Farming Link is a magazine published by DEFRA. I assume it is sent to all DEFRA registered farmers (in the UK in order to receive money under the Single Payment Scheme you must be the registered owner of entitled land; DEFRA probably uses this list or something similar).
August’s edition is hot off the press and contains an article that introduces the gamma interferon test (for bovine tuberculosis). This article correctly identifies the major benefit of the gamma interferon test, namely higher sensitivity compared to the skin test. Sensitivity is the proportion of infections that are identified as positive according to the test. So the gamma interferon test correctly identifies more infections than the skin test. The appendices of Specificity Trial of the BOVIGAM® IFN-Gamma Test in GB Cattle give a sampling of figures for sensitivity: around 77% for skin test; around 88% for gamma interferon.
Currently in some situations it is up to the farmer whether a herd should be tested using the more sensitive gamma interferon test or the traditional skin test. Carl Padgett, president of the British Cattle Veterinary Association, is quoted as saying “If you have a herd with a brand new infection, you want to get rid of that infection as quickly as possible before it spills over to neighbouring farms or into the local wildlife population. So you want to take out as many animals as possible that might be infected rather than keeping them in the herd”. Sounds like jolly good advice to me.
But then the article goes on to say that in some herds with recurrent TB problems “the [gamma interferon] test may pick out more infected cattle than the standard skin test”. Well yeah, it’s more sensitive, that’s what it’s supposed to do. The articles says that in these circumstances the gamma interferon “might not be in the farmer’s interests”. In other words DEFRA is saying that because the skin test leaves more infected animals in the herd it could be better for the farmer; the farmer will have to replace fewer heifers (expense) and therefore should opt for skin test. In any case I don’t really buy the expense argument. Animals that test positive are compensated for at the market rates, isn’t there an equivalence between money and heifers? Isn’t this what economics is about?
What is DEFRA’s agenda here? Reducing bovine TB, or minimising the number of positive animals it has to purchase?