Experts ponder using fire to help save deer from deadly plague
Mark D. Zabel wants to set some fires.
Zabel and his colleagues are developing plans to burn plots of National Park Service land in Arkansas and Colorado. If the experiments turn out as the researchers hope, they will spare some elk and deer a gruesome death.
Across a growing swath of North America, these animals are dying from a mysterious disorder called chronic wasting disease. It’s caused not by a virus or bacterium, but a deformed protein called a prion.
When ingested, prions force normal proteins in the animal’s body to become deformed as well. Over the course of months, prions can gradually wreck the animal’s nervous system, ultimately killing it.
This year is the 50th anniversary of the discovery of chronic wasting disease. In the September issue of Microbiology and Molecular Biology Reviews, Zabel, an immunologist at Colorado State University, and his former graduate student Aimee Ortega survey what scientists have learned about the slowspreading plague.
It makes for ominous reading. “There’s a lot that we still don’t know and don’t understand about the disease,” Zabel said in an interview.
Once chronic wasting disease gets a foothold, it can spread relentlessly. It’s now documented in 24 states, and continues to expand into new ranges. In some herds, as many as half of the animals carry prions.
It has only been in recent years that scientists have gained crucial clues to how the disease spreads. Direct contact, it turns out, isn’t the only way that the prions get from one animal to another.
Sick animals and cadavers spread prions across the landscape. Plants and soil may remain coated with deformed proteins for years, perhaps even decades. Zabel suspects that the only way to rid the land of them is to set controlled fires.
It was at Colorado State University, in 1967, that wildlife biologists first observed some captive mule deer developing a strange new disease. The animals lost weight and awareness of their surroundings. The symptoms slowly worsened until the mule deer died.
“They’re not hard to pick out at the end stage,” Zabel said. “They have a vacant stare, they have a stumbling gait, their heads are drooping, their ears are down, you can see thick saliva dripping from their mouths. It’s like a true zombie disease.”
It wasn’t until much later that researchers discovered that chronic wasting disease belongs to a small group of conditions caused by prions. But other prion diseases are known only to affect livestock or people, not wildlife.
Scrapie, for example, is a deadly disease that afflicts sheep. A number of studies indicated that bone meal contaminated with scrapie prions passed the prions to cows. The cows developed a prion disease of their own, called bovine spongiform encephalopathy, nicknamed mad cow disease.
In rare cases, people who ate beef from the sick cows developed prions in their own brains. As of 2016, 231 people had died from the condition worldwide.
Scientists long suspected that deer and related species developed chronic wasting disease by picking up scrapie from sheep flocks kept at Colorado State University. The disease then turned up in other states and Canada as animals were shipped to private game farms. A shipment of elk to South Korea brought the plague there as well.
But Zabel now believes that the birth of chronic wasting disease may be more complicated. “I’m starting to second-guess the original hypothesis,” he said.
Prions are misfolded versions of a naturally occurring molecule called cellular prion protein. Experiments carried out in Zabel’s lab, published in January, suggest that cellular prion protein in deer and related species may be unusually prone to misfolding.
“We were able to generate a new prion,” Zabel said. “Maybe this is a spontaneous disease.”
That result might explain a startling finding last year: researchers came across a prion-riddled reindeer in Norway, the first time chronic wasting disease had been found in Europe. Since then, two more have been found, and Norway in April approved the culling of over 2,000 reindeer to stop the spread.
“The jury is still out on the origin. This new outbreak in Norway just complicates the matter,” Zabel said.
Zabel and other scientists are trying to figure out how chronic wasting disease has become so successful. One factor is how the
prions spread through an animal’s body. They aren’t limited to the brain in deer, elk or moose. The prions also sweep through lymph nodes and the spleen.
As a result, Zabel and his colleagues have found, infected animals can release huge numbers of prions. “We found it in urine, in saliva and in feces,” he said. “They shed continually until they die.”
Other members of a herd can get sick by making direct contact with a shedding animal. But the way the disease is spreading across North America suggests that the prions are also using other routes to get to new hosts.
If deer got sick only by direct contact, for example, you would expect the outbreak to be most severe in the Midwest, where populations are densest. But some of the worst outbreaks are in the Rocky Mountains, where there are fewer animals.
Mathematical models suggest that animals are getting sick from prions in the environment. In additional to the prions shed while a sick animal is alive, its cadaver can release another bounty of deformed proteins onto the ground.
Some studies suggest that these prions can end up on grass and other plants, which are then eaten by healthy animals. Some prions in the soil may bind to minerals. It’s possible that animals may sometimes pick them up if they eat bits of dirt.
Compared with viruses or bacteria, prions are impressively rugged. To decontaminate prioncovered lab equipment, scientists have to heat them to 600 degrees Celsius, or 1112 Fahrenheit.
In a forest or on a prairie, a prion may be able to hang around for years, still able to infect a new animal. As herds migrate along the same route year after year, the supply of prions in the environment may keep increasing.
“It’s starting to make sense,” said Zabel.
Scientists don’t know enough about the ecology of prions to predict how bad chronic wasting disease will become in future decades. Mountain lions and other predators may lower the infection rate by picking off sick animals as they wander away from their herds.
Scientists have also found genes that give some animals resistance to prions. It’s hoped that resistant animals will reproduce enough to main the populations of herds.
Still, Zabel worries, the supply of prions in the environment someday might push many herds past a tipping point. “That could result in herd decimation and population declines,” he said.
Zabel is also concerned about the potential threat chronic wasting disease might someday pose to humans.
So far, there have not been any documented cases in which people got sick from eating meat from sick animals. “That doesn’t mean it won’t happen,” Zabel warned.