WHAT IS THE GAMMA BLOOD TEST?
The interferon-gamma test (IFN-γ or ‘gamma’) is a supplementary blood test used alongside the tuberculin skin test.
The two tests identify slightly different ‘sub populations’ of TB-infected animals; for example the gamma test can identify cattle at an earlier stage of infection, as well as infected animals that simply fail to respond to the skin test. Hence using both tests together increases the probability of detecting all infected animals in the herd. The sooner these animals leave your holding, the sooner they stop spreading TB bacteria. When does APHA use the gamma test? The gamma test is used as a supplementary test alongside the tuberculin skin test in specific TB breakdown herds. Since 1st April 2017, gamma testing is compulsory for TB breakdowns with lesions and/or culture positive animals in the High Risk Area (HRA) of England where any of the following three criteria are met;
• The most likely transmission route for the affected herd was contact with infected cattle and measures are in place to prevent further spread of disease from this source;
• The infected herd is located in a badger cull zone where at least two seasons of culling have taken place
• There is clear evidence that repeated skin testing of the herd has failed to resolve a TB breakdown In England and Wales the gamma test is repeated as long as standard interpretation skin test reactors and/or animals with visible lesions of TB at slaughter continue to be identifi ed in those herds.
Gamma testing practicalities
Gamma testing is usually completed before the first Short Interval Test (SIT) in new TB breakdowns. This may be at the Check Test in the case of a slaughterhouse reactor or pre-movement test failure. The blood sample is taken before the injections of tuberculin on day 1 of the test (TT1) or aft er the reading of the skin test on day 2 of the test (TT2). Animals under six months of age are automatically excluded from gamma testing as their immune systems are still developing and this can interfere with the test, leading to false positive results.
How accurate is the gamma test?
The gamma test is more sensitive than the skin test (~90% compared to ~50%), meaning it is less likely to ‘miss’ infected animals. It is also less specific than skin testing. Gamma tests will produce an average of 1 false positive result for every 30 uninfected cattle tested. The tuberculin skin test has a higher specificity of 99.98% which equates to one false positive per 5,000 disease-free animals tested. That is why the gamma test is, in most cases, only applied to lesion and/ or culture positive TB breakdown herds. In these herds the risk of taking out low numbers of false positives is outweighed by the need to identify and remove all infected cattle, and a test-positive animal is in any case more likely to be infected. A relatively small proportion of gamma test reactors show visible lesions at slaughter. This can occur for one of the following reasons;
• The animal was in the early stages of infection with M. bovis when it was detected by the test, at which point the TB lesions were too small or localised to be seen by the naked eye. This is particularly relevant for the gamma test, as it tends to identify animals earlier in the course of infection than the skin test and before any visible lesions have developed.
• The animal had visible lesions at post mortem inspection, but they were missed during a relatively quick post-mortem inspection at the slaughterhouse.
• The gamma test result was a genuine false-positive. As stated above, we can expect this to happen in approximately 3.5% of TB free animals
Furthermore, the gamma test is always used aft er the TB breakdown herd has been subjected to at least one round of skin testing. This means that, by the time the blood test is deployed, many of the worst affected animals (i.e. those with the most extensive lesions of TB) will have been removed from the herd.
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