Chronic wasting disease (CWD) is a progressive and degenerative brain disease caused by abnormal proteins known as prions. The disease, which is always fatal, has been identified in deer, elk and other cervids in 26 states — including Minnesota — as well as in Canada, Scandinavia and South Korea. It appears to spread through infected feces, urine and saliva.
CWD is very difficult to contain. An infected animal may not show visible signs of the illness for up to two years — until a few months before it dies. During that time, the animal continuously sheds infectious prions into the environment. The prions are extraordinarily resilient. They can persist in the soil for years, withstanding both extreme heat and extreme cold. Research also suggests that the prions can be taken up from the soil into the roots, stems and leaves of plants — offering another potential pathway for spreading the disease.
CWD has not yet crossed to humans, but some infectious disease experts are concerned that it may. They point to other prion diseases that have already made that crossing. One is variant Creutzfeldt-Jakob disease (vCJD), which humans can acquire by eating meat from cattle infected by bovine spongiform encephalopathy (BSE) or “mad cow disease.” Another is kuru, which was identified among people in New Guinea who ate the brains of their infected loved ones during funeral rituals. (That practice was stopped in the 1960s.)
In an article published recently in the journal mBio, a group of experts describe their concerns about CWD and call for greater funding for researching and diagnosing the disease. One of those experts is Michael Osterholm, director of the University of Minnesota’s Center for Disease Research and Policy (CIDRAP). MinnPost recently spoke with Osterholm about CWD. An edited version of that conversation follows.
MinnPost: How much of a threat is chronic wasting disease to humans right now?
Michael Osterholm: We’re in a real stage of discovery. Even though this disease has been present in deer dating back literally to 1967, when it was first identified in captive mule deer in a Colorado research facility, the number of animals that are infected has grown dramatically. In 2000, it was found in five U.S. states and one Canadian province. By 2010, it had spread to 17 states and two provinces. It’s now found in 26 states and three provinces.
MP: Including Minnesota.
MO: Yes. It’s now also been documented in South Korea, where we know that we transferred the prion in infected cervids there. But it’s also been found in Finland, Norway and Sweden, which may be an entirely different emergence, not related to what happened in Colorado. The disease is transmitted horizontally, through infectious body fluids, such as saliva, urine and feces. One of the problems is that once the prion is excreted into the environment, it may last for many, many years. This thing is heat stable up to 1,100 degrees Fahrenheit.
MP: That makes it amazingly resilient.
MO: Yes, and that’s a real challenge.
MP: What makes you concerned about it jumping to humans?
MO: We already have this type of illness in our communities. We now have well over 200 [documented] cases of variant Creutzfeldt-Jakob disease. And there have been at least 492 cases of what we call iatrogenic CJD cases. Over 95 percent of those cases were related to the administration of prion-contaminated human growth hormone and gonadotropins, as well as from cadaver grafts [such as cornea transplants] and blood transfusions, where people didn’t realize that these donations were coming from CJD-positive individuals.
MP: Medical facilities test for that today, right?
MO: Yes. There is a screening now, and also a great deal of emphasis is placed on not using material like this from anyone who died of an illness that potentially could be Creutzfeldt-Jakob disease.
MP: Just to be clear. Although people have become infected with Creutzfeldt-Jakob disease from eating BSE-contaminated beef, there have been no cases of people becoming infected with CWD from eating contaminated deer meat.
MO: That’s where people take some comfort. But I remember doing risk assessment estimates on BSE back in the 1980s, and [experts wondering] whether or not human cases would occur. We had numerous examples of public health officials, as well as agricultural officials, saying, “Don’t worry. It’s never happened. It’s not going to happen.” And, of course, in 1996 we saw the first human cases. So today we can take no comfort in having no human cases yet. That doesn’t tell us anything. If you have a [prion] disease, you may have a 10-or-more-year incubation period. … Also, the prions continue to change. As they’re passed from animal to animal, we’re seeing selection of prions that are what we call more zoonotic, meaning they have a higher potential to transmit to another animal species — or humans — than, say, the prions that were documented five or more years ago.
MP: What else are you concerned about with CWD?
