Death by brain-eating amoeba is an inside job

Immune response may be the real killer after infection with N. fowleri

amoebas (yellow); brain (blood vessel and blood cells shown in blue and orange)

BRAIN BOUND  Naegleria fowleri amoebas (yellow) get into the brain through the nose and attack tissue in the brain (blood vessel and blood cells shown in blue and orange), but the major danger comes from the immune system’s reaction, scientists suggest.

London School of Hygiene & Tropical Medicine/Science Source

On July 9, just a few days after swimming in Minnesota’s Lake Minnewaska, 14-year-old Hunter Boutain was dead. Doctors believe the culprit was the water-dwelling amoeba called Naegleria fowleri. Most people know it by its other name: The brain-eating amoeba. 

It’s the stuff of horror films — the tiny amoeba crawls up the nose to the brain where it wreaks havoc, ultimately killing 97 percent of its victims. But while the amoeba is certainly devastating for the small number of people it infects, calling it a “brain eater” may not be quite right. The immune system’s response to infection — not the amoeba itself — is the real killer, some scientists suspect.

The single-celled N. fowleri, which thrives in warm freshwater, is “destructive, nobody doubts that,” says Abdul Mannan Baig, a physiologist at Aga Khan University in Karachi, Pakistan, where numerous N. fowleri infections have been reported. But it needs a new name, he says. “When you say ‘brain-eating amoeba,’ it’s more a tabloid term.”

N. fowleri often makes news because it kills healthy people — often boys — by sparking a brain infection called primary amebic meningoencephalitis. But these cases are rare. From 1962 to 2014, only 133 people in the United States are known to have been infected, the Centers for Disease Control and Prevention in Atlanta reports. While the bulk of these cases come from southern states, officials have noted the amoeba’s northward creep. Since the infections can be hard to diagnose, the real number of cases may be higher.

Its rarity makes the infection, first signaled by headache, fever and nausea, and perhaps even an altered sense of smell, difficult to study. But experiments on animals have suggested that the amoeba damages tissue directly by releasing harmful proteins, says cell biologist Jesús Serrano-Luna of the Center for Research and Advanced Studies of the National Polytechnic Institute in Mexico City.

But Baig and others suspect that the death from the amoeba is actually an “inside job” caused by the immune system’s attempt to fight the amoeba, he writes in the August Acta Tropica. When immune cells were absent from a lab dish, N. fowleri amoebas took eight hours longer to damage human blood vessel cells, Baig reports.

Baig contends that aggressively blunting the immune response, and the brain swelling that goes with it, might help more patients survive. Reducing brain swelling caused by inflammation may have been part of the reason why 12-year-old Kali Hardig survived the amoeba in 2013.

“It was amazing at the time,” says CDC epidemiologist Jennifer Cope, who consulted on Hardig’s treatment. “She was the first U.S. survivor in 35 years.”

Cope says several aspects of treatment probably saved her life, including a quick diagnosis, a new antiamoeba drug and an emphasis on lowering the pressure inside her skull. “Our skull is great. It protects us most of the time,” Cope says. “But when it comes to the inflammation in the brain and swelling, that’s when our hard skull works against us.”

When N. fowleri attacks the brain, it sets off a cascade of inflammation that then leads to dangerous brain swelling and death, she says.  “‘Brain-eating’ is not entirely a misnomer,” she says. “But it’s also not the whole story.”

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