Until very recently it was thought that just one bacterium was to blame for causing Lyme disease in humans. But it turns out that a second, related bug can cause it too.

In 2013, during routine testing of bacterial DNA floating around in the blood samples of people suspected of having Lyme disease, researchers at the Mayo Clinic in Rochester, Minn., realized they were looking at something different.

"We detected this result which was positive, but it was clearly different from what we would have expected for Borrelia burgdorferi, which at that time was the only known cause of Lyme disease in the U.S.," says Dr. Bobbi Pritt, a microbiologist at the Mayo Clinic.

When they sequenced the genome of the bacterium, they realized it was different enough to be considered a new species. It's been dubbed Borrelia mayonii, after the Mayo Clinic. News of the new species was published this month in the journal The Lancet Infectious Diseases.

The new species hadn't appeared during routine tests on thousands of other samples over the course of a decade. Then, over the span of two years, it appeared in six patients out of about 9,000 tested for Lyme disease. They were all residents of Minnesota, Wisconsin or North Dakota.

The patients' symptoms were a little different from the usual Lyme disease cases. Instead of the telltale bull's-eye pattern associated with B. burgdorferi, rashes on these patients were diffuse or spotty. In addition to the fever, headache, rash and neck pain that accompanies the usual form of Lyme disease, patients who had contracted the new species of bacteria also experienced nausea and vomiting.

"This organism doesn't behave completely like the Lyme disease that we all know," says Pritt.

There were other odd symptoms. A child was nearly impossible to wake up from sleep. An adult had trouble with vision, and was seeing double. Two patients were hospitalized. Pritt says all have now recovered, except for one with continued arthritis.

Because the species had not been identified in the thousands of samples that were routinely tested before 2013, Pritt says, it's likely that it either recently emerged as a new organism through mutations of an existing bacterium, or it recently came into contact with people.

"Maybe it infected woodchucks and no one ever tested them," she says. "But what we can say is, it's a species that no one has ever described before, and it's clearly infecting patients."

The bacterium was isolated alive from the blood of two patients, and was subsequently found in black-legged ticks in two parts of Wisconsin.

Both species of Lyme-causing bacteria are carried by the tick Ixodes scapularis, also known as the black-legged or deer tick, which has continued to spread its range across the U.S.

"Lots of people are encountering ticks where they didn't encounter them 20 years ago," says Rebecca Eisen, a research biologist with the Centers for Disease Control and Prevention who published the latest map of tick populations in the U.S.

"It's a living organism, so the range of the tick changes, and so the likelihood of people encountering these ticks changes," says Eisen.

This new bacterium appears to be relatively rare so far. Entomologists found that of 600 ticks collected across Wisconsin, 3 percent tested positive for the new species of bacteria. The older bacterial species is typically found in 30 to 40 percent of black-legged ticks, says Pritt.

Doxycycline is used to kill both species of bacteria. The issue, Pritt says, is for patients and physicians to recognize the slightly different constellation of symptoms associated with this form of Lyme disease, so treatment can be started promptly.

It's also unclear what is behind the higher bacterial load detected in the blood samples of patients with B. mayonii, and if it's responsible for the more severe symptoms that the six patients experienced.

"It just seems like every time you turn around, there's a new tick-borne disease. So when people go outside, I think they have to think about protecting themselves from ticks," says Pritt.

The best way to avoid picking up an infection from a tick is to avoid tick-infested areas, use repellent and remove ticks as soon as they're spotted.

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