demonstrated to be clear of the disease at post mortem in each of the last five years.
Answer
It is a misconception that failure to find post-mortem evidence of bovine TB in animals that have previously had a positive reaction to a TB test means such animals are clear of the disease. It is frequently not possible to see typical signs of disease with the naked eye, or to culture the bacterium from tissue samples. Diagnostic tests are more effective at identifying infected cattle than post mortem analyses. The primary purpose of post mortem analyses is to support epidemiological investigations and the management of TB outbreaks, not to validate ante-mortem tests.An important point to note is that the TB testing programme aims to identify and confirm bovine TB on a herd rather than an individual animal basis. Meaningful "confirmation" rates for gamma interferon reactors, in particular, cannot be provided, as a significant proportion of gamma interferon positive animals are not subject to laboratory culture, as infection has already been identified in the herd.