Steve Connor: Is this a food scare or should I stop eating lamb?

'We must all decide whether to eat sheep that might, or might not, be infected with BSE'

Following the BSE outbreak, and its subsequent link with new-variant CJD, it was agreed on all sides that greater openness must ensue between the scientific community, politicians and the public. Yesterday's reports linking BSE with sheep are a case in point. BSE has not yet been found in sheep, but the Food Standards Agency (FSA) has indicated that experiments under evaluation may well come to that conclusion. Yet what is the consumer to do with this incomplete information?

One can be forgiven for a feeling of déjà vu over the continued doubts about whether sheep have been infected with "mad cow" disease. After all, the theoretical possibility was first raised publicly as long ago as 1996, when the Government admitted that bovine spongiform encephalopathy can pass from cattle to humans. The prospect of BSE in sheep has since cropped up as one of those occasional food "scares" that leads to inevitable condemnation by farmers and the food industry.

Three years ago, a particularly scary scare was generated by one of the Government's own scientific advisers on BSE. He warned, in a radio interview, that a "distinct possibility" existed that BSE had infected sheep fed on contaminated meat and bonemeal made from rendered cattle carcasses in the late 1980s and early 1990s.

Professor Jeffrey Almond, then a member of the Spongiform Encephalopathy Advisory Committee, went a step further in suggesting that, if BSE were found in sheep, it would provoke a "national emergency". He was, in his own words, publicly "roasted" by everyone from the meat industry to professional commentators for daring to suggest such a thing.

If we have learnt any lessons from BSE, it is that health risks are never an easy topic for open discussion. Indeed, no direct evidence shows that BSE is infecting sheep grazing in Britain today. However, an old scientific adage is relevant: absence of evidence cannot be taken as evidence of absence. In other words, we may have no evidence of BSE in sheep because we have not looked hard enough.

The FSA has now hinted that such evidence might be about to emerge. Referring to an experiment which tested about 3,000 brains of sheep that had died in the early 1990s, the agency said yesterday that the results so far "could be compatible with BSE having been in sheep at that time".

Solid theoretical grounds clearly exist for believing that BSE might have infected an unknown number of sheep. We know that many sheep were fed with contaminated feed before, and even after, it was banned in 1988. In fact, some sheep might have been eating BSE-contaminated feed as late as August 1996 when the feed ban was finally implemented in full.

We also know that BSE can cross the species barrier into sheep. Experiments have shown that certain genetic strains of sheep can develop the cattle disease after being fed half a gramme of BSE material. We also know that the clinical symptoms of the disease look just like scrapie – an endemic brain disease of sheep that is harmless to humans. It is therefore plausible for BSE in sheep to be masked by the many thousands of cases of scrapie that occur each year.

Also, when sheep contract BSE under laboratory conditions, the infective agent infects a wider range of tissues than the cattle version of the disease. Most worryingly, BSE in sheep infects the lymph nodes. These occur in muscle and often end up on the plate of someone eating lamb.

We can be fairly sure that the measures designed to stop the spread of BSE in cattle are working. The rapid decline in the epidemic is testament to that. But if BSE gets into sheep, there is a real possibility that the agent will be transmitted from sheep to sheep, just like scrapie. This will make it more likely that the disease will become endemic.

Of course, all of this will be academic if sheep do not have BSE. The experiment which has generated the latest "scare" involves injecting the brain material of sheep diagnosed with scrapie into mice. And the FSA is warning us that we do not as yet know the outcome of this experiment, which is why it issued its carefully crafted statement about results being "compatible" with BSE.

This might be seen as an exercise in openness, which is what the agency is keen to promote. Or it might be seen as a case of making sure that the agency is not accused at a later date of standing idly by knowing that an experiment might be about to show that sheep were infected with BSE while the meat industry was starting a campaign to promote the sale of "light" lamb, whatever that is.

Either way, it still leaves us all in that difficult position of deciding whether we should eat lamb that might, or might not, be infected with BSE. I have been reporting on these matters for several years now, but yesterday's news leaves me in the position of having to assess the probability that it will turn out that British sheep are currently infected with BSE.

I have occasionally eaten lamb and will do so again, not because I think it is totally safe but, considering all the risks in life we face, this is one I can live with. Whatever the risk today, it was much, much bigger in the late 1980s and early 1990s, and it's too late to worry about that now.

steve.connor.@independent.co.uk