Mad Cow Disease of Deer Can be Controlled by Targeted Culling, Long-term Study Finds

Tactically culling deer populations infected with chronic wasting disease -- the deer equivalent of mad cow disease -- is an effective method of keeping the disease from spreading, according to the results of a long-term study conducted by researchers at the University of Illinois.

Chronic wasting disease, also referred to as CWD, has effectively spread to deer populations across the central US and Canada since it was first documented in Colorado in the late 1960s. Little is known about CWD's potential to infect humans, but no one seems eager to let the disease run rampant to find out.

"CWD is a prion disease (like mad cow disease) and it's 100 percent fatal. There's no current way that we can actually make the deer better, so it's important that we keep it from spreading too far throughout the population," said Mary Beth Manjerovic, a post-doctoral researcher at the Illinois Natural History Survey (INHS). "And then there's also the connection to mad cow disease. We don't have enough information yet to really understand what the impact to human health could be."

But the survey found that a systematic culling of deer in areas where CWD is present keeps it from spreading to new locations without negatively affecting overall deer population.

Between 2002 and 2012, the Illinois Department of Natural Resources annually tested 7,000 deer (either hunted, culled or incidentally killed) for CWD infection. The department also conducts aerial surveys of where deer congregate and sends sharpshooters into areas where the disease is present to cull deer.

That approach seems to be working to keep the disease from spreading beyond a 1 percent infection rate, while not harming overall deer population numbers, said lead researcher Jan Novakofski, a professor of animal sciences at University of Illinois.

"We know a lot about how far deer typically move," Novakofski said. "If they're sick, they're going to spread the disease that far. So if you find a deer that's sick, you draw that small circle and you shoot there."

Novakofski called the approach "a textbook scientific strategy for control. You reduce contact and you reduce the spread of infection with the smallest overall impact on healthy deer."

The decade-long study also revealed that hunters are killing more deer in Illinois, regardless of the presence CWD, and that deer populations have risen in spite of that.

In 2001, before the appearance of CWD in Illinois, hunters killed 147,830 deer, the researchers said. In 2012, hunters bagged 181,451 deer. The rise in kills by hunters suggests that the healthy deer population is thriving.

"We wanted to know whether Illinois hunters have fewer deer to hunt now than they did before CWD," said Nohra Mateus-Pinilla, a wildlife veterinary epidemiologist at the INHS. "We found that hunter harvest has increased, and the prevalence of CWD has been maintained at low levels for 10 years in Illinois."

The Illinois researchers compared their state's efforts with that of neighboring Wisconsin. While the CWD infection rate in Illinois has remained steady at 1 percent, in Wisconsin, where the strategy shifted from culling to simply relying on hunters to thin populations, the CWD infection rate rose from 1 percent to 5 percent, the researchers said.

"We can't find an environmental or other variable that explains the increase in prevalence except a change in management," Novakofski said, adding that keeping the disease under control is in the best interest of humans and animals alike.

"We all hope that there is never a case of chronic wasting disease in humans. We all hope that it never spreads to people or agricultural animals," he said. "If it ever does, the investment in maintaining prevalence at a low level in Illinois will be repaid a thousand-fold."

Novakofski and his colleagues' research appears in the journal Preventative Veterinary Medicine.