The biggest outbreak of Elizabethkingia in recorded public health history just got bigger.

The Michigan Department of Health and Human Services (MDHHS) confirmed Thursday that an older adult from the western part of the state has died after contracting the obscure blood infection, which has sickened more than 50 in Wisconsin. Seventeen of those patients died, though it’s not clear whether the infection was to blame. All of the victims were people with underlying health conditions, including the latest one in Michigan.

Now, Michigan health officials are trying to figure where the person came in contact with the bacteria. Elizabethkingia anophelis is commonly found in soil, rivers and reservoirs, and usually has not been dangerous to people — until last November, when people in Wisconsin started falling ill.

When they do occur, most cases of Elizabethkingia (named for Elizabeth O. King, the Centers for Disease Control microbiologist who first isolated the bacterium) happen in ones and twos, usually in hospital settings, in people whose immune systems are already weak. An outbreak like this one is thought to be unprecedented — according to the Milwaukee Journal-Sentinel, most prior outbreaks had fewer than 10 patients.

Officials don’t know where this outbreak is coming from, or why it’s so deadly.

“It’s one of the largest [Elizabethkingia outbreaks] that I’m aware of, and certainly the largest one we’ve investigated,” Michael Bell, deputy director of the Centers for Disease Control and Prevention’s Division of Healthcare Quality Promotion, told The Washington Post last week.

In those it sickens, Elizabethkingia causes fever, chills and shortness of breath. Though strains of the bacterium have been found in all 18 fatalities linked to the outbreak, officials can’t say for sure whether the disease itself is responsible for the deaths.

Wisconsin health officials have been investigating the outbreak in the state since early January, and the following month they issued a call for other health providers to review their own records for signs of the illness, according to MDHHS. On February 29, a Michigan lab identified Elizabethkingia in a recently submitted blood sample from a deceased patient. Workers sent it on to the CDC for further testing, which confirmed the diagnosis.

All of the bacteria samples isolated from the more than four dozen patients share a particular genetic fingerprint, suggesting they may be coming from the same source, Bell said.

But finding the source can be difficult. The cases are scattered across 12 Wisconsin counties — Columbia, Dane, Dodge, Fond du Lac, Jefferson, Milwaukee, Ozaukee, Racine, Sauk, Sheboygan, Washington and Waukesha — and now one in Michigan.

Though it’s possible that the person who died in Michigan may have gotten the infection while visiting Wisconsin, CDC spokesperson Melissa Brower told the Associated Press that shouldn’t be assumed.

“We really don’t know how this person in Michigan may have contracted it,” she said.

The common link between the cases in Wisconsin is proving difficult to pin down. Not all of the patients had visited the same hospital, or even been to a hospital at all. Some lived in nursing homes, other in their own houses.

“That leaves us looking at a huge number of potential risk factors,” including medications, foods and environmental sources, Bell told The Post. “It’s frustrating. The fact that all these cases share a fingerprint has us wanting to really track down the source.”

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