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.

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. 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. 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.