The revelation that lambs at a government experimental station appear to have caught BSE from their mothers coincides with plans to relax anti-BSE controls in cattle and was not mentioned at a meeting of the Food Standards Agency in London this week.

Scientists will now seek to estimate from ongoing experiments whether there was ever enough infection in flocks to make the disease survive for long. No evidence of BSE has emerged from testing sheep in abattoirs or on farms, although this did not begin until well after the BSE epidemic in cattle was in steep decline.

Safety advisers have previously warned that any sheep with BSE entering the food chain would be potentially far more dangerous than a single cow, since there are far more parts of the animal that can carry infection.

The report on infected lambs, in the journal Veterinary Record, also comes as officials review contingency plans in case BSE is ever found in sheep on normal farms. The present worst case scenario assumes that around 25m sheep might have to be destroyed. There would be severe shortages of sheep meat, since an entire year's crop of lamb, some older sheep bred for mutton and many breeding ewes would have to be killed.

But the plans have been based on hypothetical models. Now scientists from the government's Veterinary Laboratories Agency have revealed that two ewes fed 5mg of BSE-infected material had lambs that died of BSE after showing signs of infection in their tonsils, 546 days after birth.

Their mothers had shown no outward signs of the disease at lambing, one showing them 73 days after lambing, and the other 198 days after.

But it is still not certain that the lambs were infected while in the uterus, or shortly before or after lambing. The disease may have spread through the birthing fluids or in some other way. The evidence so far suggests this is far more likely than the lambs catching the disease from other apparently unaffected sheep.

It is already known that BSE-like diseases can be transmitted via blood in humans as well as animals, but there has been no evidence that it has been handed from dam to calf in cattle, or mother to baby in people.

The sheep involved were of a genetic type that in lab tests previously appeared most susceptible to BSE. But it is unclear how many such sheep are in flocks on farms. There are 15 different genetic types, and unlike in BSE in cattle, genetic type seems important.

Unfortunately at present there would be no way of identifying resistant sheep in time for them to go into food, while banning others.

The fear about sheep has existed for years because, until the late 1980s, they were fed the same sort of feed as was fed to cattle. However if it was ever in sheep, there is no suggestion that it ever existed on a large scale.

There is some good news. The lambs that seem to have inherited BSE showed a brain signature similar to BSE in cattle. Officials have been worried that some BSE in sheep, if it existed, might have been masked by a similar disease called scrapie, not known to be dangerous to humans. The relatively small scale of the vCJD epidemic in humans so far might give some reassurance too, given the size of an enormous BSE cattle epidemic.

Peter Jinman, a leading veterinary surgeon on Seac, the scientific body advising the government on anti-BSE measures, said: "This clearly is an important finding. It is another part of the jigsaw." Seac would consider the implications next month.

The Food Standards Agency said the study "adds to the scientific knowledge in an area of continuing scientific uncertainty". It did not advise the public against eating sheep, but would continue to recommed "precautionary and proportionate measures".

The environment department, Defra, pointed out that nearly 2,700 scrapie samples had been tested for BSE since 1998 with no sign of the disease, although two samples with anomalous results were still being tested, using mice.

BSE has been shown to spread naturally between sheep for the first time. It passed from mother to lamb, before or during birth, in an experimentally infected flock. But if the study shows the infection spreads more generally within the flock, that means BSE could still be lurking in Europe’s sheep, possibly posing a greater health risk to people than that from “mad” cows.

Scientists found in 1996 that sheep develop a disease similar to BSE if they eat infected cattle tissue. But feeding cattle remains to sheep was banned in Britain in 1988, and in the EU in 1994. All the sheep infected before then should be gone by now.

So there should be no more BSE sheep – unless they can transmit BSE to each other. Cattle cannot do this, but sheep transmit a related disease called scrapie between themselves, apparently when they eat placentas and other birthing remains in the field. If BSE also spreads “horizontally” in this way – between other members of the flock – it might have kept spreading in sheep even after the feed ban.

And because the symptoms of BSE in sheep resemble scrapie, “mad” sheep might not have been noticed. Nearly 2700 sheep with apparent scrapie have now been tested for BSE in the UK. None so far had clear BSE, though two are being tested further. BSE-infected goats, which are biologically similar to sheep, were found in France and possibly the UK in 2005.

BSE-infected sheep are potentially more dangerous to human consumers than BSE-infected cows, as they carry the infection in more of the tissues people eat.
Mother to lamb

Sue Bellworthy and colleagues at the UK’s Veterinary Laboratories Agency (VLA) report that two ewes experimentally infected with BSE in a flock in Warwickshire in 2000 gave birth to lambs in 2003 that died of BSE this year. This is the first confirmation of “vertical” transmission of BSE from mother to offspring. It has been suspected but never proved in cattle.

In sheep, given how scrapie spreads, “this was expected,” Danny Matthews, a BSE expert at the VLA, told New Scientist. “But vertical transmission alone would not be enough to keep BSE going in the sheep population after the feed ban.” Transmission would be limited to one family line, which would die out as animals die of BSE or are eaten.

The experimental herd is now being watched to see if adults can transmit BSE horizontally to other ewe’s lambs now being born and raised within the flock. So far none has, and no uninfected adult sheep have caught the disease from experimentally infected sheep. But it’s still “too early to say”, cautions Matthews.

BSE transmitted between sheep

Incident at government test farm fuels new food fear