A blood infection caused by the Elizabethkingia anophelis bacteria has killed 18 people in two US states. Officials from the federal Centers for Disease Control have confirmed that the latest death, in Michigan, is linked to the bacteria that has already killed 17 in the neighbouring state of Wisconsin.

The outbreak is the largest recorded in published literature, according to officials. The total number of cases reported so far in Wisconsin is 54. Most of those infected are more than 65-years-old. None have been children. The infection appears to be spreading rapidly north from the first hotspot in south-eastern Wisconsin, and is now in 12 counties.

Federal and state officials are trying to track the source of the infection. "We are keeping every possibility on the table," Wisconsin state health officer Karen McKeown told the Chicago Tribune. "But it does not seem to be spread from person to person."

Governor Scott Walker recently met Wisconsin's Emergency Management team and members of the federal Department Homeland Security to plan the containment of the growing outbreak. The CDC currently has eight investigators on the ground in Wisconsin.

The Elizabethkingia is common in the environment. It is a genus of bacteria that has been detected in soil, river water and reservoirs. However, it rarely makes people sick. Illness associated with Elizabethkingia typically affects people with compromised immune systems or serious underlying health conditions. Until now, most outbreaks have been linked to healthcare sites or procedures.

Symptoms include fever, shortness of breath and chills. There may also be cellulitis, a redness and swelling from a skin infection that may feel hot and tender to the touch.