Minnesota wildlife health specialists have learned something from a year ago when a deadly bird flu outbreak began killing wild birds like bald eagles, hawks and owls: the virus remains a serious threat even if signs of its presence ebbs at times.
Now, with spring migration ahead and reports of new cases in Central and South America, attention is heightened. Migratory waterfowl, which naturally host some form of the virus, are known drivers of transmission.
"This outbreak continues to unfold like no other outbreak we have ever seen," said Victoria Hall, executive director of the Raptor Center at the University of Minnesota. Each year the center treats upward of 1,000 sick and injured birds.
The three most common species treated are great horned owls, bald eagles and red-tailed hawks. All were seen exponentially more in 2022, said Hall, attributed to an outbreak that originated in Europe. As of March 6, the center has tested 1,051 birds, with 215 positive cases. All but one raptor with bird flu died.
Great horned owls especially have been hard-hit by several factors, Hall said. They are cagey hunters and more likely to eat an infected waterfowl, or hunt in areas where geese and ducks live and are shedding the virus through bodily secretions and feces. The virus is endemic to swans, ducks and geese, some of which show no signs of illness. Plus, Minnesota has a significant waterfowl population that overwinters.
The owls in some cases were bringing the virus back to their nests — and their young, which already had hatched when the outbreak was peaking last spring.
"Entire family units would come in sick at one time, which we did not see with other species," said Hall, recalling a case of four or five owls together. Since last March, 92 of 215 positive cases have been great horned owls. Only one survived and was released back into the wild.
"Any time you are taking out breeding pairs, you have to think about population impact," she added.
As the virus shifts, specialists have shifted tactics, too, Hall said.
The center is working with the Center for Disease Control and wildlife managers with the Department of Agriculture, in addition to the Minnesota Department of Natural Resources, dialing in on how to respond and sharing knowledge.
The Raptor Center is testing for antibody levels in birds arriving at the center for reasons other than the flu to learn if raptors are surviving infection in the wild, Hall said.
"There is so much about wildife we are learning in real time, how this virus is working its way through," Hall said. "We can generate some of the science and tell the community what we are seeing."
For its part, the DNR partners with federal wildlife officials to test waterfowl seasonally for avian influenza and has for several years. DNR staff were out last week in the south metro capturing dabbling ducks like mallards, with a goal of 110 swab samples this winter, said Wildlife Health Program Supervisor Erik Hildebrand.
The agency also investigates when there are reports of five or more sick or dead birds over a short period of time in a localized area. Carcasses are collected for testing at the University of Minnesota Veterinary Diagnostic Laboratory. Bird flu was confirmed in the deaths of American crows found in Hennepin County in mid-to-late January.
The Wildlife Health Program has to be mindful of all wildlife. Last May, DNR specialists investigated and confirmed the first case of bird flu in a mammal in Minnesota, when a sick fox kit was found in Anoka County. Sporadic infections have been reported, too, in foxes, skunks, bears and sea lions in the United States, Canada and other countries. Most recently, the World Organization for Animal Health reported six more U.S. cases, including a river otter in Wisconsin.
Hildebrand said people who see dead or dying birds or other wildlife should call the DNR's hotline at 888-646-6367.
"I hope we don't see what we saw last spring," he added.