Three U.S. cows have thus far been found to have BSE. The first, a Holstein in Washington, was born in Canada. The other two were found in Texas and Alabama. And, according to a reports, USDA chief veterinarian John Clifford was reported to say that these later two cows appear to have had an atypical strain of BSE, not related to the common form found in Canada and the England.
These so-called atypical cases match results in seven cases discovered in France, says Jean-Philippe Deslys, project coordinator for NeuroPrion, the European Network of Excellence, but are dissimilar to three other French cases as well as a handful of cases discovered in Italy, Germany and other nations. The U.S. cases are so-called "high-weight" on a molecular basis, while the other atypical cases are "low-weight." (Results of ongoing experiments on this topic are due to be presented at Prion2006 in Turin, Italy in October.)
Deslys says it could mean one of three things: that the "classic" BSE infection has mutated, much like a virus; that these cases are linked to scrapie in sheep or are truly spontaneous like most CJD (Creutzfeldt-Jakob disease) cases in humans and present little or no infective risk; or most ominously, that a new pattern of infectivity has emerged and may not be identified for years.
Ed Curlott, a spokesman for USDA's Animal and Plant Health Inspection Service, agrees that the major issue is the source of infectivity. "We have no data on transmission at all," he says. "Right now, we are just following the science."
That may take several years, says Deslys, one of the world's top researchers in prion diseases. In the meantime, he recommends, steps should be taken to increase testing of animals sent to slaughter, and feed bans and other means of controlling amplification of the disease should be maintained or strengthened. "Risk assessment of this development is going to take at least a few years," he says. He notes that, to date, all atypical cases have been found in relatively older cattle, most more than six years of age.--Michael J. Baker is a Beef Cattle Extension Specialist for Cornell University, Ithaca, NY.