Chronic wasting disease

• 1. Introduction

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  Chronic wasting disease, nicknamed zombie deer disease, was first recognized in 1967 in mule deer at a government research facility in northern Colorado. The animals there slowly wasted away, and at first the problem was described only as a wasting syndrome. Today the illness turns up in wild and captive herds on more than one continent. This documentary asks what causes it, how it passes from animal to animal, and why the ground itself may play a part. It also asks why a disease of deer has made some public health experts uneasy about people, and what hunters, governments, and scientists are doing in response.

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• 2. A Misfolded Protein

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  Scrapie in sheep and mad cow disease in cattle belong to the same family as chronic wasting disease. So does Creutzfeldt-Jakob disease in humans. These illnesses are called transmissible spongiform encephalopathies, or TSEs, and all are driven by misfolded proteins called prions. In deer, the culprit is a distorted version of the prion protein, written PrP. The healthy form is found most often in the central and peripheral nervous system.

  The healthy protein is labeled PrPC, with C for cellular. The abnormal form is PrPSc, with Sc for scrapie. Once the abnormal form appears, it converts healthy protein into more of itself, which sets off a chain reaction. In 2001, Prusiner described the pattern this way: "In the prion diseases, the initial formation of PrPSc leads to an exponential increase in the protein, which can be readily transmitted to another host." As the abnormal protein builds up in the brain, widespread neurodegeneration follows.

  A gene called PRNP holds the instructions for the protein. It is highly conserved among mammals, and it has been sequenced in deer. Small differences in that gene change how an animal fares. Elk carry one of three versions at codon 132, written LL, MM, or ML. Elk with LL seem to resist clinical signs, while those with MM or ML have much shorter incubation periods.

  White-tailed deer vary at codons 95 and 96 instead. Deer with the 95H polymorphism that become infected select for altered strains of the disease. Deer carrying the 96S allele reach clinical disease more slowly.

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• 3. Signs and Diagnosis

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  Fifteen months is the youngest age at which any deer has shown clinical symptoms of chronic wasting disease. Illness appears 18-24 months after exposure, so most cases occur in adults. The disease is progressive and always fatal.

  The earliest sign is trouble moving. Weight loss over time is the most obvious and consistent symptom. Most animals also change their behavior. They interact less with other deer, seem listless, lower their heads, tremble, grow nervous, and walk in repetitive set patterns. Many grind their teeth and salivate heavily, and most drink and urinate more than usual. Loss of fear of humans and visible confusion are also common.

  The Animal and Plant Health Inspection Service, known as APHIS, summarized the progression in a single line: "Behavioral changes, emaciation, weakness, ataxia, salivation, aspiration pneumonia, progressive death."

  A necropsy, the examination of a dead animal, is not definitive by itself. Many animals die early in the illness, and the findings are nonspecific. General poor health and aspiration pneumonia are common, and the pneumonia may be the true cause of death. Laboratories therefore test brain, lymph, and neuroendocrine tissue for abnormal prion protein using immunohistochemistry. A positive result in the obex, part of the brainstem, is considered the gold standard. Other methods include western blotting, enzyme immunoassay, protein misfolding cyclic amplification, and real time quaking-induced conversion.

  As of July 2019, tests at the Canadian Food Inspection Agency were not sensitive enough to detect prions in animals younger than a year. As of 2015, no commercially feasible test existed for living animals, even though an antemortem test had been called urgent as early as 2001. A bioassay using transgenic mice that express the cervid prion protein can reveal infection. It raises ethical issues, though, and it cannot be scaled up.

  Tonsil biopsy has been a reliable live test in mule deer and white-tailed deer, but not in elk. Biopsies of the rectal mucosa have detected the disease in all three. Their accuracy depends on the animal's age, genotype, and stage of disease. Meanwhile, the prions themselves can survive sterilization in an autoclave.

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• 4. Contact and Contamination

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  An infected deer's saliva can carry the prions, which turns shared food and water into a hazard. Contact with infected animals, their tissues, or their fluids can pass the disease along, and a deer does not need to look sick to be contagious. The increased drinking and salivation of sick animals may make the problem worse. Research also indicates that deer and elk excrete prions, and that other animals take them in by eating grass growing in contaminated soil.

  Ticks add a surprising twist. They harbor meaningful quantities of infectious prions, and deer swallow them during mutual grooming, a habit common among females and between females and their young. In Rocky Mountain elk, infected females, many of them without symptoms, passed prions to their offspring at a rate of 80 percent, regardless of gestational period. Researchers caution that this does not prove the fetus develops the disease. Transmission before birth is still thought to be relatively unimportant in keeping epidemics going.

