As 'zombie deer disease' spreads, scientists search for answers

As ‘zombie deer disease’ spreads, scientists search for answers, This story originally appeared on Yale Environment 360.

Late last year, federal officials discovered the carcass of a mule deer near Yellowstone Lake in a remote area of ​​Yellowstone National Park. The cause of death was chronic wasting disease (CWD), experts said, making this the first confirmed death of an animal in the park due to the disease.

The discovery was alarming, but not a surprise. First discovered in deer in Colorado in 1967, CWD has since spread, primarily through wild and captive deer populations, throughout the United States, Canada and around the world. Transporting live or harvested deer across the country has also contributed to the spread. It is now found in 34 states, five provinces and four other countries. California just became the latest state to confirm cases, and British Columbia recently found its first cases. Experts believe the disease will be present in all 48 contiguous states within the next decade.

Although the uncommon disease mainly affects white-tailed deer and mule deer, the disease infects all cervids, including elk, moose, reindeer and caribou.

Known as America’s Serengeti, Yellowstone is of particular concern to biologists because many thousands of elk, deer and elk live on and roam the rugged landscape. Studies show that herds affected by CWD decline by 3 to 20 percent annually.

The disease was expected to be discovered in Yellowstone because there are 22 state and one federally managed elk feeding operations in Wyoming. Feeding activities bring together thousands of animals, which promotes the transmission of diseases.

The slow deterioration of the nervous system causes animals to drool, stumble and stare blankly in the days before they die.

CWD “has enormous ecological consequences,” says Thomas Roffe, a veterinarian and retired chief of wildlife health at the U.S. Fish and Wildlife Service. “I have warned that if we get a disease that we cannot get rid of, cure or vaccinate, we will have a real problem. And CWD falls into that category.”

CWD is unlike any other natural disease. It is caused by a cell protein that becomes abnormal or misfolded. When this so-called prion comes into contact with normal proteins in an animal’s brain, it causes those cells to also fold incorrectly, causing the symptoms of CWD. The brains of infected animals deteriorate to a spongy consistency. The slow deterioration of the nervous system causes animals to drool, stumble and stare blankly in the days before they die. That is why it is also called ‘zombie deer disease’ and ‘the disease from outer space’. It can take several years to kill an animal.

CWD is always fatal, there is no treatment or vaccine and it is difficult to detect. “CWD-infected animals can shed infectious prions before clinical signs appear,” said Brandon Munk, a California Department of Fish and Wildlife veterinarian who oversees the state’s CWD surveillance and response efforts. “These prions can persist in the environment for years, making it very difficult to prevent or control their spread once introduced.”

There is a long list of unknowns about CWD, and several major efforts have been made to understand the nature of the disease as it continues to spread. One of the main concerns is that CWD prions will cross the species barrier to humans, similar to the way the AIDS virus jumped from chimpanzees to humans. In humans, the prions could cause a variant of Creutzfeldt-Jakob disease, a rapidly progressive neurodegenerative disease.

“We are completely unprepared” if people contract the disease, said Michael Osterholm, an infectious disease expert who led the response to Covid-19 as director of the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota. leading its CWD effort. “If we saw an overflow now, we would be in free fall. There are no contingency plans for what to do or how to implement follow-up actions. It is a slowly evolving disaster.”

Last year, Minnesota funded a CIDRAP project to bring together 70 public health experts from around the world to begin creating a plan in the event of an overflow. Osterholm said that recently research shows that the prion is evolving and is becoming increasingly capable of infecting people. “The prions we see today will much more easily enter a humanized mouse [one that has been modified with human genes so its immune system responds in ways similar to a human immune system],” he said. “They are much more conducive to jumping into a human cell today than ever before.”

However, a recent one study by the National Institutes of Health found that the likelihood of a spillover was low after researchers introduced high levels of prions into human brain cells, which did not become infected.

The abnormal proteins that cause CWD can remain infectious for at least 15 years and can be transported via water and dust.

A case study published last April in the news Neurologymeanwhile, noted that two men who ate venison from a herd known to be infected with CWD contracted rapid-onset Creutzfeldt-Jakob disease. One of the men died from the disease within a month of showing symptoms, while the other died later. The five authors speculated that the disease could have been caused by their frequent consumption of deer meat (prions have been found in the muscle tissue of deer). The cases received wide press coverage, but other experts noted that the article was merely an observation and that the possibility that the disease arose from eating deer meat required more research.

