By David Beasley

ATLANTA (Reuters) - Researchers have discovered a new bacteria that causes Lyme disease in humans, a U.S. health agency said Monday.

The Centers for Disease Control and Prevention, working with the Mayo Clinic and health officials from Minnesota, Wisconsin and North Dakota, discovered the new bacteria, called Borrelia mayonii, the CDC said in a statement.

Previously, only one bacteria, Borrelia burgdorferi, was believed to cause human Lyme disease, which is transmitted through bites from by the blacklegged "deer" tick, the CDC statement said.

The new bacteria has so far only been detected in the upper Midwest region of the United States, the CDC said.

Researchers discovered Borrelia mayonii after studying blood samples of patients in Minnesota, Wisconsin, and North Dakota who were suspected of having Lyme disease from 2012-2014, the CDC said. Samples from six out of 9,000 patients showed "unusual results" and prompted further study, the health agency said.

The newly discovered bacteria is similar to Borrelia burgdorferi in that it initially causes fever, headache, rash, and neck pain and later arthritis associated with Lyme disease, according to the CDC.

But Borrelia mayonii adds nausea, vomiting and widespread rash, and causes a higher concentration of bacteria in the blood, the CDC said.

There are an estimated 300,000 cases of Lyme disease in the U.S. annually, according to the CDC. The disease is rarely fatal, and most patients recover after a few weeks of oral antibiotics, the agency said.

The patients infected with the new bacteria were treated successfully with the same antibiotics used for those with Borrelia burgdorferi, the CDC said.

It is too early to say whether the newly discovered bacteria will be more or less dangerous than Borrelia burgdorferi, said Jeannine Petersen, a CDC microbiologist.

"We have fairly limited information in that our study described six patients," she told Reuters. "We need more patients in order to capture the full spectrum of those who might have less severe symptoms and those who might have more severe ones."

It is likely that the bacteria is a "recently emerged" organism since it did not appear in previous, extensive testing of blood samples of patients suspected of having Lyme disease, Peterson added.

(Editing by Karen Brooks and Marguerita Choy)