ARCHIVE: Bovine TB: The tuberculin skin test

The primary screening test for TB in cattle in Great Britain is the single intradermal comparative cervical tuberculin (SICCT) test, which is commonly known as the tuberculin “skin test”. The tuberculin skin test, which is used throughout the world to screen cattle, other animals and people for TB, is the internationally accepted standard for detection of infection with Mycobacterium bovis (M. bovis), and is considered the best test currently available for detecting TB in live animals. Many countries have eradicated bTB through the systematic application of the tuberculin skin test alone and the slaughter of all test reactors.

The test involves injecting a small amount of tuberculin (a sterile antigenic extract obtained from a culture of M. bovis or other organisms of the same family of bacteria) into the skin of the animal. In most cattle infected with M. bovis, this will cause the animal's immune system to react to the tuberculin and cause a localised allergic reaction (swelling) of the skin a few days after the injection.

Cattle are sometimes infected with other types of mycobacteria which may cause the animal to react to the test. So that we can distinguish between animals infected with M. bovis and those infected by other mycobacteria, we also inject the animal with tuberculin produced from Mycobacterium avium, an organism that can cause TB in birds. The size and nature of the reactions to both tuberculins (‘avian’ and ‘bovine’) is compared (hence the term single intradermal comparative cervical tuberculin test) to determine whether the test result is considered positive, negative or inconclusive.

The Veterinary Laboratories Agency (VLA) has produced a report analysing the comparative field performance of tuberculins (PDF 164 KB) produced by VLA Weybridge and by Dutch manufacturer ID Lelystad between 1 January 2005 and 31 March 2007.

Further information on the diagnostic accuracy of the skin test can be found in Defra’s online booklet ‘Dealing With TB In Your Herd’ (PDF 643 KB).

How the test is carried out

Testing is carried out by government approved TB testers and supervised by government-approved veterinarians. The six basic steps are:

The tuberculin skin-test

1. The animal is identified (by its ear tag) and its identification recorded.

2. Two injection sites are selected in the middle third of the side of the neck, one above the other, separated by about 130mm. (if it is a small animal, the two sites will be on either side of the neck.)

3. Hair is clipped around the sites to a radius of about 2 centimetres.

4. A fold of skin at both sites is measured with calipers and the measurements recorded.

5. Tuberculin is injected into the skin; the upper site is used for the avian tuberculin (or the left hand side on small calves).

6. After 72 hours, the tester returns, confirms the animal identity, measures the same fold of skin at both sites and records the thickness of the skin fold.

All cattle herds in GB, except some beef fattening units, are regularly tested for bTB. Statutory compulsory skin testing of cattle herds at the Government’s expense takes place every one to four years according to the proportion of herds in a specific area (animal health parish) that have suffered a confirmed bTB breakdown over the previous 2, 4 or 6 years. A review of TB parish testing frequencies across GB is carried out by Animal Health (formerly SVS) on an annual basis. Ad hoc adjustments can take place locally for individual parishes in response to an emerging TB situation. Furthermore, individual herds situated in 2-, 3- and 4-yearly testing areas undergo annual skin testing if they are considered to represent a high public or animal health risk (e.g. producers of raw cows’ drinking milk in England and Wales, suppliers of milk for the production of unpasteurised cheese, dealers’ herds, bull hirers and regular purchasers of Irish cattle).

On 27 March, new statutory measures for the pre-movement testing of cattle were introduced in England to help reduce the risk of spreading bTB between herds in high-risk areas and to herds in areas previously free from the disease. Wales introduced pre-movement testing on 2 May 2006 and Scotland introduced both pre- and post- movement testing on 23 September 2005. Please see our pre-movement testing pages for further information.

This programme of regular on-farm skin testing is supplemented with the post-mortem inspection of all cattle processed at slaughterhouses, carried out by the Meat Hygiene Service. Animals found to have with suspect bTB lesions at routine slaughter are immediately reported to Animal Health and back-traced to the herd of origin, which may then be subjected to tuberculin check testing.

Interpreting the results

Usually, a 'standard interpretation' of the skin test results is applied. If the reaction to bovine tuberculin is more than 4mm greater than the reaction to avianthe animal is considered to be infected with bTB and is called a “reactor”.

If the reaction to bovine tuberculin is between 1 to 4mm greater than the reaction to avian tuberculin, the animal is considered an 'inconclusive reactor' and will be retested after 60 days. Inconclusive reactors may also be tested after a 60-day interval as part of a herd test (if the herd is already under TB restrictions) or subject to a gamma interferon blood test (if the herd is in a 1 or 2-year testing parish and the animal remains an inconclusive reactor after the first re-test.

If evidence of bTB can be found in skin test reactor by post mortem examination and/or laboratory culture of tissues, the results of other cattle included in the herd test that disclosed the infected animal are re-assessed using the so-called “severe interpretation”. This involves lowering the cut-off for animals to be classed as reactors so as to enhance the sensitivity of the test. As a result, further animals may be classed as reactors or inconclusive reactors. Severe interpretation is also used in the follow up tests carried out at 60-day intervals before TB restrictions can be lifted from infected cattle herds. Further information on testing procedures for TB-restricted herds can be found online in Defra’s online booklet ‘Dealing With TB In Your Herd’ (PDF 643 KB).

What happens to reactors?

Reactors are isolated from the rest of the herd, valued and slaughtered. The farmer is compensated for the market value of the animal based on current table valuations. The herd loses its Officially Tuberculosis-Free (OTF) status and herd movement restrictions are applied. This is a herd incident, commonly referred to as a 'herd breakdown'.

Herd movement restrictions can only be lifted and OTF status restored after all the animals have passed two consecutive skin tests 60 days apart. Only one such test is required where bTB is not confirmed by post-mortem examination or in laboratory tests.

Page last modified: 12 December 2008