JOPLIN, Mo. - In the aftermath of the Joplin tornado, some people injured in the storm developed a rare and sometimes fatal fungal infection so aggressive that it turned their tissue black and caused mold to grow inside their wounds.
Scientists say the unusually aggressive infection occurs when dirt or vegetation becomes embedded under the skin.
In some cases, injuries that had been stitched up had to be reopened to clean out the contamination.
The Centers for Disease Control and Prevention said Friday that it was conducting tests to help investigate the infections, which are so uncommon that even the nation's largest hospitals might see only one or two cases a year.
"To my knowledge, a cluster like this has not been reported before," said Dr. Benjamin Park, head of the CDC team that investigates fungal diseases. "This is a very rare fungus."
Three tornado survivors who were hospitalized with the infection have died, but authorities said it was unclear what role the fungus played in their deaths because they suffered from other serious ailments. "These people had multiple traumas, pneumonia, all kinds of problems," said Dr. Uwe Schmidt, an infectious-disease specialist at Freeman Health System in Joplin.
The infection, formally known as zygomycosis, develops in two ways: when the fungal spores are inhaled or when a tree branch or other object carrying the fungus pierces the flesh.
Most people who get sick by inhaling the spores already have weakened immune systems or diabetes.
But healthy people can become sick if the fungus penetrates their skin. The fungus blocks off blood vessels to the infected area, causing tissue to turn red and begin oozing. Eventually it becomes black.
If diagnosed in time, the infection can be treated with intravenous medications and surgical removal of affected tissue.
But it's considered exceptionally dangerous, with some researchers reporting fatality rates of 30 percent for people infected through wounds and 50 percent for susceptible people who breathe the fungus in.
Small numbers of cases have been reported after some disasters, but Park said it was the particular circumstance of the wound - not the disaster itself - that created the risk.
The Missouri Department of Health and Senior Services has received reports of eight suspected deep-skin fungal infections among survivors of the May 22 twister, which was the nation's deadliest single tornado in more than six decades. All the patients had suffered multiple injuries.
On Friday, Joplin officials raised the death toll from the twister to 151, a figure that includes the deaths of the three people with the fungus. Officials say more than 1,100 people have been treated for injuries after the storm, many of them from objects sent flying by the twister.
A week after the tornado, patients began arriving with fungal infections. Doctors had to reopen some wounds that had been stitched closed because the injuries had not been adequately cleaned, Schmidt said.
Doctors "could visibly see mold in the wounds," he said. "It doesn't have any circulation. It has to be removed."