CWD: Potential Human Infection & Disease Management

In this third and final installment of my series on Chronic Wasting Disease (refresh your memory of the first two segments here: Part 1 & Part 2 ), I’ll dive into the research seeking to determine whether humans can become infected by eating venison from CWD-positive deer. I’ll also wrap up with a brief discussion of the various management practices aimed at preventing the spread of CWD, as established by state wildlife agencies and other groups.

The Species Barrier

Back in 2018, panic broke out amongst some of my fellow wildlife biologists and deer managers as some initial and unpublished — in other words, not yet confirmed or finalized — data were released from a lab in Germany indicating monkeys could be intentionally infected with CWD in a laboratory setting.

The Canadian-German research team was asking the elephant in the room question surrounding CWD — whether humans can be infected with the disease after consuming tainted meat from CWD-positive deer, elk, moose or caribou. Up to that point, scientists approached the topic with caution but regularly reported that “it is currently unknown whether CWD poses a risk to human health.”

The ultimate question is whether an infectious CWD prion, when consumed, can jump the “species barrier” and cause infection. The species barrier to disease is a natural system that systematically prevents disease from spreading from one species to another. Just as the only constant in nature is change, there are instances where variants of a disease are successful at jumping the species barrier. One of the most well-known instances is Bovine Spongiform Encephalopathy (BSE), commonly referred to as “mad cow disease.”

You may recall the hysteria that arose after the mad cow disease cases hit the news back in the '90s. The resulting human form of the disease, known as variant Creutzfeldt-Jakob Disease (CJD), is in the same family of diseases as CWD, known as Transmissible Spongiform Encephalopathies (TSEs). Mad cow disease was able to bridge the species barrier from cattle to humans because humans fed BSE-positive material from infected cattle (as a protein source) back to cattle, and the infected cattle were then consumed by humans. This practice was immediately halted and the problem was generally fixed. Those who recall the tragic deaths during these outbreaks now fear a similar variant of CWD could jump the species barrier to humans.

Back to the leak of results from the German lab. Since researchers can’t ethically (or legally) use human subjects in lab settings to intentionally infect them with disease, they often turn to non-human primates. In this case, the experiment was conducted using macaque monkeys that are “close to humans but not completely comparable to humans,” as noted by the principal researcher in the study. If you notice the way that was worded, you can easily identify the fact that even if the macaque monkeys became infected with CWD, researchers still could not conclusively say whether humans are susceptible to infection. However, it is the closest model available. You can see the gray area here.

After stirring up fear and anxiety over the situation, the researchers never did publish their findings in a peer-reviewed scientific journal, but instead they opted for non-scientific outlets such as YouTube.

Adding to the red flags, upon hearing about their initial, yet-to-be-published results, I sought out the funding source for the research (follow the money). I found it concerning that one of the co-funders of the 2017 study, the Alberta Livestock Meat Agency, had closed its doors in October 2016 after only eight years in operation, and two years prior to the public release of the research findings. You can draw your own conclusions, but in the scientific community we’re trained to ask questions and demand only non-biased, scientific facts. I wonder, could a beef lobbying group have an ulterior motive when it comes to discouraging people from eating meat from wild or farmed deer? I’ll also allow you to come to your own conclusion as to why these researchers never published their findings in a respected, peer-reviewed scientific journal.

Fortunately, for the scientific and hunting communities, follow-up research published in 2018 led by Montana-based researchers sought to use two species of non-human primates as models to address the public’s concerns about the susceptibility of humans to CWD infection. This time, the research was funded by the National Institutes of Health, and the resulting data were published in the Journal of Virology. In their paper that summarized 13 years of study, Lack of Transmission of Chronic Wasting Disease to Cynomolgus Macaques, researchers made it clear that “the present study found no evidence for the transmission of CWD to Cynomolgus Macaques using a broad range of data, including clinical, pathological, and biochemical observations.”

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So, does that mean the debate is over? No. Remember, monkeys aren’t people, and the risk is ultimately for you to decide. As the U.S. Centers for Disease Control reports, “To date, there have been no reported cases of CWD infection in people. However, some animal studies suggest CWD poses a risk to certain types of non-human primates, like monkeys, that eat meat from CWD-infected animals or come in contact with brain or body fluids from infected deer or elk. These studies raise concerns that there may also be a risk to people. Since 1997, the World Health Organization has recommended that it is important to keep the agents of all known prion diseases from entering the human food chain.”

To be clear, scientists have yet to prove that CWD can jump the species barrier and pose a risk to human health. However, the CDC and state agencies discourage hunters from eating meat from animals that test CWD positive.

CWD Management

Strategically, I didn’t leave much room in my column for proven, best-management practices for managing CWD. That’s because they don’t exist! The fact is, CWD management is generally reactive, as all the proactive management strategies implemented by North American wildlife agencies have failed to prove effective.

Preventative strategies employed and pushed by state agencies and non-profit conservation organizations include restricting inter- and intrastate movement of live and harvested animals, feeding and baiting bans, surveillance/testing strategies, sharpshooting, targeting specific sex or age-class animals for harvest, liberalized hunting seasons and bag limits, and bans on urine-based deer lures, just to name a few. The truth is, none of these strategies have been conclusively proven efficient or effective in the scientific community.

In fairness to the policy makers, it is unrealistic to expect a positive, pro-active management impact on a disease that all experts acknowledge is both notoriously difficult to diagnose in the early stages of the disease and impossible to treat and/or cure. Could this be their way of admitting that no one really has the answer when it comes to CWD?

As the saga continues, I’d like to remind everyone that I trust in nature’s resilience. As I said in a previous column, survival of the fittest is the cardinal rule in nature. As a result, I firmly contend that nature has a handle on this, even though it may not be pretty at times.