  Once prions reach the environment, they can stay infectious for many years. Rotting carcasses, the gut piles left when hunters field dress their kills, and the urine, saliva, feces, and antler velvet of infected animals can all create reservoirs. An APHIS scientist noted that the lifespan of CWD prions is unknown, but scrapie prions have been measured surviving 16 years. The abnormal protein dissolves only in the strongest solvents and resists digestion by protein-cutting enzymes.

  In 2013, researchers at the National Wildlife Research Center in Fort Collins, Colorado, infected white-tailed deer through the nasal passage using prions mixed with clay. That mattered because prions had been shown by 2006 to bind to sandy quartz clay minerals. They cling so tightly to soil particles that the ground can become a source of infection, especially for grazing animals that touch it constantly.

  The American crow has also been evaluated as a possible carrier. CWD prions remain viable after passing through the bird's digestive tract. Crows range widely and gather in communal roosts, in cities and countryside, that can last for years. Conservative estimates put the fecal deposits at one winter roosting site, over a single winter, between 391,552 and 599,032 kilograms.

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• 5. Mapping the Spread

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  In the early 1980s, wild elk, mule deer, and white-tailed deer in Colorado and Wyoming were found to carry the disease. Canada recorded no case until 1996. The first positive farmed elk herd in the United States was detected in South Dakota in 1997.

  In May 2001, infected free-ranging deer were found in the southwestern corner of Nebraska. The zone where infected deer, moose, or elk had turned up came to be called the endemic area, covering northern Colorado, southern Wyoming, and western Nebraska. By 2006 it had widened to six states, including eastern Utah, southwestern South Dakota, and northwestern Kansas. Separate pockets appeared in central Utah and central Nebraska. The true limits are uncertain because the disease is at low incidence and sampling may miss it.

  The map kept filling in. In 2002, wild deer tested positive in south-central Wisconsin, northern Illinois, and an isolated area of southern New Mexico. In 2005, white-tailed deer in New York and in Hampshire County, West Virginia, joined the list. Michigan's first confirmed case, in 2008, was a deer on an enclosed breeding facility. In February 2011, Maryland reported its first case in a white-tailed deer killed by a hunter.

  In June 2015, a male white-tailed deer on a breeding ranch in Medina County, Texas, tested positive. State officials euthanized 34 deer to contain a possible outbreak. In February 2018, Mississippi announced its first positive deer.

  Canada's story ran in parallel. Farmed elk in Saskatchewan and Alberta were infected. A retrospective study showed that mule deer exported from Denver to the Toronto Zoo in the 1980s had the disease. In 2018-12 percent of the mule deer tested in Alberta had a positive result.

  The disease also crossed an ocean. Live elk imported from Canada for farming in the late 1990s carried it to South Korea, where it was found in captive animals by April 2016. Eventually, free-ranging and captive animals were affected in 33 US states and five Canadian provinces.

  A tally from 2022 counted the disease in 29 states, infecting moose, deer, and elk in 391 counties. Kansas led the state-by-state list with 49, ahead of Nebraska with 43 and Wisconsin with 37. In June 2023, Florida recorded its first case and a wild animal in Oklahoma tested positive. In October 2023, a case was reported in Yellowstone National Park, whose ecosystem supports the greatest and most diverse array of large wild mammals in the continental United States. Kentucky confirmed its first case in December 2023.

  In January 2024, British Columbia's surveillance program identified its first two positive deer samples, in the Kootenay region. In April 2025, Georgia confirmed a second case, in a 4.5-year-old male deer in Berrien County, about 400 yards from the first.

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• 6. The Quebec Cull

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  Between 2007 and the autumn of 2018, Quebec ran 9500 tests without finding a seropositive animal. Then, in September 2018, a managed operation in Grenville-sur-la-Rouge produced a positive case. Within two months, 3500 animals were slaughtered, and the enterprise shut down permanently.

  The Canadian Food Inspection Agency ordered the cull. It also ordered 25 centimeters of soil decontaminated in certain places across the 400-hectare operation. Each animal was tested before it could be sent to market. The routine cull for market had been 70 to 100 animals per week. As the producer closed, it neared 500 per week. Other Quebec producers complained of a glut of supply.

  Authorities declared a 400 square kilometer quarantine area in which all hunting and trapping were banned. The government culled hundreds of wild animals over two months. A year later, 750 wild specimens had been culled in a 45-kilometer-radius zone called the enhanced monitoring area. None tested positive.

  In August 2019, it came to light that Canada had once buried or incinerated every animal on infected farms. Since 2014, the Food Inspection Agency has allowed animals from infected farms to enter the food chain, because there is no national requirement to test animals for the disease. From one infected herd in Alberta, 131 elk were sold for human consumption.

  For the fall 2019 hunting season in western Quebec, the provincial ministry relaxed its rules for the white-tailed deer hunt. In certain municipalities in the Outaouais valley and the Laurentides, any white-tailed deer could be taken with any weapon. The ministry hoped to receive more samples to test. The quarantine around Grenville stayed in place, and it barred removing the head, spinal column, internal organs, and testicles, among other parts, from the monitoring zone.

