Prions, the misfolded proteins that cause transmissible spongiform encephalopathies such as scrapie, can infiltrate organs affected by chronic inflammation and proliferate, according to research done in mice. Until now, prions have been found only in immune system and nervous system tissues. The new finding raises the possibility that prions, which also cause bovine spongiform encephalopathy, may spread to other tissues in cattle and other animals that have conditions such as pancreatitis and nephritis.
"We are doing similar experiments in sheep, but no results are available (yet)," said Adriano Aguzzi, MD, PhD, director of the Institute of Neuropathology at the University Hospital in Zurich, Switzerland, who led the study. "We haven't done anything in cattle, since we lack the facilities."
Dr. Cyril Gay, national program leader for Animal Health and Safety at the Department of Agriculture's Agricultural Research Service, points out that, so far, similar prion migration has not been seen in animals other than mice. "A mouse is not a cow," Dr. Gay said. "There is a lot of research that needs to be done in the relevant host."
The researchers investigated whether chronic inflammation influences prion pathogenesis, because studies have shown that B lymphocytes and other components of the immune system are involved in prion replication.
After injecting scrapie prions into mice, researchers found that prions could accumulate in organs—including the kidney, pancreas, and liver—affected by chronic inflammation. When inflammation subsided, however, the prions disappeared from these organs. "That is a bit surprising in the sense that it says that the accumulation of prions in some tissues is not permanent," says Dr. William Hueston, director of the Center for Animal Health and Food Safety at the University of Minnesota.
The findings, published in the journal Science in January, sparked media frenzy. "Illness May Help Mad Cow Disease Spread, Study Finds" read a Reuters headline. "New Study Raises Mad Cow Alarm" announced another news outlet.
Nervous system tissues, such as the brain and spinal cord, and intestinal tract tissues from cattle, have been excluded from the food chain with the aim of protecting public health. Many reporters focused on the possibility that these efforts may not be enough. Eating prion-contaminated tissues is thought to cause the new variant strain of the rare Creutzfeldt-Jakob disease in humans.
Dr. Hueston, an expert on BSE, says the fear factor hype about the recent prion study is unwarranted and could itself have a negative impact on public health. In his opinion, press releases from research institutions and scientific journals that overemphasize certain findings are contributing to the problem.
"I think these scientific findings are interesting, but in the global context of public health, they are not that significant," he said. "We must consider these discussions in light of the success of current intervention strategies where BSE is at record lows in the UK and human cases of vCJD also are declining."
"This paper doesn't represent a crisis in the making," Dr. Hueston says. "At some point in time, the question we are going to have to face, as a nation and the world, is the responsibility of the media as it relates to public health and risk communication. There are hundreds, if not thousands, of new scientific papers coming out every week about a whole myriad of diseases with potential human health implications. (The media) can't look at every scientific paper with an eye on how (they) can create a headline that fuels public fear."
Even if prions can reside in certain inflamed tissues, it is unclear what risk this poses to human health, according to Dr. Hueston. "However, there is a cost in the anxiety we create in the minds of people," he says. From the broader perspective, fear contributes to emotional stress. If people start worrying about something they have eaten, this can translate into mental health or even physical health problems, and these have an immediate and direct public health cost to the nation, Dr. Hueston notes.
Currently, the USDA does not plan to impose additional restrictions on tissues that enter the food supply. As new information emerges, countries will need to reassess their regulations.
"If, down the road, it is demonstrated there is some prion accumulation in nonneural tissues, secondary to inflammatory processes, (in cows) with BSE, then the question people obviously will ask is 'should we exclude all additional tissues from the (food supply)?' That decision will be based on an assessment of the overall risk," Dr. Hueston says.
"If BSE is widespread or rampant in a country, then some additional measures may well be justified," he continued. "If BSE is extremely rare, the unintended public health consequences of additional control measures must be weighed against any real public health benefit that might accrue from these actions."