In the US, chronic wasting syndrome is becoming more common in deer, which has sparked worries that humans could contract the deadly neurological condition. However, a new study indicates that the illness may have a difficult time infecting people.
The misfolded brain protein known as a prion is the cause of chronic wasting disease, or CWD, rather than a virus or bacteria. A recent study demonstrating that CWD prions do not infect human brain tissue uses tiny, lab-grown organs called organoids, which validates earlier findings.
The study’s 180-day duration saw brain organoids subjected to large dosages of prions derived from mule deer, elk, and white-tailed deer remain clear of illness, according to research published in the June 2024 Emerging Infectious Diseases. Organoids exposed to human prions, on the other hand, which result in Creutzfeldt-Jakob disease, contracted the illness rapidly. The results imply that a significant species barrier keeps CWD from migrating from deer to people.
The National Institutes of Health’s Rocky Mountain Laboratories in Hamilton, Montana, employ scientist Bradley Groveman. “This was a model that could really help tell us… whether or not it was a real risk,” Groveman says.
However, brain organoids may not have all the characteristics that would make them prone to infection and are not a perfect replica of the genuine thing. In addition, fresh strains of prions may emerge, some of which may aid deer prions in attaching themselves to human brain proteins that are beneficial.
According to Cathryn Haigh, a cell biologist at Rocky Mountain Laboratories, researchers must continue gathering information and evaluating novel prion strains on organoids or in other studies in order to monitor the risk to humans. “I don’t think it will ever be possible for us to say that [human infection] is not possible.”
A contagious illness
CWD-affected deer are extinct. There’s no way to become better.
The disease, which affects moose, deer, elk, and other cervids, is caused by prions that cause a healthy brain protein called PrP to twist into an aberrant shape. These twisted proteins group together, destroying brain cells and resulting in symptoms including a lack of fear of people, tripping, listlessness, and abrupt weight loss. After infection, animals usually begin exhibiting signs 18 to 24 months later.
People’s concern over the risk posed by prions from deer, elk, and moose has grown, partly due to the disease’s ongoing expansion throughout North America. Indiana received its first report of CWD cases on April 5, while California officials reported the state’s first cases in two wild deer on May 6. Since the first recorded occurrence occurred in 1967 in a captive deer from Colorado, the disease has been found in wildlife in 34 U.S. states, as well as in portions of Canada, South Korea, and northern Europe.
Furthermore, according to Debbie McKenzie, a prion biologist at the University of Alberta in Edmonton, Canada, a larger percentage of deer have the illness. “1 in 100, 1 in 1,000 deer would be afflicted for a very long time. However, in certain deer populations right now, the prevalence of CWD is higher than 75%.
A contagious illness
Chronic wasting disease has expanded significantly throughout North America since it was first identified in Colorado in 1967. Only Colorado and Wyoming have documented examples of wild deer and elk (dark gray) prior to 2000. As of right now, four Canadian provinces and thirty other states have the disease (light gray). Animals kept in captivity are also affected; outbreaks are now occurring in some institutions (red circles), and infected populations have been eliminated in others (yellow circles).
Chronic wasting disease’s geographic distribution in North America
It’s more likely that contaminated deer meat may end up on a plate if certain hunters aren’t inspecting their kills. (In the US, state-specific regulations and guidelines for hunters differ.) Additionally, since proteins aren’t impacted by heat the same way bacteria or viruses are, even cooked meat may be dangerous.
Lessons learned from earlier instances
The 1980s and 1990s saw an outbreak of mad cow disease, another prion disease, which was brought on by people eating meat from infected cattle. This, according to Haigh, helped raise awareness of chronic wasting disease. Even though the deer disease was identified earlier than mad cow disease, at the time most people weren’t concerned about the danger to humans.
However, Haigh notes that the idea that a bovine prion may infect humans and cause illness “put into the consciousness that this is a possibility.” “And now, one of the animals that we eat has contracted another disease.”
Previous studies have indicated that prions might find it difficult to transition across different animals (SN: 4/4/14). Research with mice engineered to carry the human form of PrP has suggested that human transmission is a potential scenario, albeit a less likely one than with prions derived from cattle. However, research on macaques, a popular animal substitute for humans, suggests that it is uncommon for chronic wasting syndrome to spread from deer to people.
However, Groveman notes that there is still much uncertainty regarding the possibility of defective prions spreading from wildlife to people, especially given that venison is a common food in North America. Despite a few jump scares, doctors warn that being vigilant is essential to identifying any possible transmission early.
A case report that was presented at the annual conference of the American Academy of Neurology in Denver in April captured the attention of reporters. It detailed a neurological condition that killed two hunters. The people had previously eaten deer meat from a population where chronic wasting illness was prevalent. But according to the report and the U.S. government, both most likely passed away from a distinct prion illness called Creutzfeldt-Jakob disease, which can occasionally occur. the Centers for Disease Prevention and Control.
“There haven’t been any documented instances of CWD infection in humans to date,” according to CDC epidemiologist Ryan Maddox from Atlanta.
Obstacles to infection
Although the mad cow prion afflicted hundreds of individuals beginning in 1994 (symptoms may not appear for ten or more years), McKenzie notes that the barrier to human transmission is quite high. As of 2022, there have only been 178 cases of Creutzfeldt-Jakob disease caused by contaminated meat in the United Kingdom, the heart of the outbreak, and 55 cases worldwide, despite the fact that millions of people are thought to have been exposed to sick cattle during the outbreak in the 1980s and 90s.
However, it’s still unknown what exactly might have rendered some people more prone to infection. According to McKenzie, there’s a chance that those who had the infection were subjected to extremely high doses or that the PrP protein in their brains was specifically shaped to interact with cattle prions. “There had to be additional factors involved in their susceptibility as well.”
Additionally, prions strains, which twist PrP proteins in various ways, may be relevant. According to Groveman, one thing that can stop prions from spreading to new species is how the proteins misfold. Determining the variations in deer prions’ bending and curling behaviors may aid in identifying which may cling to human or animal PrP.
That being said, compared to regular PrP, prions are more difficult for scientists to characterize. It is more difficult to investigate the possibility that prion infections could spread from one species to another because of this ignorance.
For example, chronic wasting illness is caused by at least five separate prion strains, each of which affects a distinct range of cervid species. Furthermore, according to McKenzie, “we really don’t understand enough about how strains are generated in an animal.” Although certain animals may be able to infect other species in the lab, CWD has only ever been discovered in cervids in the field.
It is therefore feasible that new strains that are more dangerous to humans could evolve, even though the results in brain organoids imply that at least some of the current strains don’t pose a high hazard to people. According to McKenzie, “I still think that [chronic wasting disease] can jump into humans.” Still, she believes that these kinds of occurrences would be uncommon in better news. “I don’t believe an epidemic will occur.”
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