WASHINGTON-The CDC’s emerging-disease watchers are hoping the other hoof won’t drop. Though there’ve been no confirmed U.S. cases of the variant Creutzfeldt-Jakob disease that’s been linked to eating British beef, the CDC has investigated nearly 100 cases to ensure they are classical.   Dr. Lawrence Schonberger of the CDC says he’s seen nothing disquieting. But CDC stepped up its surveillance in five areas-Connecticut, Minnesota, Oregon, California, and Atlanta-in response to the United Kingdom’s 10 cases of variant CJD reported in March. They’re linked to an epidemic of bovine spongiform encephalopathy among British cattle.   The U.S. annual incidence of CJD has hovered at 0.8 to 1.1 per million-or about 250 new cases nationwide-since the CDC began monitoring the disease in 1979. “There’s no outbreak going on,” Dr. Schonberger says.   The one-in-a-million rate of familial and sporadic CJD has stayed steady everywhere it’s been monitored, including Britain. Yet Edinburgh neurologist Robert Will, who heads Britain’s CJD surveillance unit, says the 10 “mad-cow” cases there differ from classic CJD in several ways. Average age was just 27, vs. older than 60 for typical CJD. And patients lived four times longer than those with familial or sporadic CJD, who are usually dead within six months of onset.   Between 1970 and 1989 in Britain, only one CJD case was reported in a patient under 30, vs. six since 1990. The 10 British variant CJD cases, found among 207 others reported since 1990, were also singular for their psychiatric presentation-anxiety, depression, withdrawal, and behavioral changes. Nearly all were sent to a psychiatrist early on.   The first neurological involvement in four patients was limb or face dysthesia, or both. Over weeks to months, all 10 developed a cerebellar syndrome with gait and limb ataxia.   Memory was disturbed, and most developed cognitive impairment, akinetic mutism, and myoclonus sometimes preceded by choreiform movements. Eight have died.   Unlike typical CJD, however, EEGs showed no characteristic spikes. Nor did any any patients have iatrogenic risk factors. Diet histories from nine showed all ate beef or beef products over the past decade but not brain. One had been a vegetarian since 1991; one had been a butcher for two years.   Agriculture officials here say there is no need for Americans to worry because there’s no epidemic of BSE in North American cattle-not a single documented case. “There’s no evidence of BSE in the U.S.,” says USDA’s Will Hueston.   But critics note that the agency has looked at only 3,000 of the nation’s 100 million head of cattle. The British BSE epidemic is believed by many to have originated when cattle were fed meat and bone meal composed of brains and organs from sheep and other cattle. For more than two centuries, many British sheep have developed scrapie, a TSE that causes them to scrape their wool off against rocks and barns. The USDA says scrapie is present in the U.S., as are TSEs in goats, mule deer, elk, and mink. British cats have it too.   The British team was set up in 1990 to watch specifically for a CJD outbreak after BSE had already affected more than 155,000 cattle. Now, in a Lancet report, Dr. Will says, “We believe we may have identified a new clinicopathological phenotype of CJD which may be unique to the U.K.” The team concludes that “exposure to the BSE agent is the most plausible interpretation of our findings.” The evidence remains “strongly circumstantial,” says the NIH’s Dr. Joe Gibbs, but it rings true.   In addition to the spongiform changes in British patients’ brain biopsies, there was neuronal loss and astrocytic gliosis in the basal ganglia and thalamus. But Dr. Will says that the most striking neuropathologic feature was the extensive amyloid plaque formation, distributed throughout the cerebrum and cerebellum. Such plaques, common in Alzheimer’s and kuru, are rarely seen in classic CJD patients. A 41-year-old woman in the U.S., whose clinical illness was similar to that of the British patients, was judged to have classic CJD because she lacked the variant cases’ distinctive plaques.   “What we’re seeing here is really an infectious amyloid disease,” says NIH’s Dr. Paul Brown. “The distinction between the transmissible spongiform encephalopathies and Alzheimer’s is that we can transmit TSE to experimental animals.” NIH scientists say they’ve purified prion protein (PrP) from classic CJD patients and have successfully transmitted it to unaffected animals. A diagnostic test, based on artificial antibodies to the protein, is in the works (see related story). Immunostaining for a PrP in the British patients showed pericellular PrP deposits in plaques throughout the cerebrum and cerebellum.   Drs. Gibbs and Brown believe that the plaques and clinical course in the British variants more closely resemble kuru, a human TSE, than CJD. Unlike CJD, which incubates for two to four decades, kuru develops rapidly, in three to four years. The BSE epidemic peaked in Britain in 1989, the same year the government banned the use in animal feeds of any supplement from cattle CNS, tonsils, spleen, thymus, and intestines.   The only thing that’s prevented a BSE epidemic here, one researcher says, is that the U.S. has far fewer sheep than Britain and, thus, far less scrapie. Britain has four sheep for every head of cattle; the U.S. has 10 head for every sheep. -Joe R. Neel