A separate group of experts at the University of Minnesota is studying the ecology of CWD, which is key to understanding where it has spread and could spread and how best to prevent or limit it. “We’re looking at how prions move through the environment,” says Peter Larsen, an associate professor at the University of Minnesota’s College of Veterinary Medicine and co-director of the Minnesota Center for Prion Research and Outreach.

“There are a lot of prions being released into the environment,” he said. “Where are they going and what does the exposure look like? These things can circulate in the environment for a long time, but how long do they remain infectious in the environment?”

So far, data from studies of scrapies – a fatal prion-related degenerative disease that affects sheep and goats but not the people who consume them – shows that prions can remain contagious for at least 15 years. They can be transported by water and by dust in the air.

As 'zombie deer disease' spreads, scientists search for answers 3

One recent one study discovered that plants can be vectors of the disease, taking up prions from the soil through their roots, carrying them to above-ground growth and infecting animals that consume them. So far, however, that movement has only been observed in laboratory settings, not in the wild. In some places, the import of hay from prion-infected areas is banned.

In some parts of the country, there are percentages of 30 to 40 percent of a deer herd, Larsen said. And the infected deer “release these prions into the environment every day through their feces and urine. If you think about an alfalfa field, you can get a hundred deer in the field for days. That’s a lot of prions in the environment.”

How the prions will evolve is another big unknown. A study showed that prions passing through the digestive tract of voles evolved and became infectious to raccoons.

A mitigating factor in Yellowstone may be the presence of wolves, which are not susceptible to the disease. Some biologists think they can help prevent CWD from spreading by chasing and consuming animals weakened by the disease. Preliminary models show that wolves can slow outbreaks and reduce their size; Some experts believe that the inexorable spread of CWD may be the result of a lack of predators and scavengers in the landscape.

Experts say the key to preventing a possible spillover to humans is developing a test that hunters can perform in the field.

At the same time, predators and scavengers can spread the disease. “This is one of the most concerning aspects of CWD,” says Larsen. “Many species may be exposed to CWD prions, and we don’t know how those prions will evolve and which mammals those prions will be able to infect.”

Prions pose a unique challenge because, unlike bacteria or viruses, they are virtually indestructible. Cooking doesn’t kill them: on the contrary, it concentrates them. Also, they cannot be killed with antiseptics or radiation.

The only known spread of a prion disease among humans occurred in the late 1990s and early 2000s, when prions infected livestock and caused bovine spongiform encephalopathy (BSE), or mad cow disease. More than 200 people who ate the contaminated beef died from variant Creutzfeldt-Jakob disease, mainly in Europe. However, some researchers believe prions can cause other diseases, including Parkinson’s and multiple sclerosis.

Nevertheless, it is estimated that between 7,000 and 15,000 infected animals, mainly deer, are unknowingly consumed by humans each year. Deer can be tested after they have been shot, but it can take up to a week for results to come back and many hunters have no problem with it. The key to preventing continued spread and possible spillover to humans, experts say, is developing a test that hunters can perform in the field and provide results in hours rather than days or weeks.

Elk CWD USGS
A bull elk with CWD in Wind Cave National Park, South Dakota. Credit: USGS

Another challenge in detecting the presence of prions in humans is their long latency. Infected venison may not cause neurological disease for years, after which the previous meat consumption and the disease may appear unrelated.

Around the world, hunters are on the front lines of the disease. While some people no longer eat wild game, Steve Rinella is the host of the television series MEAT EATER, says most hunters he speaks to say they’re not concerned about it since CWD hasn’t been observed in humans. “That would change dramatically if a hunter were to get CWD,” he said. “It would be a nightmare if a hunter got CWD.”

The threat poses a unique concern for indigenous people who rely on game animals. “White-tailed deer are an important cultural and food resource for the Leech Lake Band of Ojibwe,” said Tanya Roerick, wildlife program director for the tribe. “Not for sport, but to maintain their family and spiritual ties.”

CWD has been found in three places within 15 miles of the reservation, she said, and threatens the Ojibwe way of life. The tribe has offered free CWD testing of deer harvested by tribal members for years. That oversight, Roerick said, “will continue for the foreseeable future.”

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