Virus may be cause of mad cow
Mad cow disease and other related brain disorders may be caused by a virus and not the weird, misshapen proteins, known as prions, that scientists think are responsible, according to a study released Monday.
Researchers reported that they found virus-like particles in mouse nerve cells infected with two brain-wasting diseases similar to mad cow disease, but found no traces of the particles in uninfected cells.
Lead author Dr. Laura Manuelidis, a neuropathologist at Yale University, said the finding suggested that prions in infected brains were the result of a viral infection and not the cause of the disease.
“We found something that people have been ignoring,” Manuelidis said of the virus particles. “What we hypothesize is the simplest, most parsimonious point of view.”
Several brain researchers were skeptical about Manuelidis’ findings, published in the Proceedings of the National Academy of Sciences.
“It’s very remarkable that we only see these particles after infection of these cells,” said Bob Rohwer, director of the Molecular Neurovirology Laboratory at the Veterans Affairs Medical Center in Baltimore, who was not involved in the study. “But the evidence that they are in fact the infectious agent responsible is still highly circumstantial.”
Mad cow disease, known formally as bovine spongiform encephalopathy, is a cattle disease that destroys brain tissue by causing abnormal tangles of protein fibers and creating microscopic holes in the brain.
It is part of a family of slow-developing brain diseases, including scrapie in sheep and Creutzfeldt-Jakob disease in humans. Human infection in recent years has been connected to eating infected meat. There have been dozens of fatalities over the years.
The prion theory of mad cow disease proposes that a normal protein spontaneously misfolds, starting a cascade of abnormal changes in other proteins.
In Manuelidis’ previous experiments, prions did not appear until late in the progression of spongy brain diseases.
To find what appears at an earlier phase, Manuelidis’ team homogenized mouse brains infected with scrapie and Creutzfeldt-Jakob disease and injected them into nerve cell cultures.
The only new objects were dense spheres that looked like small viruses, she said.
She later added a compound to spur the growth of prions about fivefold to see if that would increase the level of infection. The level did not show a significant increase, suggesting that prions were not the active agent of infection, she said.
To prove that the virus-like particles are solely responsible for the infection, Manuelidis plans to isolate the particles and see if they can start an infection when injected into healthy cells.