TRAVERSE CITY, Mich. — A fungal disease that has killed millions of North American bats is spreading and has been detected in half of the United States, officials said Thursday.

Wildlife agencies in Michigan and Wisconsin said they had confirmed diagnoses of white-nose syndrome in tested bats, further evidence of the ailment’s rapid expansion since it first was documented in a cave near Albany, N.Y., in 2006. Cases have turned up in most states east of the Mississippi River, and as far west as Missouri and Arkansas.

Officials acknowledged they had no cure and could take only limited steps to protect the winged mammals that provide an enormous economic and ecological benefit by feasting on nuisance insects that gobble crops and trees.

“We face the loss of multiple bat species and the benefits they provide to our ecosystems and our people,” said Erin Crain of the Wisconsin Department of Natural Resources.

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White-nose syndrome is named for the fuzzy spots it plants on victims’ muzzles, wings and tails. It doesn’t affect people or other animals but interrupts bats’ hibernation, sapping their energy and fat stores, which can cause starvation and dehydration.

More than half of the 45 bat species in the U.S. hibernate during winter. Many seek out caves or mines, an ideal environment for spreading the killer fungus as bats clump together on the moist walls. No one has come up with a treatment that would kill the fungus but not the bats, said Dan O’Brien, a wildlife veterinarian with the Michigan Department of Natural Resources.

One step that can be taken is to prevent humans from spreading the fungus, officials said, which often sticks to their clothes and gear while exploring. That may explain how the disease reached the U.S., O’Brien said. It’s believed to have come from Europe, which has different bat species that are not greatly affected.

The disease has been confirmed in 25 states and the fungus has been found in three others, according to the U.S. Fish and Wildlife Service, which estimates more than 6 million bats have died.

A 2011 study led by Justin Boyles of Southern Illinois University calculated that bats save the agricultural economy $22.9 billion a year by eating crop-damaging insects and reducing the need for pesticides. They also eat mosquitoes, some of which carry West Nile virus.