MO: In bovine spongiform encephalopathy, the primary source of the prion was in the brain, in the central nervous system, meaning the spinal nerve. And unless that material got into the meat, the actual muscle of the animal, the risk [of infection] was very low because there was virtually no prion activity in the muscle. That is very different with cervids. It’s now well documented that there are very high levels of prions actually in the muscle. And these prions, as opposed to what’s in the brain and in the spinal column, look more zoonotic to us, meaning they’re prions that are closer to the kinds that we would expect to be potentially transferred into humans. We don’t know what the risk is, but I would never tell you it’s zero. I’m not going to tell you it’s going to happen tomorrow, but at the same time, having been through BSE and remembering how all those people provided all those assurances only to be proven wrong 10 years later, I worry about it.
I also worry because the last thing we want to do is reduce hunting. Hunting right now is a very critical population management tool. Wisconsin last year killed 340,000 deer. We killed over 200,000 deer in our state. If you don’t reduce the deer populations, the spread of the disease would even be more dynamic.
MP: In the paper, you and your colleagues call on state and federal agencies to immediately fund research and develop new diagnostic tests for CWD.
MO: There have not been adequate resources at the national level. In many cases, [state agencies] have had to take funds out of deer hunting license fees just to try to manage the situation. There needs to be much more put forward in terms of research.
MP: Why the urgency?
MO: Well, it’s a very important economic issue. Hunting in Minnesota [contributes] an estimated $1 billion to the state’s economy. That’s just one state. So there’s real potential here for economic harm if the disease continues to spread and people defer or not hunt because of chronic wasting disease. But also, there’s the fact that the numbers are increasing. We [the public health community and the Alliance for Public Wildlife] estimate that last year between 7,000 and 15,000 chronic wasting disease-affected animals were consumed. That number is increasing quite dramatically — by an estimated 20 percent per year. There are counties in Wisconsin right now where half the deer or more are now positive for this.
MP: And it’s not always obvious which deer are infected.
MO: Unless you have testing that is readily available, which in most cases it’s not, hunters wouldn’t know whether their animal is positive or not. Many of these animals show no signs or symptoms. We need a point-of-hunting testing device, much like you might take a home pregnancy test, where basically you could be in the field and any hunter could use this test and know right away if their animal was infected or not.
MP: Because right now it takes weeks to know, right?
MO: It takes weeks, and in many cases it’s not readily available. So that’s a challenge. In some locations, like Minnesota, a real effort has been made to try to get testing done as quickly as possible in CWD-infected areas. So in some cases, you may get it back in a few days. But for many parts of the country, testing like that just doesn’t exist. That’s something we have to remedy. We need to both build testing capacity and rapid turnaround. We need to do hunter education as to why they should be testing their animals, and we need to devise new and better tests. That’s the kind of research that we need to support.
MP: What should hunters and others be doing?
MO: If they’re hunting in a chronic wasting disease-infected area, where it’s documented that the prion is there, we encourage them strongly to have that animal tested before it’s consumed. One of the challenges we have today, which we recognize is a real problem, is that these animals are often taken to local meat packing plants, where you don’t get back the same animal you put in. You get back the weight of meat. It gets mixed in with other deer. … Imagine trying to clean [prion-contaminated] meat packing plant grinders. There is no way that it could be adequately sterilized.
MP: You also talk in the paper about hunters leaving their deer’s entrails in the woods. That practice can also spread the disease.
MO: It’s not just the entrails. Where do you dispose of all the rest of the carcass — the hide, the bones, et cetera? And many deer carcasses are moved great distances. If you look at the upper Midwest, people from most of the lower 48 states hunt here. And when they get a deer they take it back home. So you have people moving the carcasses to other locations, even after they’ve basically gutted it. There still is potentially infectious material there, wherever they deposit it.
MP: But you don’t want people to stop hunting.
MO: No. But I would not eat venison from an animal killed in a chronic wasting disease [surveillance] area without it first being tested. Absolutely not. [A map showing the four CWD surveillance areas in Minnesota can be found on the Minnesota Department of Natural Resources’ website.]
MP: What else do you want the public to know about CWD?
MO: I think the message here is that although CWD is a real concern, we can manage it in terms of human risk by developing the testing and making it available. And then, you know, we need more hunter education. We also have a lot of work yet to do on the meatpacking side to understand what the risks are there. And we need national leadership badly.
FMI: You can download and read the mBio paper at the journal’s website. mBio is published by the American Society for Microbiology.