The bad blobs -- known as Naegleria fowleri or N. fowleri -- thrive in warm, fresh water all over the world. But the key word here is warm. The amoeba loves heat. In the U.S., it inhabits the relatively hot waters of lakes, hot springs, and poorly maintained pools in Southern or Southwestern states.
All six of the 2007 cases were in Florida, Texas, and Arizona (the victims' names and swimming sites come from local media reports):
- May/June 2007: Angel Arroyo Vasquez, age 14, of Orlando, Fla., was swimming in an apartment swimming pool.
- July 2007: Will Sellars, age 11, of Orlando, Fla., was swimming and wakeboarding in Lake Conway.
- August 2007: Richard Almeida, age 10, of Kissimmee, Fla., was swimming and wakeboarding at Orlando Watersports Complex.
- August 2007: John "Jack" Herrera, age 12, participated in water activities during summer camp at Lake LBJ in Texas.
- August 2007: Colby Sawyer, age 22, ruptured his eardrum while wakeboarding at Lake LBJ in Texas.
- September 2007: Aaron Evans, age 14, was swimming at Lake Havasu in northeastern Arizona.
Why the deadly amoeba struck these six and not the thousands of other people exposed in the same places at the same times is a mystery, says CDC epidemiologist Jonathan Yoder.
"Humans are the accidental host -- we are not part of this amoeba's life cycle," Yoder tells WebMD. "But when it finds a nice warm environment like your nose, it looks for a food source."
How Brain-Eating Amoebas Attack
"It actually is using the brain for food," Yoder says. "So it is a very tragic situation for the person unfortunate enough to have that happen."
After the amoeba enters the nose, it finds its way to the olfactory nerve. N. fowleri appears to be attracted to nerve cells, so it follows the nerve into the brain. That's when bad things happen.
The amoeba has mouth-shaped structures on its surface called food cups. It's perfectly capable of chewing up brain and blood cells with these food cups, but the blob finds it more efficient to secrete enzymes and proteins that dissolve brain cells so it can suck up the debris with its food cup.
Obviously, this causes a lot of damage. And it happens fast: Victims usually die seven to 10 days after infection, although symptoms may not appear for up to 14 days.
Initial symptoms include headache, fever, nausea, vomiting, and stiff neck. Later symptoms include confusion, inability to pay attention to people and surroundings, loss of balance, seizures, and hallucinations. Death follows the first symptoms by three to seven days.
The disease is technically called primary amoebic meningoencephalitis, or PAM.
"We want to make sure people understand this is a tragic event," Yoder says. "When it happens to a person, particularly if it is a child, we don't want to minimize the tragedy."
At least eight people have survived PAM. All were treated with powerful drugs soon after infection. Unfortunately, most victims aren't treated in time. There are rapid tests for N. fowleri infection, but because the infection is so rare, doctors usually don't suspect a brain-eating amoeba until it's too late.
Brain-Eating Amoeba Not on the Rise
Last summer's six cases were a lot compared to most years. But the CDC says there's no evidence that the brain-eating amoeba is on the rise. There were eight cases in 1980, seven cases in 2002, and six cases in 1978, 1986, and in 1995. Since 1937, there have been only 121 known cases.
So far, there haven't been any cases in 2008. But the CDC warns people either to avoid swimming in warm, fresh water or to wear nose plugs if they do. N. fowleri does not live in salt water or in properly maintained swimming pools, although it has been found in domestic water supplies.
"People should assume there is a risk of swimming in warm, fresh water," Yoder says. "And we think that things people do to minimize entry of water into the nose might provide some reduction of risk, such as using a nose clip. We can't say there is scientific evidence this works, but that is a commonsense approach."
The CDC also suggests that people avoid digging or stirring up sediment while playing or working in warm waters. The CDC further suggests that people avoid thermally polluted water, such as the water near power plants, although Yoder says the CDC has not yet looked at how much thermal pollution affects amoeba populations in public waters.
States where N. fowleri has caused disease include Arkansas, Arizona, California, Florida, Georgia, Louisiana, Missouri, Mississippi, North Carolina, New Mexico, Nevada, Oklahoma, South Carolina, Texas, and Virginia.
Infections have been seen around the world, including 16 cases traced to the same swimming pool in the Czech Republic.
There's no question of eradicating the blob. N. fowleri turns into its cyst form when conditions aren't right -- and can survive for years in the soil.
The CDC reports details of the six 2007 cases, and analyzes PAM trends, in the May 30 issue of Morbidity and Mortality Weekly Report.