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Common Amoeba Blamed In Swimming Deaths Is Mystery To Doctors

By NEIL JOHNSON | The Tampa Tribune

Published: September 6, 2007

TAMPA - A freshwater amoeba suspected in two Central Florida deaths within a month is largely a mystery to doctors because cases of the brain infection it causes are so rare.

The Florida Department of Health is investigating the two deaths to see whether the victims died from a rare infection caused by a microscopic creature that lives in every body of freshwater in the state.

The health department is aware of only 13 Florida deaths caused by the amoeba in the past 30 years, said Carina Blackmore, a state environmental epidemiologist. That isn't enough cases to learn much about the amoeba or its victims.

Doctors do know the amoeba, called Naegleria fowleri, enters the body through the nose and travels to the brain, where it causes an infection and swelling called amoebic encephalitis. It is nearly always fatal, though it can be treated with antibiotics if caught quickly enough.

It strikes quickly once symptoms appear – headache, fever, nausea and vomiting, followed by lethargy, stiff neck and confusion. Death can occur seven to 10 days after symptoms show up.

"We don't know how long the incubation period is. We don't expect it to be long," Blackmore said.

Naegleria lives in the top few inches of muddy sediment at the bottom of any body of freshwater and thrives when the water temperature gets to 80 degrees or above, as it does in late summer in Florida. The warm temperatures coupled with increased swimming activity in late summer expose more people to the bacteria.

Stirring up the sediment suspends the amoeba in the water, so it can reach the nasal passages. Florida's shallow lakes are perfect for exposure to the amoeba because they warm quickly and it's easy to stir up the bottom.

But scientists do not know how many of the creatures need to be in the water to present a danger or whether there is a safe concentration. They also do not know how many amoebas an individual has to inhale to become infected.

Though mostly young people have died from the amoeba, doctors don't know whether age is a factor or whether younger people simply tend to be more active swimmers and do things to stir up the water, Blackmore said.

"We don't know why these few people were affected and not the thousands of others who swim in freshwater," she said.

Health officials say to avoid swimming in stagnant lakes, ponds or retention areas. The amoeba does not occur in well-maintained swimming pools.

Flowing rivers tend to be cooler than shallow lakes or ponds and may not harbor as many of the amoeba, though scientists are not certain, Blackmore said.

Doctors suspect cooling water during the fall and winter contains fewer of the amoebae because fewer infections occur during cool months. That may be because not as many people swim and no one knows how cool the water must be to be safe.

"We don't know the magic temperature when they go into an inactive winter form," Blackmore said. "We can say we see human cases in the late summertime, and those have had exposure to shallow, warm lakes and swimming activity."