The slow march of chronic wasting disease

Part of what made mad cow disease (bovine spongiform encephalopathy, or BSE) such an insidious health problem was its slow creep. The peak incidence of the notorious disease it caused in human beings occurred in 2001. The peak of the disease in cows, however, had been much earlier, in 1993. And those sick cows had typically picked up the infectious agent from contaminated feed years before.

The infectious agent -- a misfolded protein called a prion -- caused disease very gradually. Opportunities to have headed off the epidemic early existed, but they were ignored for reasons that seemed economically and scientifically plausible (to the decision makers, anyway) at the time.

Now epidemiologists are growing more worried about another prion epidemic that has been gathering steam for decades: chronic wasting disease (CWD). The differences between it and mad cow disease are important but so, too, are the parallels.

Both are diseases that affect animals with major economic value. Both, at least in the early stages of their epidemics, seemed like they would be containable and non-threatening to humans.

CWD might nonetheless be scarier than BSE because it is much more easily transmitted, and has already moved between different species. In recent years, it has spread to 17 American states and two Canadian provinces. Nothing yet shows that it causes disease in humans, but that's no guarantee that problems won't eventually come to light. And even if humans didn't create the CWD problem as they did with mad cow disease, that will be little comfort if poor wildlife management decisions allow the epidemic to spread unchecked.

I've had occasion to think about CWD while hanging out recently with my friend and sometime collaborator Jay Ingram. (I'm writing these words while lecturing at the Banff Science Communications program, which Jay co-founded.) Jay, an icon of science popularization in Canada, was the long-time host of the CBC science program The Daily Planet and, before that, the radio show Quirks and Quarks. He has just published his 12th book, Fatal Flaws: How a Misfolded Protein Baffled Scientists and Changed the Way We Look at the Brain (HarperCollins 2012).

Jay's book reviews the array of peculiar neurological diseases known to be caused by prions -- not just BSE and CWD, but also scrapie in sheep and kuru in the Fore tribe in New Guinea, who had briefly taken up the custom of eating the brains of late loved ones. It then also explores the idea (which is still just an intriguing possibility at present) that some other neurological disorders might be thematically linked to kuru and BSE by an underlying pathology of bad protein folding. (By coincidence, I touched briefly on this subject, too, in a column from this past February.)

Jay is unabashed in calling CWD "the most important prion disease today." More abnormal prion biomass is tied up CWD than in any other such known disease in the world, he says. "It's more numerous, it's more threatening to wildlife, and it may even be threatening to people."

Chronicles of a wasting disease

The prions that cause CWD currently afflict only certain members of the deer family, or cervids: mule deer, white-tailed deer, elks, and mooses. It is found only in North America and in South Korea. (The disease reached South Korea through deer imported for their antler velvet, which local folk medicine prizes as an alleged aphrodisiac.)

The disease was first identified in 1967 in deer held at a research center in Colorado. After popping up in other captive deer and elk elsewhere, it eventually started appearing in wild populations in the early 1980s.

Because the 1967 sick deer may have kept near sheep, some researchers have always suspected that scrapie prions passed from the sheep might have triggered the emergence of a new abnormal prion in the deer. However, epidemiologic models hint that the disease might have been around longer -- perhaps even by decades -- and simply never been noticed. CWD's origin is therefore still a mystery.

As the name implies, symptomatic animals with CWD gradually lose drastic amounts of weight, show zombielike listlessness and repetitive behaviors, and drool and urinate copiously. The disease is always fatal: once animals become symptomatic, they typically die within a few months.

Yet the incubation time for the infection is long and highly variable. In captive deer and elk, CWD most often shows up in animals between the ages of two and seven, but it has presented in an elk 15 years old. When the animals become infectious is unclear, but most researchers assume that it long predates the appearance of outward symptoms. And even a few years, Jay observes, "is plenty of time for a deer to move hundreds of kilometers from where it was originally infected."

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From an infectious disease standpoint, CWD is far more frightening than BSE. In BSE, the infectious prions are normally confined to tissues of the nervous system. People get exposed to BSE prions in mad cow beef only because slaughterhouse conditions often splatter brain matter indiscriminately, and it only takes a milligram or so of infected tissue to transmit the prions.

In CWD, however, the bad prions manifest throughout an infected animal's tissues and secretions: they show up in its saliva, its blood, its bones, its feces, its urine, and its muscles.

Most often, CWD probably passes from one animal to the next through direct contact, when an infected deer licks or drools on another member of its herd. But scientists also consider it highly possible that some animals get infected indirectly by things the sick deer contaminates. "So a perfectly healthy-looking deer can be running along the river valley with chronic wasting disease prions in it and stopping at a salt lick and spreading them around, or dying and leaving them in the soil," Jay says.

