The reindeer at the Minnesota Zoo are all still alive and healthy this fall, showing early promise that the zoo may be able to keep an encroaching deer virus at bay.
The virus — epizootic hemorrhagic disease (EHD) — killed four of the zoo's reindeer last year. The disease has long been widespread in the southern United States but has only recently showed up in Minnesota, carried in by a biting gnat called a midge.
Zoo veterinarians tested a new vaccine this summer on roughly 30 of their animals, including reindeer, bison, moose and pronghorn, all of which can be killed by EHD.
It's too early to tell if the vaccinations were successful, said Alex McFarland, veterinarian intern for the zoo. Lab work, which will be completed in the coming months, will show if the vaccine did, in fact, raise antibodies or if any of the animals had already built up resistance to the virus from when it spread last year, he said.
"The big thing is, we did not see any mortalities or illnesses this year," McFarland said. "There's no way to know until the results come back if that was because of the vaccine or something else."
If the vaccines worked, they could be a useful tool in keeping hoofed animal populations alive in zoos and farms throughout the Midwest as the midge continues to spread.
EHD has been creeping closer to Minnesota for years as the state's climate gets wetter and warmer.
It was found for the first time in the state in 2018, on a deer farm in Goodhue County. An outbreak in 2019 killed at least 20 wild deer in Stearns County. And the disease is likely responsible for about 20 dead deer found this month in Houston and Winona counties, according to the Department of Natural Resources (DNR).
The known cases are likely an undercount because deer carcasses are often found when they are too decomposed to test, said Barb Keller, big game program leader for the DNR.
"It's entirely possible that it's been in Minnesota in years past and we just haven't picked up on it," she said.
The problem is that the midges that transmit the virus from deer to deer have been surviving farther north, said Taylor Yaw, who leads the zoo's health department.
"Normally, you think of this as a southern disease in places like Texas and Georgia," he said. "The midges like these warm, humid areas. Now that geographic range is pushing north."
Fortunately, the disease can only cause so much damage to wild deer in a single year. The bugs are killed off by the first or second freeze, Keller said.
"In certain years we might see severe outbreaks, but the damage is generally localized and the disease is not 100 percent fatal, so both individuals and populations can recover," she said. "If we have a later and later frost then the time period for the midges to transmit the disease would be longer."
Droughts and dry years, such as this one, also make deer more vulnerable, Keller said.
The midges live near open water. As ponds, creeks and other water sources dry up, deer are forced closer together around the available standing water. The virus causes a fever, which forces sick deer to seek out more water than they normally would. The midges bite the sick deer and easily spread the virus to the others around that source of water, Keller said.
EHD works quickly, killing deer within a few days. But those that survive build up a strong immunity to it.
Because of that, biologists aren't as worried about EHD's spread into the state as they are about the other disease stalking the Midwest's deer: chronic wasting disease. Chronic wasting disease is always fatal to deer and it spreads through bodily fluids — as well as the soil, water and food that any infected deer's fluids touch — making transmission much more difficult to control.
EHD is not dangerous to people. Still, the DNR recommends that hunters do not eat any animal that looks sick.