What happens to road-killed deer, and does it help stem CWD spread?

Heather Paul / Flickr cc

Roughly 1.8 million deer-vehicle crashes are reported each year in the United States, typically peaking during mating season and the end of Daylight Saving Time in November, although the vast majority—typically those that don't involve motorist injuries or extensive vehicle damage—likely go unreported.

While a road-killed deer or other cervid (member of the deer family) may be a safety hazard and unwelcome sight for motorists, chronic wasting disease (CWD) experts view it as a potential reservoir of deadly infectious CWD-causing misfolded proteins called prions, which can persist in the environment for years and pose a risk to other animals.

The disposal of those deer carcasses could have implications for both human and animal health, but management practices vary widely by jurisdiction, with state, county, and city officials often taking different, usually budget-based approaches with differing levels of effectiveness, a concept that is itself only vaguely defined.

And with most deer dying naturally throughout the landscape and CWD management efforts largely failing to stem disease spread, it begs the question of how much it matters.

'Deer strewn all over the roads'

In Wisconsin, where an estimated 30,000 deer-car collisions are reported each year, deer carcass-disposal methods appear nonexistent, said Scott Hygnstrom, PhD, professor emeritus of wildlife damage for the School of Natural Resources at the University of Wisconsin-Stevens Point.

"I don't think any of them are disposed of in approved locations," he told CIDRAP News. "The WDNR [Wisconsin Department of Natural Resources] used to pick up road-killed deer, and township and county road crews used to at least slide them off the roads and shoulders and into the ditches, but a governor a few administrations ago banned the practices, and now there are dead deer strewn all over the roads in various shades of decay for everyone to see. We could do better."

In neighboring Minnesota, the Minnesota Department of Transportation (MNDOT) leaves road-killed deer found on state highways and interstates in the same area or as close as possible, MNDOT Spokesperson Anne Meyer told CIDRAP News in an email. About 1,200 deer-car collisions—almost certainly an enormous undercount—are reported to authorities each year in the state, per the Minnesota Office of Traffic Safety.

"Crews will drag the animal into the ditch, back slope in the tall grasses, or cover them with compost (typically wood chips, mulch, or dirt) if they are in a visible area," she said. "For some areas of the metro, this may not be possible—We do have a deer composting area located in Anoka [a Twin Cities suburb]."

But if the area is mowed by an adjacent landowner or is by a driveway, MNDOT takes the deer to a different location as close to the original site as possible. "In the urban area, we look to take [them] to a landfill, or we will take them to the closest rural area," Meyer said. "No difference between freeways and non-freeways."

Disposal methods vary by jurisdiction

Most state roads, however, are maintained by counties and cities, and their carcass-management practices range widely, said Michelle Carstensen, PhD, wildlife health program supervisor at the Minnesota Department of Natural Resources (DNR).

And the DNR can only make recommendations because it has no jurisdiction over the practices. "In areas like southeast Minnesota, for example, counties just leave them lay on the side of the road on certain county highways," Carstensen said. "They have no staff that goes out and picks them up."

If the public complains about dead deer, cities or counties without staff or the right equipment sometimes hire a contractor to haul them away.

"We request that they don't move carcasses out of areas where they died for reasons of disease spread, especially chronic wasting disease, and we request best disposal methods" such as disposition in lined landfills or incinerating landfills, or, barring that option, onsite burial, she said.

"But that's not necessarily what always happens. Some are allowed to lay, some are covered with woodchips, some are brought to a landfill, some are buried in a pit on city or county land. When people get involved in moving them, that's where the risk becomes more enhanced, because where are the carcasses going?"

It's a common practice for predator hunters and fur-trappers to collect road-killed carcasses for use as an attractant. Many times, the location where the carcasses are [used] as bait is counties away, or maybe even a state away, from where they're picked up.

Russ Mason, PhD

Russ Mason, PhD, retired from the Michigan Department of Natural Resources, said there's value in leaving road-killed deer close to where they're found.

"But leaving carcasses on the roadside where they're easily found by scavengers, probably increases the likelihood of CWD spread through contaminated droppings or scat," he said. "As well, and perhaps more important, it's a common practice for predator hunters and fur-trappers to collect road-killed carcasses for use as an attractant. Many times, the location where the carcasses are [used] as bait is counties away, or maybe even a state away, from where they're picked up."

Which method is best?

The University of Minnesota's Veterinary Diagnostic Laboratory in St. Paul operates a digester for animal remains that uses alkaline hydrolysis. This process can neutralize prions, but it is highly expensive, labor-intensive, and low-throughput. "It does denature prions, but it was never meant to be a mass disposal mechanism for a pile of deer that don't have CWD," Carstensen said.

And the result of alkaline digestion, a strong basic solution, requires special handling, said Jean Bonhotal, MS, director of New York's Cornell Waste Management Institute. "If you put it into a tanker that is not glass-lined, it will eat pinholes through the metal truck body," she said.

In New York, all landfills are toxic waste–level facilities that divert the leachate (wastewater from the landfill) to a wastewater-treatment plant so it doesn't get into the groundwater, but Bonhotal said, "I don't think anybody has the answer to whether CWD survives in the landfill. Can it leach it into the water and go back to the wastewater treatment plant? And does that cause a problem in the long run?"

Many states compost dead deer, using 2-foot layers of woodchips all around the animal, to discourage scavengers like dogs, eagles, coyotes, and bears from digging, with some also using cement barriers and electric fences in case carcasses don't always get covered right away, she added.

Bonhotal doesn't recommend putting carcasses directly on the ground or turning the compost.

"I don't like the idea of just leaving a deer in the ditch," she said. "If I put an animal on the ground and then cover it with woodchips, there's no air going under that animal, so it will liquify and go into the ground, versus the air coming around and creating that chimney effect and naturally aerating them. I don't advocate that people do any turning, because we don't want to open up those smells, because they will attract animals and turn people off to the process."

Ideally, a composted carcass is ultimately reduced in size by at least half by shredding before it is incinerated, because the heat generated by composting is insufficient to disable prions, she said.

Roadkills don't make the best sentinels

But road-killed deer are rarely tested for CWD. For example, the Minnesota DNR doesn't believe that the very small potential benefit of sampling the animals justifies the considerable risk and expense of dispatching staff to accident sites.

It's a needle in the haystack trying to find a roadkill that has CWD.

Michelle Carstensen, PhD

"It's a needle in the haystack trying to find a roadkill that has CWD," she said. "Not that we don't care; it's just that it's not super valuable for a sampling scheme, and very labor intensive and a risk to send our staff out there for a couple of roadkill and not get hit on the side of the road, too."

Carstensen said she would welcome a DNR collaboration with CWD experts, MNDOT, and county representatives to develop best management practices (BMPs) for road-killed cervid carcass disposal that could also be shared outside the state. "Because it's not just us who have this problem," she said. "So some BMPs could be really helpful and made available to all of these jurisdictions, and maybe folks would start to do things a little safer."

Mason, from Michigan, said that, to date, CWD management has been viewed much too narrowly. "Obviously, it's a hot-potato issue," he said. "The problem is that whatever chance we have of effectively slowing the spread of this disease depends on more than deer hunters, deer hunting practices, and the wildlife agencies with the responsibility for managing deer."