Experts on brain-riddling spongiform diseases have grown steadily more uneasy over signs that so-called mad cow disease, linked to a lethal human illness, may be lurking in sheep. Not only would that open up a new front in the battle to purge bovine spongiform encephalopathy (BSE) from livestock, but it would also suggest that far more people--at least those who eat lamb, anyway--are at risk of contracting the human killer, variant Creutzfeldt-Jakob disease (vCJD). A study in the 10 August issue of Nature now offers evidence that BSE is not rampant in sheep after all, although scientists are far from ready to let their guard down.
As of 30 June, 75 confirmed or probable vCJD cases had been diagnosed in the United Kingdom. Cases have increased an average of 23% each year since 1994 (ScienceNOW, 4 August), and projections have raised the specter of tens or even hundreds of thousands of deaths in the coming years from exposure to BSE.
With BSE in British cattle on the wane (see chart), some experts now worry that BSE might be circulating in sheep, where it could be masked by scrapie, a related disease with very similar symptoms (Science, 17 March, p. 1906). Although there is no evidence to date that sheep can become infected naturally with BSE--nor that scrapie itself is transmitted to humans--sheep experimentally infected with BSE develop scrapielike symptoms. This raises fears that if a BSE epidemic occurred in sheep, it might be confused with scrapie. Indeed, there has been speculation that BSE may be to blame for a scrapielike illness recently reported in some sheep in Vermont.
Like BSE, scrapie, which has flourished in British flocks for more than 250 years, is thought to be caused by infectious prion proteins. If BSE jumped into sheep from cattle, there might be a spike in the incidence of reported "scrapielike" cases during the height of the BSE epidemic in cattle between 1990 and 1995. To investigate this possibility, a U.K. team from the Institute for Animal Health in Compton, the Veterinary Laboratories Agency in Surrey, and the University of Oxford analyzed scrapie incidence between 1962 and 1998, using data from an anonymous mail survey of 11,554 sheep farms. Their numbers show that scrapie incidence has risen slowly, but there is no noticeable jump in scrapie cases before, during, or after the height of the cattle BSE epidemic in the early 1990s. The authors interpret this to mean that the BSE epidemic in cattle did not lead to a comparable BSE epidemic in sheep.
Although the survey doesn't eliminate the nightmarish possibility of BSE in sheep, "one can at least be confident that there has not been an epidemic in sheep equivalent to that in cattle," says epidemiologist Peter Smith of the London School of Hygiene & Tropical Medicine, acting director of the U.K.'s Spongiform Encephalopathy Advisory Committee. Even so, researchers plan to keep a wary eye on sheep. As they know all too well, the history of the BSE epidemic and the emergence of vCJD illustrate the high price of complacency.