August 7, 2008

Another Reason to Avoid Beef?

Prions, the proteins that cause mad cow disease, can be transmitted by feces, a new study has found, raising questions about safe disposal..

A recent study shows that prions can be transmitted by feces. Prions are infectious agents composed solely of protein. They cause a variety of brain−wasting diseases in humans and animals. The most familiar of these is probably mad−cow disease or bovine spongiform encephalopathy (BSE). The disease was given its long name because of the appearance of brain tissue from infected cows; it has so many holes in it that it resembles a sponge. The exact mechanism of how prions cause disease is not fully known, but seems to involve errant folding of proteins.

Hamsters have been used to study prions in the laboratory for over 25 years because they are much easier to work with than either sheep or cows.

Two other diseases caused by prions, scrapie in sheep and chronic wasting disease in deer, spread naturally within flocks and herds of animals, but how they are spread is unknown, as is the exact method of transmission of BSE among cows.

The study, by Nobel prize−winner, Dr. Stanley B. Prusiner and colleagues at the University of California, San Francisco, established that prions are spread by feces in the laboratory but does not show whether or not this occurs naturally. The study raises questions about the safe disposal of waste from cattle and sheep.

The experiments by Dr. Prusiner's team determined that prions are found in the feces of orally infected Syrian hamsters and that those feces can spread the infection. Infection rates as high as 100% were found. Fecal material collected as late as 42 days after infection of the hamsters was still capable of transmitting the disease. Hamsters have been used to study prions in the laboratory for over 25 years because they are much easier to work with than either sheep or cows. This research was published in the July 1st edition of the Journal of Infectious Diseases.

According to the researchers, "[C]onsidering the long−term survival of prions in soil, the data presented in the current study have raised imperative questions about the safe disposal of excrement from BSE−infected cattle, and animals with chronic wasting disease and scrapie, especially during the early, asymptomatic stage of disease."

The oral prion concentration given to cause the original infection in the hamsters was extremely high; so the high level of prions found in the stool of these hamsters may simply represent the natural excretion of such a high dose. The findings do suggest that additional studies should be done to clarify matters. If you needed another reason to avoid eating beef, here it is.

Prions were first discovered by Dr. Prusiner. In 1972, he became interested in what were known at the time as "slow−virus" diseases. These are a group of invariably fatal brain diseases of humans and animals, whose progression may span decades and whose cause was unknown. His interest was spurred by a patient who had Creutzfeldt−Jakob disease (CJD) the human version of BSE and who eventually died from it. Dr. Prusiner began researching scrapie, a similar disease of sheep. Over the years, he discovered that the causative agent was made solely of protein, containing no DNA or RNA. This result flew in the face of basic biological wisdom which had it that infections were always caused by something that contained nucleic acid. Even when Dr. Prusiner had sufficient evidence to publish his findings in 1982, they were greeted with a combination of skepticism and disdain by the scientific community. It wasn't until the 1990s that prions gained widespread acceptance. Dr. Prusiner won the Nobel Prize in Medicine in 1997.

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