Interpretation of TB Skin Test results
Interpretation of the TB test looks at the reaction at each injection site and compares the reaction at the two sites. Interpretation can be altered depending on the level of TB in the area, the disease situation on farm, supplementary test results (such as IFNG) and other disease risk factors. Guidelines have been developed over many years from experimental work in laboratories and millions of actual tests on farms. Using the readings obtained at the TB test, the vet performing the test classifies each animal as negative, inconclusive, or positive (reactor) to give the herd keeper an early
Testing Requirements for OTS/OTW Herds
My Herd has OTS Status
If TB is not confirmed* at post-mortem or laboratory examination, and there is only 1 skin positive animal in your herd, it may be possible to remove restrictions after only one clear herd test - an RH1 (if all other requirements have been fulfilled). This test must be carried out a minimum of 60 days following disease disclosure and isolation of the reactor animal.
If a positive TB result was first discovered at an individual animal test or at slaughter, rather than at a herd test, an initial RHT herd test may also be required (see RHT section of TB Breakdown Test Types page). If the herd restriction is due to disclosure of one or more Inconclusives, restrictions could be lifted after the suspect animals are tested clear, at least 42 days after the initial test.
After TB restrictions have been lifted, one further Check Herd Test (CHT) will be arranged for your herd 5 to 6 months after movement restrictions are lifted, to check that no infection remains and to make sure that any original cause of infection has not resulted in the infection of more animals in your herd.
My Herd has OTW Status
If TB has been confirmed* in your herd, or when more than 1 animal has been positive in your herd at the skin test, or more than 5 animals have been LRS at the abattoir, it may be possible to lift restrictions after two clear herd tests in a row (RH1 and RH2), involving every animal that is 42 days of age or over at the time of the test. These herd tests must be carried out a minimum of 60 days apart. We may apply a more severe interpretation than in routine tests to make sure reactors are found at an early stage helping your herd become free of infection as soon as possible. We may remove further high-risk animals from the herd. These may include animals that have previously been inconclusive, or animals considered at particularly high risk due to their presence in heavily infected batches of cattle.
If a positive TB result was first discovered at an individual animal test or at slaughter, rather than at a herd test, an initial RHT herd test may also be required (see RHT section of TB Breakdown Test Types page).
After TB restrictions have been lifted, 2 further Check Herd Tests (CH1 & CH2) will be arranged for your herd - CH1 will be 5 to 6 months after movement restrictions are lifted, and CH2 will be 5 to 6 months after CH1. This is to check that no infection remains and to make sure that any original cause of infection has not resulted in the infection of more animals in your herd.
*TB is confirmed in a herd when 2 tests (from skin test, post-mortem, histology, and bacteriology) are positive in one animal, or an animal has a bacteriology positive result for M. Bovis.