* 25 people in Wisconsin, Minnesota have been infected
* Bacterium causes flu-like illness known as ehrlichiosis
By Julie Steenhuysen
CHICAGO, Aug 3 (Reuters) - Researchers in Minnesota have discovered a new bacterium carried by deer ticks that has caused flu-like symptoms in at least 25 people in Minnesota and Wisconsin.
The bacterium, which is yet unnamed, is part of the Ehrlichia genus. Other species in the genus, such as Ehrlichia chaffeensis and Ehrlichia ewingii, are transmitted by ticks in the southeastern and south-central United States.
"Before this report, human ehrlichiosis was thought to be very rare or absent in Minnesota and Wisconsin," said Dr. Bobbi Pritt, a microbiologist at Mayo Clinic who helped to coordinate a multi-agency team that identified the new bacterial strain.
Pritt and colleagues published an account of four cases on Wednesday in the New England Journal of Medicine.
Ehrlichia infect and kill white blood cells, causing fever, body aches, headache and fatigue. In severe cases, other organs such as the lungs, kidneys and the brain may be affected and in rare cases, the infections can result in death.
Researchers have not seen any infections that did not originate in Minnesota or Wisconsin.
All four of the patients described in the New England Journal of Medicine were infected in 2009 with the newly described ehrlichia species.
All four had fever, fatigue and headache and one patient also had nausea and vomiting. And all responded to treatment with the antibiotic doxycycline, a drug that is used with other forms of ehrlichiosis infections.
The new bacterium was discovered by Carol Werner, a former technologist at Mayo Clinic Health System's Eau Claire hospital, who noticed an abnormal result in a genetic test for ehrlichiosis.
Mayo then alerted the U.S. Centers for Disease Control and Prevention, and several universities and public health departments helped confirm the bacterium was a new strain.
Researchers said it is very similar genetically to an ehrlichia species found in Eastern Europe and Japan called E. muris, but they are not sure how it got to the upper Midwest.
"This bacterium could have been present at very low levels for years and we didn't have the tools to detect it," Pritt told a news briefing.
Since the finding, researchers at the Mayo lab have tested thousands of blood samples as well as thousands of deer ticks from across the United States.
"For some reason, it seems to be localized to this area," Pritt said.
The team said doctors in the two states need to be aware of the infections.
(Editing by Doina Chiacu)