The species, tentatively called Bartonella melophagi, has been found in women suffering from muscle fatigue and weakness, and even from a case of pericarditis, an inflammation of the membrane surrounding the heart, researchers from North Carolina State University and the U.S. Centers for Disease Control and Prevention report in the January issue of Emerging Infectious Diseases, published by the CDC.

In nature, sheep are the most likely hosts for B. melophagi. Transmission among sheep is thought to be by way of the wingless parasite louse flies known as a keds.

But how the bacteria transmits to humans is unknown, the report says.

The research marks the first time this particular strain of Bartonella has been cultured from human blood and associated with human illness.

"A small number of Bartonella in the bloodstream can cause infection, and this fact, coupled with the large variety of transmission routes by which people can become infected, make the diagnosis, treatment and prevention of the illnesses caused by this bacteria a real challenge," Research Assistant Professor Ricardo Maggi of North Carolina State's Department of Clinical Sciences says.

"I think it's critical that we discover more about how this bacteria infects people, and how Bartonella infection relates to the subsequent development of progressive illnesses in humans," he says.