Scientists say a new type of bacteria is to blame for at least four cases of a potentially severe tick-borne illness in Minnesota and Wisconsin.

All four patients experienced fever, malaise, headache and other symptoms, but recovered after treatment with antibiotics, according to a report in Thursday's New England Journal of Medicine.

The disease, called ehrlichiosis, was caused by a previously unknown bacteria that appears to be spreading in the region, according to the report.

The report was led by scientists at the Mayo Clinic and Minnesota Department of Health. They said the newly discovered species, which has yet to be named, has been found in more than 25 people. It is carried by deer ticks, which also spread Lyme disease.

"Before this report, human ehrlichiosis was thought to be very rare or absent in Minnesota and Wisconsin," said Dr. Bobbi Pritt, a Mayo Clinic microbiologist who helped lead the study, in a statement released by Mayo. As a result, she said, "physicians might not know to look for Ehrlichia infections at all."

In severe cases, the illness can affect the lungs, kidneys and brain and require hospitalization. The infection is rarely fatal, Mayo officials said.

The investigation began after a Mayo Clinic technologist in Eau Claire, Wis., noticed a strange test result in 2009 and "raised a red flag," officials said. Last year, Minnesota health officials put out a health advisory warning that it was seeing increasing reports of ehrlichiosis in people.

Mayo scientists said they screened thousands of blood samples from across the country but found the new bacteria only in specimens from Minnesota and Wisconsin.

The researchers also reported finding the new bacteria in 17 of 697 deer ticks collected in Minnesota or Wisconsin.

Maura Lerner • 612-673-7384