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Bovine Tuberculosis

Answer

The performance of the single intradermal comparative cervical tuberculin (SICCT) test under GB field conditions is continuously monitored using a number of TB epidemiological parameters. Quoted sensitivity and specificity ranges for diagnostic tests are established from research studies and published in the scientific literature. For bovine TB, the most sensitive tests for diagnosis of infection depend on the identification of the immune response mounted by an animal in response to being exposed to the causative organism, as detected through tuberculin skin testing or gamma interferon blood testing. Historically, between 30 and 40 per cent. of SICCT test reactors have demonstrable evidence of infection when slaughtered. The primary purpose of post mortem and bacteriological culture of test reactors is not to establish the presence or absence of disease, but rather to identify the severity and strain of infection—this is useful for epidemiological purposes and supports subsequent disease control efforts. TB skin test reactors that fail to disclose disease in the slaughterhouse or laboratory should not be considered as being uninfected. In essence, post mortem examination and microbiological culture are not the gold standards for diagnosis of M. bovis infections as both methods are less sensitive than immunological tests such as the SICCT test. Moreover, these tests are used in different cattle populations. The SICCT is primarily used as a screening test for cattle showing no clinical signs of disease, whereas post mortem and culture are used as tests on a subpopulation of cattle selected on the basis of positive results to a screening test. The different nature of these populations will affect the performance of any tests used on them. A direct comparison of the performance of tests that ignores these differences in populations is, therefore, not valid.

Secondary information

Type

Written question

Reference

5426; 682 c35-6WA

Session

2005-06

Related items

Bovine Tuberculosis
Wednesday, 22 March 2006
Written questions
House of Lords

Subjects

Monitoring Standards Testing Bovine tuberculosis

Link

View this Written question on www.publications.parliament.uk