Chronic wasting disease research is continually evolving, leading to lots of changes in how we view and manage the disease among Michigan’s deer herd
By Russ Mason
The fourth International Chronic Wasting Disease Symposium was held in Denver this year. It was attended by about 450 prion biologists, wildlife veterinarians, agency managers, tribal representatives and farmed cervid owners from across North America and Europe. The conference theme was barriers to management, reflecting the seemingly inexorable spread of CWD to (now) 30 states, three Canadian provinces and several countries in Northern Europe and Asia. Here’s my take on the information, parsed into what I categorized as good news, cuts both ways and bad news.
Good News
We understand CWD much better than we did just three or four years ago. Mainly, this reflects the development of cervidized and humanized transgenic mouse models that have improved the speed and efficiency of research.
We have a definitive and ultrasensitive testing method (‘Real Time Quaking Induced Conversion’ or RTQuIC) that can be used for CWD detection in animals and environmental samples. Once this technique is approved by USDA Veterinary Services, one company (MN-PRO) will use it to make testing vastly more convenient, faster and less expensive for hunters.
In addition to RTQuIC, the University of Missouri and the Missouri Department of Conservation have developed a microfluidic microelectromechanical systems (MEMS) biosensor technology for rapid and accurate CWD detection and now is seeking an industry partner to commercialize the technology.
Anatomical investigations have confirmed that CWD prions invade muscle (meat) only during late-stage disease (when animals are visibly ill). The implication is that although many infected animals go untested and are eaten, overall exposure risks probably are low. Moreover, several investigators confirmed that barriers to human transmission remain strong. However, “strong” likely isn’t a synonym for impenetrable, as demonstrated with bovine spongiform encephalopathy (BSE or mad cow disease, which has killed roughly 250 people).
Creutzfeldt Jacob Disease (CJD – human prion disease) has increased over the past 20 years (now accounting for 1/20,000 deaths). This increase has led some to hypothesize that the increase in CJD could be related to CWD. So far, no link is apparent, as demonstrated by an ongoing study of mortality in hunters from Colorado (since 1995, 1.6 million hunters, 1.3 million person/years), Wyoming (since 1996, 600,000 hunters, 4 million person/years) and Wisconsin. In the Colorado sample, there were 19 CJD mortalities (19.4 mortalities expected by chance); in the Wyoming sample, there were 7 CJD deaths (5.4 expected – higher, but not a statistically significant increase). Only hunters who had consumed CWD-positive venison were included in the sample in Wisconsin. Out of 1,561 hunters, there were no CJD deaths. An important caveat is that CJD and CWD encephalopathies are virtually indistinguishable. Because the time course of human prion disease is long (> 40 years in the case of Kuru), these longitudinal studies are continuing.
Thanks again to the MN-PRO folks; prior to this year’s hunting seasons, a practical guide to decontaminating cutting tools, grinders and surfaces will be available for hunters and meat processors.
The SOP4CWD warehouse now includes surveillance and management data sets from most states east of the Missouri River and is now expanding to include data from western states. The SOP4CWD dashboard has developed a glossary of common terms and definitions for cross-jurisdictional use and, because the Wildlife Management Institute is curating the site, data sets are updated and regularly refreshed. Predictive modeling tools (including those developed by scientists at Michigan State University and Cornell) are available on the website and include packages that:
- Maximize surveillance effectiveness relative to available funding.
- Direct sampling by identifying high-risk locations predicts the impacts of regulations and habitat characteristics on disease prevalence and spread.
Data from Tennessee and Mississippi were used to demonstrate that buck scrapes and licking branches are good surveillance points for CWD prion detection. Prions are detectable for at least 1.5 years and multiple bucks visit the same licking branches and scrapes (> 39 bucks in one instance).
Interestingly, studies documented the presence of CWD prions on licking branches in areas where CWD had not yet been detected in deer. Not surprisingly, licking branch assays were more sensitive than assays of scrape soil samples (other more troubling data indicated that soil type influences CWD detection but not infectivity). The management implication is that licking branches and scrapes could be useful for surveillance in areas where landowners are unwilling to allow more invasive methods.
Dogs can be trained to detect CWD-positive feces. Within broad constraints, dog breed appears unimportant. Detection accuracies in farmed cervid samples reached 91%; detection in wild-sourced feces was lower (70%). When feces samples from the colon were used as stimuli, accuracy improved to 93%, regardless of whether donor animals were farmed or free-ranging. Most likely, ester differences between positive and negative fecal samples are being used as sensory cues (just as ester differences are used for the detection of human cancers). Ongoing field experiments focus on samples collected at salt licks and baiting sites.
MN-PRO and the Minnesota Department of Natural Resources recently used RT-QuIC in a forensic examination of an illegal carcass dump on private lands. Despite carcasses being more than six months old and spread over an 11-acre area, RT-QuIC was used to detect CWD, and microsatellite genetic methods were used to trace the carcasses to a captive cervid facility in the vicinity. Interestingly, the RT-QuIC analyses also revealed that fly larvae in the bone marrow of CWD-positive carcasses were CWD-positive. This result adds to a growing list of insect parasites (nasal bots, ticks) and scavengers (flies) that may represent competent mechanical vectors for CWD spread.