A persistent but picky menace

What makes this contamination scenario so plausible is that abnormal prions are stunningly hard to destroy. They retain their infectious properties even after treatments with heat, chemicals, and enzymes. No one knows how long they might remain virulent in nature.

In 1985, the Colorado Division of Wildlife tried to decontaminate the pens at the research facility in Fort Collins where infected animals had been kept, and where newly introduced animals somehow kept catching CWD. Workers treated the soil with chlorine, carted most of it away, repeated the chlorine treatment on what was left and let the pens sit vacant for a year. They then brought new deer into the pens... and watched them develop CWD, too.

The good news is that CWD has so far not proved to be naturally infectious to other deer species, livestock, or other animals. Cattle have proved resistant even when penned alongside sick deer or given infected feed.

In laboratory tests, scientists have succeeded in transmitting CWD into cattle, mice, ferrets, monkeys, and goats through the extreme recourse of injecting infected brain matter into their brains. Even then, the transmission rate was fairly low.

Yet the nature of the barrier that stops diseases from hopping between species is poorly understood (many experts didn't initially think the BSE prions could infect humans, which is one reason the mad cow outbreak in the U.K. wasn't taken more seriously at the time). Researchers are therefore not at all confident that CWD might not find its way into other deer species, or even non-cervids, in the future.

A danger to humans?

Of course, the most terrifying idea is that CWD would hop the species barrier into people, probably through the consumption of tainted meat. Fortunately, the limited information available to science about that prospect is mostly encouraging.

One study from 2000 found that under cell-free laboratory conditions, CWD prions can make human prions fold abnormally at a low rate. The relevance of that demonstration to human transmission is highly questionable, however.

Epidemiologists have looked into a number of cases of people who died of what was diagnosed as an atypical case of Creutzfeldt-Jakob disease after eating deer or elk meat (or other exposures to those animals' products). Their conclusion was that the evidence possibly tying those deaths to CWD was weak. The patients generally had not eaten venison from areas where CWD was known to occur, if they had eaten venison at all.

What will surely be one of the best tests of humans' susceptibility is an ongoing study of people who attended a cookout in Oneida County, N.Y., in 2005. One of the deer used for venison at the event turned out to be CWD positive, and at least 81 people who ate or helped prepare food were exposed to it. Final results of the study are not due until 2014, but Jay reports that according to the scientist overseeing it, none of the participants shows any of the symptoms that are commonly linked to prion diseases.

The economic threat

Nevertheless, even if CWD's spread is confined to cervids, it could be economically devastating. According to a 2009 report from the Quality Deer Management Association, U.S. deer hunters spent $12.4 billion on their pastime in 2006. If worries about CWD imperiled the sport's popularity, it could deal a blow to many state economies.

For example, in July two mule deer with CWD were found in Texas, raising concerns that the white-tailed deer (a much more heavily hunted species) could also be at risk. The sport is worth $2.2 billion to Texas annually.

The problem could become far worse in Canada if CWD's ongoing spread northward in Saskatchewan was accompanied by infection of a new species: caribou. The First Nation people native to the north depend heavily on caribou for their livelihoods, and the animals figure prominently in their traditions. To quote from Jay's book: "It's been estimated that in meat value alone, the northern caribou herds are worth $100 million, but their cultural value to the people who have lived with them for centuries is beyond any dollar value."

Managing the risk

I asked Jay if he thought the CWD problem in North America had reached its current state because the authorities had missed good opportunities to throttle it when it was more containable. He suspects not. "It would have required a lot more foresight than existed in the late '60s through the '80s to choke this off," he says. The long incubation period of the disease and the highly migratory nature of deer make it very difficult even now to know where the leading edge of CWD's spread may be.

Nevertheless, local authorities may feel a lot of pressure to cull herds wherever CWD-infected animals appear. Whether that will always work is uncertain. In New York State, for example, aggressive slaughter after the discovery of the first few sick animals seems to have kept the presence of CWD contained to Oneida County. In Wisconsin, on the other hand, attempts to eliminate the infected in recent years have failed to halt its spread.

To try to head off the worst-case scenarios for CWD, ongoing surveillance of deer populations and further aggressive research on the disease may be essential. Jay was particularly complimentary about some promising work at the University of Saskatchewan on the development of an oral vaccine that could be fed to deer and elk to curb the spread of CWD in the wild. "But it's going to be very tricky even with a vaccine," he cautioned.

The absolute priority is of course to prevent CWD, an illness about which we still know very little, from ever becoming an epidemic threat as mad cow disease did in the U.K. "For now it seems like a remote possibility," Jay writes in Fatal Flaws, "but then, so did BSE."

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Top image: Elk buck at sunset. (Credit: National Park Service)

This post was originally published on Smartplanet.com