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• 7. Moose in the North

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  A dying female reindeer in the Nordfjella herd of southern Norway gave Europe its first case, in 2016. Routine screening at necropsy turned up an unexpected positive. The origin of the Norwegian disease is unknown. Norway has strict rules against importing live deer, and it has run a scrapie surveillance program since 1997. No scrapie has been found in the reindeer's range, but sheep are herded through the region and are a potential source.

  In May and June 2016, two infected wild moose were found in Selbu Municipality, around 300 kilometers north of the first case. By the end of August, a fourth case was confirmed in a wild reindeer shot in the same area as the first. In 2017, the Norwegian Environment Agency issued guidelines for reindeer hunters in Nordfjella. It explained how to spot symptoms and minimize contamination, and it gave hunters supplies for taking and submitting samples.

  In March 2018, the Finnish Food Safety Authority EVIRA announced Finland's first case, in a 15-year-old moose that died naturally in Kuhmo, in the Kainuu region. The animal did not carry the transmissible North American form. Like the Norwegian variant, it was an atypical or sporadic form that occurs incidentally in individual animals. Finland had screened fallen wild deer since 2003, and none of roughly 2,500 samples had been positive. Live deer exports were temporarily banned, but sales of deer meat were not restricted, since only the brain and nervous tissue of infected moose contain prions.

  In March 2019, the Swedish National Veterinary Institute diagnosed Sweden's first case. A 16-year-old emaciated female moose in Arjeplog, in Norrbotten county, was found circling, unshy toward humans, and possibly blind. After it was euthanized, brainstem tissue tested positive by western blot, though lymph nodes did not. A second case followed in May 2019 in Arvidsjaur, about 70 kilometers east, in another emaciated and apathetic 16-year-old female.

  EU-regulated surveillance ran from 2018 to 2020 and required testing at least 6,000 deer, farmed red deer, and semi-domesticated reindeer. In September 2019, a hunter-harvested 10-year-old female moose from Arjeplog, apparently healthy, was a third positive. A fourth case was verified in September 2020, in a 14-year-old lame and unshy female euthanized in Robertsfors, in Västerbotten county, about 200 kilometers from the earlier cases.

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• 8. Human Risk and Defenses

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  In April 2024, it was revealed that two men from the same hunting group had contracted Creutzfeldt-Jakob disease. Medical researchers speculated that they caught it by eating venison from deer with chronic wasting disease. A related case dates from 2022, when a 72-year-old man died within a month of arriving with confusion and aggression. His friend had earlier died of Creutzfeldt-Jakob. His own diagnosis was confirmed as sporadic Creutzfeldt-Jakob. Researchers stressed that cross-species transmission is plausible in animal models and could not be ruled out in this cluster.

  Officially, transmission to humans or livestock has not been confirmed. Squirrel monkeys and humanized mice have been infected in experiments. So have sheep, cattle, prairie voles, ferrets, and raccoons, though raccoons only by injection into the brain, with limited efficiency and long incubation periods. Livestock housed with infected deer and elk showed no sign of disease, yet injection into the brain does cause it, suggesting only a weak species barrier.

  A 2004 study for the US Centers for Disease Control and Prevention concluded that "more epidemiologic and laboratory studies are needed to monitor the possibility of such transmissions." A 2019 study added that "the potential exists for transmission to humans and subsequent human disease." As a precaution, the studies advise hunters to avoid eating the brain, spinal cord, eyes, spleen, tonsils, and lymph nodes of animals from affected areas. A 2022 study by the National Institutes of Health indicated that the likelihood of prions infecting human brain cells remains remote.

  In 2023, Minnesota funded a project at the Center for Infectious Disease Research and Policy that gathered 70 public health experts from around the world. Their task was a response plan for a possible human spillover. Michael Osterholm, an infectious disease expert at the University of Minnesota, stressed urgency, saying the prion is evolving and becoming more capable of infecting humans.

  Prevention has been mostly local and voluntary. By 2012, APHIS had published a voluntary control system built on minimum standards and herd certification, and by August 2019 the rules sat in 9 CFR Part 55. Pennsylvania's Game Commission began placing disposal dumpsters for heads and spinal columns across affected regions in 2018. Crow Wing County, Minnesota, found the disease in wild deer for the first time in January 2019. For that year's hunting season, it offered free incineration of carcasses at the county landfill east of Brainerd.

  Research now targets better monitoring of wild herds, live animal tests, vaccines, safer carcass disposal, and ways to decontaminate soil. Scientists are also studying genetic resistance in certain deer populations, to learn whether selective breeding or natural selection could help slow the disease.

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