Several groups are investigating factors that increase the risk of infection in farmed cervid facilities. Overall, the research demonstrates that just over 50% of infected premises have one or more biosecurity problems (e.g., captive animals observed near fencing, lack of double fencing, water or food resources near fences, lack of fence buffers, above-ground carcass disposal, avian scavengers, feral cats). Most infected premises are also within six miles of an infected free-ranging cervid. A group at the University of Minnesota has developed a biosecurity standard operating procedure for farmed cervid facilities (cwdbiosecurity@umn.edu) that includes a numerical scoring system that can be used to identify the greatest risk(s) at any farmed cervid premise.
A scientist tests for bovine tuberculosis and chronic wasting disease at the Michigan State University Infectious Disease Laboratory.
Cuts both ways
Extensive testing shows that the CWD detected in Nordic countries over the past several years (i.e., infections in reindeer, moose, red deer) was spontaneous and not somehow imported from North America.
Unfortunately, spontaneous CWD was infectious among reindeer (but not in moose or red deer). Although factors that promote the emergence of spontaneous prion diseases are not well understood, the current speculation is that overpopulation and/or other stressors (including anthropogenic influences) might be catalysts, just as intensive breeding of sheep encouraged ovine (sheep and goat) scrapie during the 17th century. If confirmed, this speculation could be bad news for states east of the Mississippi where whitetails are over-abundant.
The development of a CWD vaccine for use with captive cervids is advancing. However, complexities in vaccine/delivery technologies mean that a vaccine for free-ranging cervids remains decades away — if it will ever happen at all.
Previous investigations had suggested that feral swine could represent reservoir hosts for CWD. New research shows that while swine can become infected, they appear to be dead-end hosts (i.e., infected swine don’t transmit disease). On the other hand, emerging evidence suggests that raccoons are competent disease reservoirs (one step forward, one step back).
Bad news
We have been unable to quantify the minimum effective dose for CWD infectivity.
Grilling, broiling or boiling infected venison makes CWD prions more available for detection by RTQuIC. The why remains unclear, but the obvious inference is that cooking infected venison may free prions, making them more available for infection.
Expanding host ranges for CWD prions are expanding the potential for new CWD strains. At present, seven separate strains have been identified. Moreover, CWD strain diversity is subject to Darwinian selection at the level of individual neurons. In other words, when an animal is infected, multiple CWD strains compete to infect each new cell. If the dominant strain is blocked experimentally, then other strains become dominant.
Prion biologists now suspect that there are silent (i.e., asymptomatic) human carriers of CWD (just as there are for BSE). Whether silent cases can activate is unknown. These same experts also suspect that CWD prions in spleen tissues might be more zoonotic than in neural tissue.
Both BSE and bovine scrapie transmit between mothers and their offspring in utero. Now, evidence from Tennessee, West Virginia and Missouri whitetails show that the same is true of deer. Infected, pregnant does have prions in amniotic fluid and umbilical tissues, and the offspring of infected does are infected prenatally nearly 100% of the time.
In Wisconsin’s southeastern CWD core zone, one in three deer are estimated to be CWD-positive. Now, studies show that CWD force of infection is selecting for deer with a 96S protein polymorphism because these animals show a substantially longer time to death (4.3 years) than other infected deer. As a result, this once uncommon polymorphism has become common. Within the foreseeable future, a reasonable expectation is that most deer in CWD zones in Wisconsin (and perhaps elsewhere) will be 96S, so the number of hunters exposed to CWD is predicted to increase markedly.
During a state (Missouri, Ohio, Wyoming, Colorado, Mississippi) and provincial (Alberta, Saskatchewan) roundtable, panelists agreed that social and political resistance fed by inaccurate social media were major challenges to CWD management. Missouri, Mississippi and Ohio reported that none of the measures implemented to date (except for targeted culling) had any demonstrable effect on CWD prevalence and spread. Colorado, Wyoming and the Canadian provinces reported better success in managing prevalence (albeit not spread) for two reasons. First, cervid populations were smaller, and second, most hunting occurs on public lands where regulations can be more consistently implemented. All panelists stated that reducing deer densities and buck age structures were likely the most effective components of CWD management. Baiting and feeding restrictions, scent bans and carcass disposal in the absence of population reduction were likely ineffective or worse (because such regulations perpetuate the false impression that CWD can be managed by these measures alone).
Conclusions
On the one hand, what we understand about CWD — where the disease came from, its impacts on cervid populations and whether CWD is zoonotic — is considerably better than just a few years ago. Soon, rapid, inexpensive and convenient CWD detection strategies and clear, practical guidance for decontaminating equipment will be available, meaning that hunters can avoid exposures if they choose.
On the other hand, the management fundamentals for controlling the disease on the landscape haven’t changed. At least from the experts’ point of view, meaningful reductions in deer numbers and/or the average age of bucks are the key features of any successful plan to contain and manage CWD.
The elephant in the room, at least from my perspective, is whether there are still sufficient numbers of hunters willing to harvest enough deer. While I – and most managers – still believe that there are, if this belief is wrong, we (hunters and managers) will confront an unpalatable choice: Either identify other strategies to effectively reduce deer numbers or accept that CWD prevalence and spread will continue to grow.
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