Conspiracy theorists were able to add a choice specimen to their collections this week. The challenge was to explain how scientists from the Institute of Animal Health, who had run a five-year study to determine whether BSE had got into sheep, could have committed the extraordinary bungle exposed last week, when it was revealed that the brains they had been testing were not from sheep, as they thought, but from cows.

The outcome of this study had crucial implications, not only for human health but also for farming. The Government had already promised that if BSE were found in sheep the entire national flock of 40 million animals would have to be slaughtered.

But it was not only country dwellers who needed some answers. If BSE is in sheep it would be more alarming for consumers even than BSE in cows, given that the infection has been shown experimentally to spread throughout the animal. How convenient, then, that three days before the results were due to be released, one of the biggest scientific cock-ups in living memory was revealed.

A labelling error in the laboratory is the theory being touted for this disaster, but conspiracy theorists have a darker explanation. Did somebody – from the farmers' lobby, the Department of Environment, Food and Rural Affairs or the Government – having been alerted to the finding that BSE was, indeed, in sheep, decide that this was one agricultural catastrophe too far and that the results should be suppressed? How simple to declare that the samples had been mixed up and that all along the scientists had been testing cows' brains instead of sheep's.

If that sounds far-fetched, there is another conspiracy involving farming practices that is all too real – a conspiracy of inaction. This has to do with the practice of routinely dosing animals with antibiotics as growth promoters in their feed. It is not too far-fetched to say that this kind of bio-warfare down on the farm poses a potential threat to human health on the scale of BSE.

A report last week in the New England Journal of Medicine pointed up the danger. Researchers who tested samples of ground meat – chicken, turkey, beef and pork – bought in supermarkets in the Washington DC area of the United States found one in five infected with salmonella, the food-poisoning bacterium.

This is not so serious, you might think, as salmonella is destroyed in cooking and so ought to pose a threat only to lovers of steak tartare. But the alarming finding was that 84 per cent of the samples of salmonella isolated were resistant to at least one antibiotic, and half were resistant to three or more antibiotics.

Two studies in the same issue of the Journal drive the message home. The first shows that strains of bacteria resistant to a new antibiotic, licensed for use only two years ago, are present in the animal and human populations as a result of decades of using a similar drug in animals. The second shows that these resistant strains can theoretically be transmitted from person to person. The threat could not be clearer. With the world facing a new era of biological warfare, anti-biotics are almost our only defence. It is critical that nothing is done to weaken that. Yet decade after decade, farming practices have done exactly that, in both the US and the UK.

Cases of patients infected with "superbugs" resistant to most known antibiotics are surfacing with increasing regularity in hospitals in the UK and abroad. The Public Health Laboratory Service in the UK has been calling for five years for a ban on the use in animals of quinolones, the newest class of antibiotics, to ensure that the most powerful drugs were reserved for human use.

In an editorial this week, the New England Journal repeats a demand it made two years ago for a ban on the routine use of antibiotics as growth promoters in animals. So far these demands have fallen on deaf ears. We cannot afford to ignore them any longer.