How rubbish is the TB cattle skin test.

I'll put a little of this into basic terms and links to more detailed articles and pages for folks to read.
The current cattle skin test runs at about 80% accuracy which means on an infected farm for every 5 reactor cows tested it averages as missing one, that cow then goes back into the herd.
This is a false negative and there are also false positives at a lower rate.
Easily explained from 5 page to page 8 of

The cattle skin test is 50 yrs old and works in a similar way to the old Heaf test (daisy prick test to decide whether or not you need a BCG jab.) Remember having that at school as a teenager?

The heaf test worked on that fact that if your skin reacted you either already had immunity and did not require the vaccination or if you reacted severely you were sent for x-rays to check for infection.

The difference in the bovine situation is if you react to the heaf test and you are a cow you would be sent to slaughter (hence we have been steadily removing cattle with natural TB immunity from the gene pool)
The skin test used to identify bTB cannot tell which cow is carrying natural immunity and which cow will actually go on to be infectious.

Due to using this same test for over 50yrs as one vet put it..
"This test just clears anything that has no natural immunity to TB as well as some carriers, It's Darwin on it's head - survival of the sh*tist "

Detailed but brilliant read here of the skin test and its problems

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