Extract from Private Eye's "Not the Foot and Mouth Report"
".... Tens of thousands of hefted sheep which wandered a vast area of hills and mountainsides were being rounded up and placed in pens, where blood samples were taken to see whether they tested 'positive for antibodies'. If they did, those sheep and all those in the pens around them were slaughtered. The explanation offered to the public was that the tests had demonstrated the presence in the sheep of foot and mouth.
In fact these so-called Elisa tests (for enzyme-linked immuno-absorbent assay) proved nothing of the kind. The presence of antibodies merely showed that, sometime in the previous six or nine months, those sheep had been exposed to the virus and their immune systems had generated antibodies in response. It was certainly theoretically possible that some of those sheep were still actively infected with foot-and-mouth and that they could pass the disease on. But what was much more likely, since the sheep looked perfectly healthy, was that they had been exposed to the virus months before and had generated antibodies which allowed them to make a full recovery. They were now, as a result, not only free from the disease but immune to it.
In other words, what the officials of the Welsh rural affairs department were actually doing was slaughtering the very animals who were least likely to be or to become infectious. There are scientific tests which could have distinguished between any sheep actually infected with the disease and all those now immune to it. But so crude was the Elisa test used by the officials that it could not show this one way or the other..."