'Atypical' strain of BSE found in U.S. cattle

August 18, 2006

By Chris Clayton, DTN Staff Reporter, and Journal staff

The two cases of bovine spongiform encephalopathy found in U.S. cattle over the past year came from a rare strain of BSE found largely in Europe that scientists are only beginning to identify, according to research by a French scientist.

Researchers in France and Italy who presented their work at an international conference in London reported two rare strains of bovine spongiform encephalopathy that are harder to detect and affect mainly older cattle.

Thierry Baron of the French Food Safety Agency presented research indicating that a 12-year-old Texas cow testing positive for BSE last June, and the 10-year-old Alabama cow that tested positive in March, showed identical testing patterns to a small number of BSE cases in France, Sweden and Poland.

Animal scientists are calling such strains "atypical" BSE, which is different from the "typical" BSE caused by cattle eating feed with ruminant offal contaminated with a BSE protein.

They don't know whether the atypical strains are caused by something else or simply appear spontaneously in older, susceptible cattle.

Art Davis, a U.S. Department of Agriculture scientist for the Animal and Plant Health Inspection Service at the National Veterinary Services Laboratory in Ames, Iowa, said in his presentation Sunday at the London conference that the Texas and Alabama test results showed completely different prion patterns than the Washington state case discovered in December 2003.

"The classical lesions were not there," Davis said of the cases. The Washington state cow originated in Alberta, Canada, near where several other BSE cases have been found.

The "typical" BSE strain caused a mad cow disease epidemic in Great Britain beginning in the mid-1980s that killed 184,000 cattle and more than 100 people who contracted a human form of the disease caused by eating contaminated beef products.

The scientific evidence shows that in almost all cattle cases, the fatal neurological disorder was contracted through contaminated meat and bone meal fed to the cow, typically at a young age.

However, scientists finding atypical cases of BSE are beginning to question if there has been a change in the abnormal protein that causes BSE or if cattle might be susceptible to a sporadic BSE affecting older cattle.

Danny Matthews, head of transmissible spongiform encephalopathies at England's Veterinary Laboratories Agency, said recent research on atypical cases of BSE raises questions over whether older cattle can sporadically get the disease or if there are more strains of BSE than previously understood. Scientists might also be facing something new, such as "son of BSE," he said.

"We don't fully understand what atypical BSE means," Matthews said. "Is it spontaneous or another source causing it? Time will tell."

Although the test patterns in the U.S. cases and atypical cases in Europe closely matched, Baron said there were no known links among any of the positive animals. The French Food Safety Agency sent a researcher to the United States to study the positive Texas case and compare its results to known cases in France that did not match the typical BSE positive tests.

"You could place them side-by-side and not tell the difference," Baron said.

Baron also raised the prospect that the disease could be sporadic in at least a small number of older cattle. He said, however, such a conclusion would be hard to determine because of the small number of cattle with this atypical strain globally.

Dr. Sam Holland, South Dakota's state veterinarian, said there are many strains of BSE and varying degrees of infectiousness of the agent.

"What if the scenario is there is an atypical prion out there that is much less infective, has a longer incubation period and has not been recognized as part of the Great Britain BSE experience identified in 1985 and '86?" Holland said. "There could be others out there that we haven't recognized yet."

He said it is possible the atypical strains are not caused by contaminated feed.

He said it still makes sense to continue the ban on ruminant offal in cattle feed to prevent the spread of typical BSE and eventually to eliminate that disease.

"Based on what we know about BSE, it makes good sense to, number one, keep some surveillance in place; number two, watch what we import and restrict shipments and movements from places that have had those syndromes; and, number three, with what we know about BSE, it seems to be very prudent to keep our ruminant offal ban in place," Holland said. "At least for typical BSE's, it seems to be very effective. It's probably reasonable to continue the ruminant offal ban even after the last typical BSE case has been eliminated."

Editor's note: DTN, a private company based in Omaha, Neb., provides information to agriculture, energy trading markets and other weather-sensitive industries. The Rapid City Journal received a copy of DTN's story and expanded on it.

 

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