Wednesday, July 07, 2004

Tests for Tb. Accurate or Rubbish?


"The cattle test is rubbish isn't it?"

"Half the cattle slaughtered haven't got Tb at all"

"Why can't we test the badgers and only cull those with Tb?"

In answers to PQ's all logged in archived on this site, all these points were answered by Mr. Bradshaw.

8th Dec-141968. The turberculin skin test for cattle has been compulsory in Great Britain since 1950. It is used throughout Europe under directive 64/432/EEC and prescribed by the OIE (Office of International Epizootics) for international trade.

"The skin test is rubbish?" Well no actually - although Professor Bourne of the ISG frequently says it is.

"Half the cattle slaughtered haven't got Tb at all" True, but they have met and had contact with that wonderfully robust little bug mycobacterium bovis. The skin test shows exposure to this. The cattle may (or may not) go on to develop the lesions associated with full blown Tb. Exposure in the 30 - 50 days prior to the test will not show up. There is a period of latency which is why, after a herd breakdown, 2 tests 60 days apart are needed to escape restrictions. Exposure to m.bovis can take up to 226 days to show in a skin test. But if it fails the test, the bovine concerned is shot as a Reactor.

Sensitivity on a cattle herd test is 96 - 99 per cent, on a single animal less so, but on a single animal tested several times, sensitivity rises to 100 per cent. No problems there then.

In the absence of a maintenance reservoir in the wildlife, the cattle skin test is absolutely fine, and every country in the world uses it.

"Test the badgers"

First catch your badger! Cage traps are notoriously ineffective, with target rates varying from as low as 3 out of a target 10 caught, to a maximum of 8. The Brock test has been around for several years and was the subject of yet another 'trial' in the early 1990's. This one seemed to fade out of existence after a couple of years, with no published findings - other than what was known about the live badger (Brock) test before it started!

That is, although fairly accurate on a positive result the negative one is of "very low sensitivity" (PQ 6th Feb 150583)

"This affects its ability to indentify diseased animals."

Badgers to be relocated by sanctuaries are therefore (if tested at all) tested 3 times.

"The voluntary protocol was not devised or approved by Defra"

Badger in a cage - Tb takeaway?

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