How Does the Brain-Eating Amoeba Naegleria fowleri Kill You?

Naegleria fowleri is a single-celled organism found primarily in warm freshwater with the potential to cause a disease that is fatal in the vast majority of cases.

The infection that the Naegleria fowleri amoeba causes is known as primary amoebic meningoencephalitis (PAM) and it has fatality rate of more than 97 percent.

Thankfully, cases of this disease are extremely rare in the United States and other parts of the world where the amoeba is found. But only four people out of 154 known infected individuals in the U.S. from 1962 to 2021 have survived, Dr. Julia Haston, a medical epidemiologist with the Centers for Disease Control and Prevention, told Newsweek.

How Does Naegleria Fowleri Kill People?

N. fowleri infects people when contaminated water enters the body through the nose, which typically occurs when people go swimming or diving in warm bodies of freshwater, such as lakes and rivers.

"It travels to the brain along the olfactory nerve, which is a nerve connecting the nose and the brain that controls our sense of smell," Haston said. "Once the amoeba reaches the brain, it begins destroying brain tissue and causes a devastating infection called primary amoebic meningoencephalitis, which is usually fatal."

The brain-eating amoeba Naegleria fowleri
Stock image: A 3D-rendering of the brain-eating amoeba Naegleria fowleri. This ameba can cause an infection known as primary amebic meningoencephalitis that has fatality rate of more than 97 percent. iStock

"The amoebae... destroy brain tissue by releasing toxic molecules," she said. "The immune system tries to fight the infection by sending immune cells and fluid to the brain. The combination of the toxic molecules and the immune response causes brain swelling and death."

Infections caused by N. fowleri progress very rapidly, and people typically die within only a few days of becoming ill.

"The symptoms of a N. fowleri infection are similar to those of bacterial meningitis, so sometimes it is not diagnosed initially," Haston said. "There is no treatment that has been proven to prevent death; however, treatment options are available."

The fact that N. fowleri can feed on brain tissue in the human body has led to it being dubbed a "brain-eating" amoeba.

A human brain
Stock image: A 3D rendering of the human brain. Infection with Naegleria fowleri can cause devastating damage to the human brain. iStock

"Naegleria fowleri is normally a free-living amoeba that feeds on bacteria and thrives in warm natural environments," Dr. Bobbi Pritt, director of the clinical parasitology laboratory at the Mayo Clinic, told Newsweek. "Unfortunately, it can also feed on brain tissue and is perfectly suited to survive and multiply at human body temperature."

"People infected with N. fowleri die when parts of the brain that control vital functions such as breathing are destroyed. The damage is caused by the amoebae feeding on brain tissue, as well as from brain swelling associated with infection. As the brain swells, it is forced through small openings like the foramen magnum (the opening of the skull to the spinal canal) which leads to tissue death."

Why Is the Fatality Rate so High for Naegleria fowleri Infections?

The fatality rate for N. fowleri infections is very high because the amoebae multiply so rapidly within the human body and are so effective at destroying brain tissue.

"The infected person has no specific, pre-existing defense methods to the amoebae, so they are unable to contain the infection on their own," Pritt said. "In the rare instances where people survived, it was because the infection was diagnosed early on, before much damage was done, and they were treated very aggressively."

"The survivors reported to date in the United States were treated with multiple drugs to kill the amoebae, and with therapeutic hypothermia (cooling the body's temperature below normal levels) to reduce brain swelling."

What Are the Symptoms of a Naegleria fowleri Infection?

The initial symptoms of PAM, which appear anywhere between one and nine days after infection, include a stiff neck, confusion, lack of attention to the surrounding environment, loss of balance, seizures and hallucinations.

After the appearance of symptoms, the disease progresses rapidly, usually resulting in death anywhere between one and 12 days after onset.