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Calling it the largest outbreak of its kind and stressing the urgency of finding the source, the CDC has identified two more suspected cases of deadly blood infections and sent additional investigators to Wisconsin.

"This is very much a real outbreak," said Michael Bell, deputy director of the Division of Healthcare Quality Promotion for the U.S. Centers for Disease Control and Prevention.

Investigators have not been able to find a medical product, single facility, food source or other means of exposure that could explain how dozens of people — mostly elderly residents of central and southeastern Wisconsin — have become ill from a bacterium named Elizabethkingia anophelis.

Isolated tests from the two suspected cases had not yet been confirmed, and officials would not say whether the individuals were sickened or had died. As of Tuesday, the outbreak was tied to 44 confirmed cases, including 18 deaths.

Other strains of Elizabethkingia have caused smaller outbreaks in previous years, but most were tied to a single source.

Thus far in Wisconsin's outbreak, the only known commonalities are that the majority of people are 65 or older and have had serious underlying health conditions — although those conditions vary. Many have had recent contact with some type of medical facility, but others have not.

"We have a mixed group of people," Bell said. "A good number hadn't had any contact with a medical facility. They never left home."

Elizabethkingia is a gram-negative bacterium — meaning it's resistant to multiple drugs, including many antibiotics. It's found throughout the environment and is prevalent in soil and water. It can survive on skin — though it is not well-adapted to do so. Instead, it needs warm, moist places to thrive. This specific strain, E. anophelis, is also known to live in the guts of certain mosquitoes.

"This is a particularly challenging outbreak as this bacterium is everywhere in the environment," Bell said. "The number of possible risk factors is tremendous."

Healthy people can typically fend off infection, whether the bacteria is ingested or enters the body via other means. But those with compromised immune systems cannot always fight it. The bacteria colonize and spew toxins that ultimately can lead to sepsis, causing the body to shut down.

'A tricky one'

The CDC now has eight investigators in Wisconsin interviewing patients and families about where they've been, what they've eaten, what products they've used and more. They then compare them with people who had similar experiences but did not become sick.

"The CDC has got a tricky one here," said Luke Moore, an infectious disease specialist who studied an Elizabethkingia outbreak in a critical care unit in a teaching hospital in London.

In that outbreak, a different strain of the bacterium sickened 30 people and was traced to a sink used by surgeons in the hospital. Most of the people got pneumonia. Somehow the bacterium transferred from the doctors' hands to somewhere on the patients and then moved to the lungs, where it was able to colonize, Moore said.

"That's our best guess" of how the London outbreak happened, he said. "We haven't worked out in fine details how these bugs move around our bodies."

Of the cases in Wisconsin he said, "there is certainly some common aspect, some common thing that all these people are using."

Susan Dolan, president of the Association for Professionals in Infection Control and Epidemiology, said she would not be surprised if the outbreak expanded beyond Wisconsin.

Dolan discovered an outbreak of a deadly bacillus cereus bacterium in Colorado in 2010. She and her team at Children's Hospital Colorado found the bacterium had contaminated alcohol wipes. It started with just a few cases of children quickly becoming gravely ill. Over subsequent weeks and months, others around the country — including a 2-year-old from Houston — were identified with blood infections from the bacterium. The wipes, blamed in multiple lawsuits for the outbreak, were manufactured in southeastern Wisconsin, in Hartland.

"You usually start by identifying a cluster (of cases)" Dolan said. "Once you put the word out, more cases come forward.

"Maybe you have really good people (in Wisconsin) who detected it before anyone else."

Dolan said investigators will likely explore many leads that won't pan out before the real culprit becomes apparent.

"It's detective work and you try to use the literature and have your shoe-leather epidemiology guide you, and at the same time you have to keep an open mind," she said. "No one thought alcohol pads could be contaminated...people thought they were sterile, but not all of them are."