Why did deer meat from an infected herd end up in Canada's food chain?

This is an excerpt from Second Opinion, a weekly roundup of eclectic and under-the-radar health and medical science news emailed to subscribers every Saturday morning. If you haven't subscribed yet, you can do that by clicking here

Last fall, a dangerous animal sickness — chronic wasting disease (CWD) — was detected in a Quebec deer farm. It was a disturbing development — the first sign of this highly contagious infection outside of Alberta and Saskatchewan.

There were almost 3,000 deer in the herd. Eleven tested positive for CWD. The rest — more than 2,700 animals — tested negative and were released into the food chain.

It was a controversial decision, in part, because so little is known about the human health risk from CWD.

The Canadian Food Inspection Agency's website cautions that: "A negative test result does not guarantee that an individual animal is not infected with CWD."

"There is not currently a food safety test available for any prion disease," CFIA's spokesperson told CBC News in an email. "The tests that are used are the best available. In accordance with Health Canada's precautionary approach, no animals known to be infected were released into the human food chain."

CWD is similar to another frightening animal illness — mad cow disease, officially called "bovine spongiform encephalopathy" or BSE. It is a fatal infection in cattle that can be spread to humans through beef consumption.

Both CWD and BSE are caused by a strange protein — a prion — which can jump the species barrier, triggering a deadly cascade of neurological damage.

Worldwide, BSE has caused about 225 cases of human prion disease called "variant Creutzfeldt Jacob Disease (vCJD)." There is no treatment and no cure.

After an epidemic of mad cow disease in the U.K. more than two decades ago, governments developed strict controls to prevent BSE-infected cattle from being processed for human food.

But so far there are few official controls in place to keep CWD out of the food chain. 

Is CWD a human health risk?

At this point, scientists don't know whether CWD can infect humans. So far, no human cases of CWD have been detected.

But there is reason to be concerned, based on research showing that the CWD prion can cross the species barrier into non-human primates.

As a precaution, the World Health Organization and other health agencies recommend that no prion-infected meat should be consumed.

But experts say Canadians are unknowingly consuming meat from animals infected with CWD.

"Human exposure to CWD is quite widespread in my opinion," said Michael Coulthart, who tracks prion diseases at the Public Health Agency of Canada.

The risk is highest for people who consume wild deer and elk.

"The big complicating factor is that CWD-infected animals can be completely asymptomatic for a long time before they get clinical disease, so if you don't test the animals you won't necessarily have any clue that the animal is infected," said Coulthart.

"In cases like that, it's very likely that many people have been exposed through consumption of those harvested animals."

For 20 years, Coulthart's team has been tracking human prion disease in Canada. So far, there have been two cases caused by infected cattle. One infection was traced back to the U.K. and the other most likely came from Saudi Arabia, but the person may have been exposed in United Arab Emirates or U.K. (There is also a sporadic form of human prion disease that appears spontaneously in about one or two out of every million people).

"We're fairly confident that we haven't seen obvious evidence for CWD having been transmitted to humans," he said. But scientists suspect human infections from CWD might have unusual symptoms.

"It could look different. Prions have repeatedly shown themselves to be sources of scientific surprise."

I think we're teetering on the edge of a catastrophe, to use a very strong word. ​​- Dr. Neil Cashman, prion scientist, UBC

Meanwhile, word is gradually spreading about ground-breaking research by Canadian scientists who were able to infect some macaques with CWD by feeding them deer meat.

The results, if confirmed, would be the most compelling evidence so far that CWD could be a risk to humans.

But the study has not yet been published or peer reviewed, although it has been presented at two international prion conferences. The scientists, based at the Alberta Prion Research Institute, are completing follow-up experiments and plan to submit the work for publication soon.

At this point, Canada and the U.S. are the most heavily affected countries. But the disease has been found in Norway, Finland and Sweden. Scientists believe CWD is spread between animals through saliva, urine, blood and even through the soil.

Scientists demand action from Ottawa

A group of scientists, Indigenous leaders and wildlife advocates sent a letter to Ottawa this week asking the federal government "to recognize the dire nature of this epidemic," and implement controls to stop the spread of CWD, including closing game farms and prohibiting transport of CWD-infected carcasses.

The letter also demands expanded testing for CWD to ensure the safety of the food supply.

"I think we're teetering on the edge of a catastrophe, to use a very strong word," said Dr. Neil Cashman, one of the prion scientists who signed the letter.

To read the entire Second Opinion newsletter every Saturday morning, please subscribe.