If the amoeba isn’t already in dining mode at this point, the intense burst of human body heat can help it shape-shift out of dormancy. Like anyone else waking up somewhere unfamiliar, the brain-eating amoeba is desperate for a food source, so it slithers its way up the olfactory nerve until it spies a tasty-looking tangle of neurons and digs in. The host’s immune system, sensing an unwelcome visitor, sends an onslaught of white blood cells to take down the feasting parasite. That commotion leads to a swollen—and, eventually, irreparably damaged—brain.
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“We have these hard skulls to protect our brains from traumatic injuries,” says Jennifer Cope, a medical epidemiologist who studies N. fowleri for the Centers for Disease Control and Prevention. “But when you start to swell, that hard skull actually gets in the way.” With nowhere else to go, the brain stem and other nearby regions are pushed down through the bottom of the skull. For most of the brain-eating amoeba’s victims, this crowding is the direct cause of death.
Altogether, this deadly infection is known as primary amebic meningoencephalitis (PAM). It’s similar to viral and bacterial meningitis, except the invader that causes it comes from the water and eats your brain. With just about 30 cases in the United States in the past decade, PAM is an uncommon variant of an already rare disease. Since N. fowleri was first identified by Australian doctors roughly 50 years ago, there have been at least 146 confirmed cases of PAM in the United States, with only four survivors.
N. fowleri is so alarmingly dangerous because, like bacterial and viral meningitis, PAM is nearly impossible to spot until long after any of the interventions that have worked for some victims are even an option. Even when a patient does make it to a hospital within a few days of infection, PAM is often mistaken for one of these types of meningitis (distinguishing them requires an invasive spinal tap) and treated as such, to no avail. Experts agree that for every confirmed case, there are likely one or two that were misdiagnosed and recorded instead as more common meningitis fatalities.
Gray’s death fits the most common narrative of an N. fowleri fatality. Though not much is known about the organism, Cope says, “we do know it’s thermophilic—it likes heat.” Swimmers in balmy southern lakes and rivers have generally been the unlucky few each summer, but in recent years there have been some concerning outliers.
In 2013, a 4-year-old boy living near New Orleans died unexpectedly from what doctors later determined to be PAM. The CDC was called. “I asked the same questions we usually do about swimming in lakes, and [the parents] said they hadn’t been to any lakes,” Cope says. “It led to us finding that really the only thing was that he’d been playing on the backyard Slip ’n Slide—with a hose hooked up to the backyard tap.”