April 13, 2024

Chronic wasting disease: how to control it

A lethal, incurable disease similar to mad cow disease is ravaging deer species in North America and beginning to spread across the world. First identified in a single herd of captive deer in Colorado in 1967, the chronic wasting disease – CWD – has now been found in captive and wild mule deer, white-tailed deer, elk, moose and reindeer. It has been found in 32 states and has crossed international borders into Canada, South Korea and Norway, among other countries.

The disease – caused by a harmful protein known as a prion – has not yet been shown to infect humans, although fears remain. But even if that never happens, CWD could kill large numbers of deer and possibly wipe out individual populations. Wildlife management agencies could, in turn, introduce stricter hunting rules, and fears of contaminated meat could drive away potential hunters, hurting the roughly $23 billion deer hunting industry in the United States.

Since the emergence of CWD, scientists have worked to understand the disease and how it can be controlled. Over the years, three potential mitigation strategies have emerged, but each presents significant challenges. Nicholas Haley, a veterinary microbiologist at Midwestern University in Arizona, co-authored an overview of the chronic wasting disease in 2015. Annual Review of Animal Biosciences and has been working on the problem ever since. Cognizable Magazine spoke with Haley about the options and whether we will ever be able to contain the disease.

This conversation has been edited for length and clarity.

What is a prion disease?

CWD is not caused by a bacteria or virus, but by a naturally occurring protein in our cells that becomes misshapen.

Kurt Vonnegut’s novel cat cradle describes the discovery of a new form of ice, nine ice, which is solid even at room temperature. In the book, when ice nine touches water, it forces all other water to crystallize in the same way, until all the water on Earth is frozen. This is what is happening in the body. An animal is exposed to the prion, usually through ingestion, and anywhere in the body where the prion encounters the normal version of itself, the abnormal protein convinces the normal protein to take on this misfolded form.

This is particularly dangerous in the central nervous system, because these proteins can form plaques that kill the cell. Eventually enough cells die and you will have nervous system disorders. The animal begins to behave strangely and ends up dying.

Meanwhile, the sick animal can spread the prions to other animals through things like saliva, urine or feces. Prions are very resilient and can remain in plants or soil until another animal comes along and eats them.

Could we just kill all sick animals before they can spread the disease further?

Unfortunately, this only really works if done early enough. It’s like a forest fire: the sooner you put it out, the better your chances of preventing it from spreading. But if you let CWD fester for any length of time, culling probably won’t work.

New York State, for example, carried out a massive culling operation in Oneida County in 2005, after first identifying five or six CWD-positive deer. This appears to have worked, and the state still tests animals for the disease to try to detect these outbreaks early.

But when wildlife managers tried localized culling in Colorado, it didn’t seem to affect CWD long-term, possibly because the infectious protein had been in the area so long that it essentially became embedded in the landscape. The protein is incredibly stable and can exist in the soil for years. Or new, sick deer may have moved into the now-empty area from nearby populations. Deer are not symptomatic until the later stages of the disease, but are likely releasing the prion into the environment some time before then.

So if culling is only really effective from the start, are there other strategies that can help in places where CWD is already “embedded”?

My work focuses primarily on breeding animals resistant to CWD – not curing the disease, but trying to find animals that don’t get sick so easily. We are working with a deer farm used for hunting. They own a few properties, representing about 600 to 800 deer, where CWD has become common. We first identified CWD there in 2014, and within a few years, a deer on one of those properties had about a 60 to 70 percent chance of being positive for CWD.

We also carried out genetic tests on the animals. We found that somewhere around 80 to 90 percent of the deer had a specific genetic variant, or allele, of the prion protein that appears incredibly susceptible to infection. But that’s just one of five possible alleles in deer. And it appears that some alleles are more resistant to CWD than others.

Why?

It’s like a lock and a key. The CWD infectious prion is a very good key to that really ordinary lock, but with different alleles, the lock is subtly different and the key doesn’t work as well. We’re still learning exactly how it all interacts.

Over time, we began to focus on two different “good” alleles. I think our ultimate goal is to use artificial insemination and other breeding practices until we have a population of animals with only the good alleles, eliminating the one we know is terrible.

Would having only animals with good alleles stop the spread?

That could make it manageable. Animals with these good genetic variants are significantly less likely to get CWD, but they are probably not completely immune. We have been putting more animals with good variants on the farm and can see that fewer of them appear to be infected by the time they are hunted – on a property where we introduce a lot of selectively bred deer, we have not found a positive case in the last two, if not three, years.

Therefore, selective breeding could work like the Covid vaccine: it is still possible to get an invasive infection, but it has had a huge impact on slowing the disease and minimizing transmission. And at that point, there may be management tools that we could use to essentially keep it at zero. If these highly resistant animals take five years to get sick, but they are all hunted by the age of three, then eventually we won’t have any CWD, for example.

Could selective breeding also work for wild animals, not just animals in captivity?

That’s a very good question. This type of selection occurs naturally in wildlife – natural selection will favor resistant animals over time – but it is much slower. I could see the release of captive-bred animals happening in controlled situations, like when CWD completely wiped out a local population. But just putting a dollar or two into the landscape – your genes would get diluted very quickly.

And while you’re there they are Although there is precedent for raising animals on a farm and releasing them into the wild, many wildlife professionals are strongly against it. They want to keep wild populations wild. The introduction of agricultural deer would hurt it, in some ways. And it’s something you can’t take back. I understand this perspective. Instead, many wild animals are placing more hope in vaccine research.

I know we have vaccines against viruses, but is it possible to make a vaccine against a protein?

We already do. The Covid vaccine is specifically against the spike protein of the Covid-19 virus, for example, and not the virus as a whole. And prions are just other proteins. So a vaccine could theoretically work, creating antibodies that can bind to the prion protein, helping the body recognize and eliminate it.

But the problem with chronic wasting disease is that, unlike Covid, a healthy version of the problematic protein already exists naturally inside our bodies. Trying to develop a vaccine that can target the unhealthy version of the protein without attacking healthy cells, that’s the challenge.

The way the disease acts in the body can also make it difficult to create a vaccine. Researchers in Wyoming did some vaccine testing and found that when elk were injected with a specific experimental vaccine, they got sick more quickly.

What we think may have happened was this: White blood cells will naturally kill the invaders and carry the remains back to the lymph nodes to teach the body what they saw and activate defenses. Getting a vaccine can speed up this process, making white blood cells better at detecting and capturing invaders.

But the problem in this case is that the white blood cells were unable to destroy the prion after capturing it. It was still contagious. So all they did was move the prion faster to somewhere where it could spread, like ants bringing poison back to their nest and spreading it to other people.

This does not mean that it would be impossible for vaccines to work, and there are groups working on the problem. I want to be optimistic. I just have reservations about it.

Plus, even if we get an effective vaccine, we’ll also need to figure out a good way to distribute it. It would be impractical to give an injection to wild animals. There is a baited rabies vaccine that has been used in the eastern United States that can be dropped from an airplane. Hypothetically, something like this could work for CWD. But there are many things we would have to overcome.

So overall, what are the prospects in terms of managing and containing CWD?

It really depends on how people react. Unfortunately, states’ responses have been mixed. Some take this very seriously, but some states try to sweep things under the rug. I hope that within our lifetime this will happen in every state in the United States except Hawaii.

And then what? Do you think this will eventually go away? Or will we just have to live with it?

I think it will be like Covid. This will never go away. It may not be a big deal in 100 years, but it will still be there.

And fingers crossed it never jumps to humans?

Yeah, well, cross your fingers tighter.

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