This Brain-Eating Amoeba Isn’t Actually After Your Brains

3 min readHere’s what you’ll learn when you read this story:Although the single-celled, free-living protist Naegleria fowleri is typically harmless, it can become extremely deadly if it enters the nasal cavity and makes its way to the brain. Although this “brain-eating amoeba” is rare, scientists want to understand why it enters the brain, since its food source isn’t there. What they discovered is that the amoeba loves enclosed spaces, to the detriment of the infected person.Earlier this month, the Louisiana Department of Health reported the tragic death of an 8-year-old who recently swam in Claiborne Lake, near the small town of Homer. The cause was an incredibly rare one—a free-living amoeba named Naegleria fowleri entered the child’s brain via the nasal cavity, causing an infection known as “primary amebic meningoencephalitis” (PAM) that eventually led to her death. A little more than a week later, North Carolina reported another “brain-eating amoeba” infection, this time in a teenager who currently remains in critical condition.Although two infections in as many weeks may seem alarming, PAM remains incredibly rare, with only 167 total reported cases from 1962 until 2024, according to the Centers for Disease Control and Prevention (CDC). Unfortunately for those infected, PAM is incredibly deadly, and of those 167, only four survived.Despite its gruesome “brain-eating” nickname, N. fowleri is typically harmless. The amoeba lurks in bodies of fresh water and in soils around the world, and it can’t cause infection when swallowed. Instead, to harm you it must travel into your nasal cavity, cross the cribriform plate (the bone that separates the nasal cavity from the brain), and embed itself in brain tissue. But weirdly, the amoeba feeds on bacteria that isn’t found in the human brain. So why does it enter the brain at all?A new study published on the preprint server bioRxiv uncovers N. fowleri’s pathogenesis—that is, its pathway toward infection. Because of the dangers of working with an organism known for “eating” brains, scientists worked with its close (and harmless) relative Naegleria gruberi, which displays similar preferences and behaviors. In the laboratory, researchers created a variety of artificial channels only two to eight micrometers thick, similar to the spaces that amoebas navigate when entering the brain via the nasal cavity. While no amoebas entered the smallest channels, the protists could move twice as quickly in four-, six-, and eight-micrometer channels than when they were navigating freely. In other words, these single-celled protists seem to be drawn to enclosed spaces.“These amoebae...actively seek confinement, exhibiting pronounced claustrophilia, and crawl rapidly with near-perfect persistence across millimeter-scale distances when confined to tight channels,” the authors wrote. “Naegleria cells also crawled faster and more persistently in channels than on flat surfaces.”Their leading theory is that N. fowleri’s deadliness is an accident of biology, since the amoeba has an evolutionary preference for entering tight spaces of lakebed sediments to find bacterial prey. Once it finds itself in a human’s nasal cavity, it simply does what it knows—it fits into tight spaces and searches for food. Unfortunately, this leads right to the brain, causing a cascade of symptoms including confusion, seizures, hallucinations, coma—and eventually, around 95 percent of the time, death.“It’s really just an unfortunate circumstance when the amoebae get into your upper sinuses,” University of Massachusetts cell biologist Katrina Velle, lead author of the study, told Science. “They’re just doing what they’ve evolved to do—looking for nooks and crannies where their bacteria should be.”While a run-in with this “brain-eating amoeba” will likely remain incredibly rare, it could become slightly more common and widespread with climate change. Hotter weather raises temperatures in northern lakes, rivers, and streams that previously had no reported infections of N. fowleri. That uptick in temperature also increases cyanobacteria, the amoeba’s main food source. And of course, longer and more intense heatwaves mean more people seeking freshwater recreation areas to cool off, increasing the chance of more aquatic amoeba encounters.For now, there’s no cure for PAM, but understanding why the protist does what it does could help experts identify the infection before it’s too late.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.