Some experts find the data unconvincing, however, and one researcher recently presented findings he said robustly support a different hypothesis: these diseases are caused by a bacteria.

"This is really the best and final proof for the prion hypothesis," Claudio Soto, a professor of neurology at the University of Texas Medical Branch at Galveston and senior author of the new study, told United Press International.

Stanley Prusiner, of the University of California, San Francisco, won a Nobel prize in 1997 for his hypothesis that prions are the pathogen that cause these fatal, brain-wasting diseases, collectively known as transmissible spongiform encephalopathies or TSEs. These include mad cow disease, Creutzfeldt Jakob disease in humans, chronic wasting disease in deer and elk and scrapie in sheep.

The prion hypothesis has enjoyed widespread acceptance among TSE specialists, but it also has remained controversial because prions isolated from a TSE never have been shown to be infectious. Prions exist in two forms in the body: a normal form whose function is uncertain and a misfolded form that is associated with disease.

The new study, published in the April 21 issue of the journal Cell, describes how Soto and colleagues isolated misfolded prions from the brains of hamsters experimentally infected with scrapie. Using a method they developed called protein misfolding cyclic amplification, or PMCA, they significantly increased the number of prions and then placed them in a test tube with normal prions.

The misfolded prions are thought to cause the normal prions to transform into the misfolded, disease-causing variants.

The team diluted the samples over and over again, to ensure the only remaining prions were those generated in the test tube and none of the original prions from the hamster brain were left. Then they injected the samples into the brains of hamsters -- which developed symptoms of TSE.

Laura Manuelidis, a TSE expert and section chief of surgery in the neuropathology department at Yale University -- who has long doubted the prion hypothesis -- reached the opposite conclusion from Soto.

"It's about the best proof that (prions) aren't infectious," Manuelidis told UPI.

She noted the study shows prions obtained through PMCA had less infectivity than the same amount of prions from the original hamster brain sample.

"They got amplification but they didn't see any increase in infectivity," she said.

Soto's team acknowledged this problem in their journal article, noting "the reason for the lower level of infectivity ... is unknown and currently under investigation."

Manuelidis, who thinks the bulk of the evidence points toward the TSE pathogen being a virus, said she is not committed to that position.

"I'm firmly in the 'I don't know' camp," she said. If researchers could prove prions are the infectious agent in TSEs, she said, she readily would acknowledge it, but this study does not meet that criteria.

Neil Cashman, a TSE expert at the University of Toronto, said the study offered solid support for the prion hypothesis.

"I find the article convincing," Cashman told UPI. He added, however, he would like to see other groups repeat the methodology and obtain the same results, because previous similar experiments have failed to find increased infectivity.

Wen-Quan Zou and Pierluigi Gambetti, of the National Prion Disease Pathology Surveillance Center in Cleveland, argued Soto had not ruled out the possibility of other infectious agents as the causative pathogen of TSEs.

"Although the study ... makes significant progress toward proving the prion hypothesis, it still falls a bit short of achieving this goal," Gambetti and Zou wrote in an editorial accompanying the paper. "For example, the possibility remains that RNA or other molecules present in the original brain preparation might be amplified along with (prions) and might play a critical role in conferring infectivity and specificity to the new (prions)."

Frank Bastian, a pathologist at Tulane University in New Orleans, whose hypothesis is TSEs are caused by a bacteria, doubted the study for similar reasons.

Bastian said the processes used by Soto's team -- to ensure other pathogens were destroyed in the original sample of hamster brain -- may not have been entirely effective, particularly against Spiroplasma, the strain of bacteria he thinks is the cause of these diseases.

"I suggest that if Spiroplasma are in the original preparation, that they could survive" the disinfection measures, Bastian told UPI. He said he has exposed cultures of Spiroplasma to some of the same procedures and not seen a decrease in infectivity.

Bastian presented research at the American Academy of Neurology meeting last week. He said his lab had succeeded in isolating the Spiroplasma bacteria from the brains of deer infected with chronic wasting disease and the brains of sheep infected with scrapie. He said the findings offered further support of his hypothesis that Spiroplasma are the pathogens behind TSEs.

Bastian's lab is currently injecting the isolated Spiroplasma into animals to see if they contract symptoms of a TSE. "If we can fulfill that, then we have proved that it's a cause," he said. If that happens, then scientists will "have to seriously consider criteria of the bacteria hypothesis," he said.

The problem Bastian faces is that his funding may run out before he can complete the animal inoculation studies, which will take about a year. His current grant ends in July and it can be difficult to get funding in the TSEs field for studies not focused on prions.

"I'm going to go as far as I can go," Bastian said.

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Steve Mitchell is UPI's Medical Correspondent. E-mail: sciencemail@upi.com