Deadly 'Zombie Deer' Disease a Struggle for Wildlife Agencies

This chronic wasting disease (CWD) is a transmissible spongiform encephalopathy, meaning the deer are exposed to a pathogen (a protein called a prion) that causes holes throughout the brain, making the organ look like a sponge by the time of death.
By on 06/30/2020 2:00 PM

Source: MedicineNet Health News

U.S. state and federal agencies are scrambling to get a deadly deer disease under control before hunting season, though rates of infection seem to be climbing.

A brain infection is infiltrating herds of deer, moose and elk throughout at least 26 states and two Canadian provinces, according to the U.S. National Institutes of Health. The infection causes emaciated “zombie deer” to stagger and slobber through the herd, infecting others before their inevitable collapse and death from a brain riddled with holes.

This chronic wasting disease (CWD) is a transmissible spongiform encephalopathy, meaning the deer are exposed to a pathogen (a protein called a prion) that causes holes throughout the brain, making the organ look like a sponge by the time of death.

Though the CWD prion doesn't seem to be able to infect humans or other livestock now, the COVID-19 coronavirus crisis has shown how easy it is for some microorganisms to jump the species barrier. This is easier with close contact between infected animal populations and humans, as is the case with deer farming and hunting.

The US Department of Agriculture is in the second phase of a CWD study with Texas A&M University and Texas Parks and Wildlife to identify deer in wild and farm herds that are more genetically susceptible to the disease.

“While the research is still in an early stage, it's the first of its kind,” according to a June post on the USDA blog. “The hope is that continued analysis will enable us to effectively categorize animals as being minimally susceptible, moderately susceptible or highly susceptible to CWD. This will help herd owners make management decisions that may eventually help to breed the disease out of existence or at least significantly reduce its presence.”

Meanwhile, fish and game agencies throughout the states are requiring more testing of deer carcasses and directing hunters to strategically thin herds identified as hotspots for infection, as well as banning wild deer feeding.

The CDC offers hunters the following tips to avoid CWD exposure:

  • Do not shoot, handle or eat meat from deer and elk that look sick or are acting strangely or are found dead (road-kill).
  • When field-dressing a deer:
    • Wear latex or rubber gloves when dressing the animal or handling the meat.
    • Minimize how much you handle the organs of the animal, particularly the brain or spinal cord tissues.
    • Do not use household knives or other kitchen utensils for field dressing.
  • Check state wildlife and public health guidance to see whether testing of animals is recommended or required. Recommendations vary by state, but information about testing is available from many state wildlife agencies.
  • Strongly consider having the deer or elk tested for CWD before you eat the meat.
  • If you have your deer or elk commercially processed, consider asking that your animal be processed individually to avoid mixing meat from multiple animals.
  • If your animal tests positive for CWD, do not eat meat from that animal.

What Is CWD?

Chronic wasting disease first became a big problem in the U.S. in the early 2000s. The public had its initial education in prion diseases from a 2003 outbreak of mad cow disease (bovine spongiform encephalopathy, or BSE), which people contracted from eating beef contaminated with brain or spinal tissue from the animal.

This was particularly frightening because the incubation period seemed to be about 10 years, according to CDC data published on MedicineNet. This means for some exposed to mad cow, their doom was likely already sealed long before anyone realized mad cow disease was a problem for humans. According to a CNN timeline of the 2003 outbreak, the first established mad cow disease death in that period was a 20-year-old British vegetarian who contracted the deadly prions from eating contaminated hamburgers as a child.

This crisis sparked a flurry of research into prion diseases like CWD, including a 2004 study in the journal Emerging Infectious Diseases. The study showed prions from deer with CWD could alter human prion proteins in a petri dish, but there has been no evidence of CWD transmission to humans.

“The foodborne transmission of (mad cow disease) to humans indicates that the species barrier may not completely protect humans from animal prion diseases,” the study states. “Conversion of human prion protein by CWD-associated prions has been demonstrated in an in vitro cell-free experiment, but limited investigations have not identified strong evidence for CWD transmission to humans.”

Furthermore, a study by the Alberta Prion Research Institute showed CWD prions could infect research monkeys, a species many steps closer to humans than deer as far as physiology and shared proteins.

What Is a Prion?

Prions are tiny scraps of protein that, under normal circumstances, help insulate your nerves with fat, according to our best understanding.

Each nerve in your brain and body transmits a chemo-electric signal. In order for that signal to transmit from one nerve cell to the next effectively, nerve cells called Schwann's cells manufacture fat to form the myelin sheath, which performs a function similar to the colored, rubber insulation on the outside of electrical wires, according to a Medscape article by Dr. Tarakad S. Ramachandran.

Prions stimulate the production of fatty tissue for the myelin sheath, according to scientists' best understanding. A 2010 study published in the journal Nature showed mice with prion genes removed had problems with developing and maintaining myelin sheaths on their nerves, making them hypersensitive. The mice were, however, immune to prion diseases because they had no native prions in their bodies to misfold and propagate the disease.

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That explanation is far from settled science, however; other researchers speculate prions in their healthy state could have some metabolic function or carry copper molecules, a micronutrient, according to Medscape.

Part of what allows prions to function is the shape of the molecule. It's folded in a certain way that allows it to dock with other molecules to promote healthy nerve functioning. Prions become dangerous when they “learn” to misfold, either from faulty genetic instructions or from the introduction of a specifically misfolded prion.

“Prion diseases are unique in that they can be inherited, they can occur sporadically, or they can be infectious,” according to Medscape. “The infectious agent in the prion disease is composed mainly or entirely of an abnormal conformation of a host-encoded glycoprotein called the prion protein.”

In other words, a genetic mutation may cause your prion protein to misfold, or environmental triggers we don't understand yet could cause the faulty folding.

The third type of prion disease transmission – animal-to-animal infection – is more dangerous because it can potentially reach more members of a herd, and the prions have more opportunities to come into contact with other species.

Creutzfeld-Jakob Disease, a form of progressive dementia caused by prions, is a close analog in humans to the deer-specific CWD. This disease can be inherited, it can appear sporadically in people with no known risk factors, or it is possible to transmit, usually through medical procedures that could expose someone to foreign prions, according to NIH data published on MedicineNet.

Creutzfeldt-Jakob is not contagious in casual settings, as far as we know.

What Are Federal and State Agencies Doing to Stop CWD?

Chronic wasting disease is much more contagious among deer than Creutzfeld-Jakob is among humans.

Fish and game agencies throughout the U.S. are instituting bans on salt licks and feeding of wild deer, practices that help hunters maintain good hunting grounds, but that also aid prion transmission through deer saliva exchange.

Check with your state's wildlife agency for the specific rules governing hunting and feeding deer in light of rising CWD rates.

The USDA, in addition to its genetic monitoring experiment, operates a national CWD herd certification program that private deer and elk ranchers can join voluntarily to help beat back the infections.

If you have accidentally shot and/or processed a deer that seems sick or that later tests positive for CWD, a simple bleach solution can clean your tools and work area, according to a 2019 study by the National Institutes of Health.

“As the range and number of deer, elk, and moose infected with chronic wasting disease expands, so does the potential for human exposure,” the study author Dr. Brent Race said. “While no human cases of CWD have been confirmed to date, it remains prudent to handle CWD-infected tissues with caution. Here we demonstrate that bleach treatment can serve as an easy and practical method for disinfecting equipment contaminated with CWD prions.”

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