Cause of mad cow disease may be found in milk: study
By HELEN BRANSWELL
November 17, 2005

TORONTO — New research into prions, the infectious agents that cause mad cow-like diseases, has found them in the mammary glands of some sheep, raising questions about whether milk and milk products from infected animals could transmit the pathogens.

Prion experts were quick to insist the current potential risk to human health is low and may even be nil.

But they suggested the findings are a warning that if prion diseases in livestock aren't rigorously hunted for and rooted out, milk and products like cheeses and yogurt could be a potential route of transmission of prions to humans.

In humans, prions — highly infectious misfolded proteins — cause the brain wasting variant-Creutzfeldt-Jakob disease or CJD.

"I think the public health implications of this are profound . . . (and) need further investigation," Dr. Neil Cashman, Canada's leading expert on transmissible spongiform encephalopathies or TSEs, said Thursday.

TSEs are the class of diseases that includes bovine spongiform encephalopathy (BSE) or mad cow disease; chronic wasting disease, which affects wild ruminants such as elk; CJD and scrapie, which infects sheep.

"I sincerely think that the human risk is very small or zero," said Dr. Cashman, a researcher at the University of British Columbia. "But we won't know that until further work has been done. This points up a gap in our understanding of BSE in particular that needs to be answered with further research."

Dr. Cashman was not involved in the research, which was published in a letter earlier this month in the journal Nature Medicine.

The findings was reported by a team of scientists led by Dr. Adriano Aguzzi, one of the world's leading prion researchers. Dr. Aguzzi is based at the Institute of Neuropathology at University Hospital in Zurich, Switzerland.

His team had earlier shown that prions, once thought to be concentrated in the brain and central nervous system tissues of infected animals, actually migrate to other organs if those organs are inflamed because of infection.

In a study published in January, they showed prions migrated to and propagated in the pancreas, kidney and liver of infected mice when inflammation of those organs was induced. Earlier this fall they showed that mice co-infected with a prion disease and inflammation of the kidneys shed prions in their urine.

Dr. Aguzzi decided to study an animal that had more implications for human health. So he found and bought a flock of Sardinian sheep, some of which had problems with both scrapie and mastitis — inflammation of the udder.

Prions were found in the mammary glands of co-infected sheep and in macrophages — cells that the immune system uses to try to clear infection — those sheep generated, Dr. Aguzzi said in an interview from Zurich.

"It turns out that if you have an inflammation of the mammary gland, the milk is full of macrophages. So it's not hard from there to infer that eventually you will end up with prions in the milk," he said.

But Dr. Aguzzi's team has not yet isolated prions from milk, a complex task. "We are making progress. But we are not there yet," he said.

Experts believe scrapie prions aren't a threat to human health; it is believed humans are not susceptible to them.

Humans are vulnerable to BSE prions. But no one yet knows if BSE prions are drawn to inflammation in the way mouse and scrapie prions are. Those studies have not yet been done.

And there are fears that sheep and goat flocks could have been infected with BSE prions before the practice of mixing cattle offal into livestock feed was banned when it was shown to be spreading BSE.

To date only one goat, in France, has been found to be infected with BSE prions, but experts admit BSE and scrapie prions are exceedingly difficult to differentiate in ruminants such as sheep and goats.

"These data would be very worrying if we still had a big epidemic of BSE, notably in small ruminants," said Dr. Jean-Philippe Deslys, a prion researcher with the French Atomic Energy Commission in Paris.

"It indicates that we need to maintain a good surveillance, not to let this disease develop. Because we see that there are different possibilities for the agent to replicate."

Dr. Aguzzi said for now, he has no plans to drop dairy products from his diet.

"I don't want to stress too much the implications for human health. I think it's early days," he said.

"This is of course something that needs to be investigated. I would not want to provoke a wave of panic. And if that helps, I may add that I'm not going to stop eating sheep's cheese."