The bovine Tb skin test used in Ireland is the Single Intradermal Comparative Tuberculin Test (SICCT).
The sensitivity of a test is the ability of a test to correctly identify an infected animal as positive. The higher the sensitivity of a test the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result). The SICCT Tb skin test used in Ireland has a sensitivity of approximately 80% i.e. it can, on average, detect around 80% of bTB-infected cattle. In some cases, this sensitivity figure can be higher, and in others, it can be lower. There are several factors which can affect this sensitivity figure:
- The skin test cannot detect very recently infected cattle, as their immune system has generally not had the time to respond to the bTB bacteria. This is called the pre-allergic phase. At least 6 weeks must have passed after an animal becomes infected with TB before the Tb skin test will detect the infection.
- The skin test can detect around 90% of cattle where the infection has advanced such that there would be visible bTB lesions. However, as the disease progresses further, an infected animal’s immune system becomes damaged and the Tb skin test becomes less likely to work.
The specificity of a test is the ability of a test to correctly identify as negative an animal that is free from infection. The higher the specificity the lower the probability of incorrectly classifying an uninfected animal as infected. The SICCT Tb skin test has an excellent test specificity of 99.98%, thus it is rare that a non infected animal will be classified as infected.
Good quality testing facilities and assistance on the day are essential in carrying out a TB test. As with all tests, failure to perform the test properly will result in the specificity and the sensitivity of the test being reduced. My Department has robust quality control systems to ensure high quality testing. These include quality control relating to the tuberculin used in the test; the only facility in Europe to measure the potency of tuberculin in naturally infected animals, lab based potency testing and supervision of the manufacturing facility.
Quality Control in relation to equipment includes the requirement to service equipment annually, approval and inspection of servicing providers and on-farm inspection of equipment. My Department also has robust quality control in relation to test performance; testing only being performed by veterinary practitioners, tests being subject to veterinary certification, advanced IT systems for reporting of tests, IT systems for measurement and comparison of test performance, risk-based targeting of test inspections by Veterinary Inspectors, inspection and blood testing of reactor animals.
The tests recognised by the European Commission for the purposes of TB testing which can be used here are:
- The single intradermal comparative skin test (SICTT) which is the test most widely used in an Irish context.
- The gamma interferon test (a blood test) which is used as an adjunct to the SICTT in known infected herds.
Another blood test, the Tb elisa test, is also used as a tool where there are intractable TB breakdowns to try to identify chronically infected individual animals which are still clinically normal but whose immune system is not capable of reacting to the tuberculin skin test.
My Department uses the above two blood tests in addition to the skin test as appropriate. My Department has been very actively involved over many years in efforts to develop and validate new and improved Tb tests, and we remain actively involved in such efforts.