How the event then unfolds depends a lot on the temperament of the vet and the handlers. If they're excitable, it makes for a torrid experience all round. If they're calm, it's just tedious and repetitive.
While I've been looking at testing all my life, I never knew what was the finer details of what was going on until last week when I observed Sarah Ryan, one of four female vets working in Kyle Veterinary.
The method used is the comparative skin test and apparently is the best that is currently available, being more accurate than blood or tissue testing.
First off, the animal number is recorded. Then the vet shaves (with a scissors or hand-clippers) two centrally-placed patches on the neck, one below the other. The thickness of the skin is measured with a callipers and this thickness is recorded.
The vet then injects a shot of avian tuberculin (sounds like a pen being clicked) into the top patch and a shot of bovine tuberculin into the lower one.
I had assumed that the injection goes under the skin but it actually goes into it, so the needles are very short, about 2mm long.
Does it hurt? Animals will often shake their head after getting a shot, so presumably there is a little sting.
The guns hold 20 shots a time and at the bottom of each holster is a receptacle of surgical spirit so the needle is disinfected between animals.
The vet then checks that the shot has actually gone in where its supposed to, that there is a small pea-sized bump at the injection site.
This Clip, Clip, Measure, Measure, Shot, Shot, Pair of Peas sequence only takes about five seconds per animal. But some animals take longer because they are jigging about and, allowing for filling and emptying the crush and changing pens, time adds up.
Once the test is over, the farmer has to wait (and pray, if so inclined) for 3 days.
The "reading" of the test is much faster. As each animal is passing through the crush, the vet runs their hand over their necks, and well-trained fingers will immediately detect if there is a difference in size of two lumps. Cattle that are infected with Mycobacterium bovis tend to show a greater reaction to bovine tuberculin than avian tuberculin.
Sarah says a lot of farmers panic when they see a lump appearing. But their worry may be unnecessary. If the top lump is bigger, it's not a concern. Nor is it if the two lumps are the same size. However, when the bottom lump is bigger, the result is either inconclusive or positive (reactor).
So how did our test go? We had one inconclusive, who will either be sent for slaughter or can be retested in 42 days.