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Email received December 15th 2010

Dear Mary,
The flaw in this argument is that the skin test used to determine an animal's TB status relies on the animal having some resistance to TB. If the animal has no resistance to the disease then the skin test will be ineffective. This I believe can be the case with alpacas, and it is causing their keepers problems. On rare occasions it happens with cattle when an animal that has been tested clear is subsequently found to be infected at slaughter. If cattle are skin tested positive, they must have met TB by some route or other, and they must have some natural resistance to the disease. Cattle licks with extra selenium and iodine could possibly reduce the number of infected animals that go on to display clinical symptoms, but the purpose of the testing regime is to remove infected animals before they develop clinical symptoms.
Best wishes
John Tuck