An evaluation of interferon-gamma (IFN-g) testing for bovine TB in Northern Ireland

Current European legislation (EU Directive 64/432/EEC (as amended)) requires that the on-farm diagnosis of bovine tuberculosis (TB) is done using the comparative ‘skin test.’ Almost all animals that test positive to the skin test are infected. However, it does sometimes fail to detect infected animals. To help address this, European legislation allows Member States and regions to use a supplementary blood test, the interferon-gamma test (IFN-g test).

Benefits of the IFN-g test

Compared to the skin test, the IFN-g test is able to detect more infected cattle but a larger proportion of blood test positive animals are later found not to be infected (false positives). This, together with various practical and cost constraints, means that the test can only be applied in a targeted way.

The purpose of this project is to evaluate the blood test as it is currently applied in NI and compare its application within NI to its international use. The project will include an evaluation of factors that might influence test result such as animal age, sex and breed; and farm size, and history of bovine TB. A part of this will be an assessment of the disease outcome of test positive animals which were not slaughtered shortly after testing positive and the effect of these cattle remaining on farms on farm TB disease status in future. The project started in May 2011.

The nature of the blood test result means that the criteria for defining when a test is positive or not can be varied. By doing this it can be made to detect more infected cattle or to reduce the number of false positive results. An important part of this project will be to assess the effects of varying the definition of a positive test result on test accuracy.

It is hoped that the overall outcome of the project will be a series of recommendations on the best use of the IFN-g test to aid the control and eradication of bovine TB in cattle here.

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