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Dept for Environment Food and Rural Affairs

Answer

Andrew George: To ask the Secretary of State for Environment, Food and Rural Affairs what assessment she has made of the effect of repeated intradermal skin tests on the ability of the test to detect bovine TB infection in individual cattle. [172530] Mr. Bradshaw: There is often a trade-off between test sensitivity and specificity. The skin test used in GB has been fine tuned to optimise specificity (i.e. keep false positives to a minimum) while retaining a good sensitivity (i.e. probability of identifying infected herds). However, because of the dynamics of Mycobacterium bovis (M. bovis--the causative organism for bovine tuberculosis) transmission and the time it takes to mount a detectable immune response to the intradermal injection of tuberculin, the skin test cannot be relied upon to detect every animal infected with M. bovis at the time of the disclosing test. This phenomenon is not unique to the tuberculin test and, in fact, this is quite normal for any immunological test. For these reasons, it is well accepted that the tuberculin tests are more suited to the detection of infected herds rather than the detection of infected individual cattle, i.e. a small proportion of infected animals in reactor herds may escape detection when the herd is first tested (false negatives). In order to increase the sensitivity of the skin test and detect any animals in the early stages of infection, reactor herds are re-tested at 60 day intervals until no further reactor animals are found and restrictions can be lifted. Reactor herds are then re-tested 6 and 18 months after the lifting of restrictions. Additionally, if infection has been confirmed in the herd by post mortem examination or culture, a more severe interpretation of test readings is applied. This further enhances the ability of the test to detect any residual infection after the initial screening. In other words, the imperfect sensitivity of the skin test is compensated for by the application of short-interval testing and severe interpretation. Defra has also sponsored experimental work at the Institute of Animal Health (Compton), to look into the effects of repeat skin testing. The main finding of this experiment was that repeat testing at 56-day intervals before experimental infection of calves with M. bovis had no effect on the animals' ability to respond to an intradermal test carried out seven weeks post-infection. Therefore, no matter how often an animal is skin tested before it contracts TB, the sensitivity of the skin test after infection will not be compromised.

Secondary information

Type

Written question

Reference

172530; 421 c1103-4W

Session

2003-04

Subjects

Cattle Disease control Testing Bovine tuberculosis Skin

Link

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