Direct attack

This is what it looks like when parasitic worms directly invade your brain

Doctors unknowingly took time-series images of the worms invading patient's brain.

Beth Mole – Mar 11, 2025 3:14 PM | 119

Doctors in China inadvertently took time-series images of parasitic worms actively invading a woman's brain and causing rare and rapidly progressing lesions.

The previously healthy 60-year-old woman went to the hospital after having a fever and altered mental status for three days, according to a report of her case published Monday in JAMA Neurology. By the time she arrived, she was unable to communicate normally.

Figure A: FLAIR MRI of the brain before treatment showed multiple white matter lesions adjacent to the lateral ventricles. Credit: Li et al. JAMA Neurology 2025

Magnetic resonance imaging (MRI) showed white matter lesions around her lateral ventricles, large cavities in the center of the brain filled with cerebrospinal fluid that, from the side, are C-shaped. The type of MRI used, a FLAIR MRI, is used to more easily detect lesions, and the fluid-filled lateral ventricles appear as dark, curved spaces in the center. Doctors could see white blotches and smears around those dark spaces, indicating lesions. After doing a spinal tap and running tests on her cerebral spinal fluid, they suspected she might have a bacterial infection in her brain. So they treated her with an antibiotic and a fever reducer.

But two weeks later, her symptoms hadn't improved. The doctors repeated a FLAIR MRI and found that the white lesions had only gotten worse. They did more tests.

Blood tests showed elevated eosinophils, which can be a sign of a parasitic infection, among other things. They searched for genetic signatures of parasites in her cerebrospinal fluid and got a strong hit. The genetic sequencing matched Angiostrongylus cantonensis, also known as rat lungworm.

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Sickening cycle

This ghastly parasite causes outbreaks and sporadic cases in various places around the world, including China and other parts of Asia. You may remember hearing of it amid an uptick in cases reported in Hawaii a few years ago or a report in 2023 that it had invaded the Southeast US.

A “semi-slug,” carrier of the rat lungworm disease. Credit: KAWIKA KAINA

The worm gets its name from its complicated invasion of rats, its true target, in which it spends much of its life cycle except for a nauseating interlude in its intermediate victims, slugs and snails. The cycle begins in rats, where first-stage larvae are released into the environment via the rodent's feces. These larvae then either bore into or are eaten by slugs or snails. In them, the larvae develop into third-stage larvae (L3). Rats get infected with these L3 larvae when they eat an infected slug or snail. From the rat's gastrointestinal tract, the larvae tunnel their way to the animal's brain, where the larvae incubate and go through L4 and L5 stages to become young-adult worms. The worms then move to the rat's lungs, where they become sexually mature, mate, and lay eggs—hence the name "rat lungworm."

To bring this disturbing life cycle full circle, the worm eggs hatch into stage-one larvae in the lungs and migrate up the rat's throat. The poor critter then ends up swallowing them. The first-stage larvae then take a ride through the rat's gastrointestinal tract and are excreted, lying in wait of a snug or snail.

Humans are accidental hosts and get infected with the L3 larvae by accidentally eating infected slugs or snails on raw or undercooked produce or by eating other undercooked animals that eat slugs or snails, such as frogs, land crabs, or freshwater prawns and crayfish. Humans can also get infected by eating raw produce—like leafy greens—that have been contaminated with slug slime because the L3 larvae are shed in the gastropods' slime trail. Although, as a recent case report noted, "the infectious dose of slime is not defined."

Rare lesions

*Angiostrongylus cantonensis* third stage (L3), infective larva recovered from a slug. Image captured under differential interference contrast (DIC) microscopy. Credit: CDC

When the woman's doctors detected rat lungworm, they quickly asked about what she had eaten before getting sick. They learned that she had eaten a "substantial amount" of uncooked leftover crayfish (Procambarus clarkii) the night before she developed a fever.

Once eaten by a human, the L3 larvae try to complete their life cycle by burrowing into tissue out of the intestines to get to the brain, traveling either through the blood or along nerves. The migration along nerves can cause sensory abnormalities, like burning, tingling, or pain. Once in the central nervous system, the invasion can cause confusion, encephalopathy, seizure, cranial neuropathy, or eye problems. In a brain, the L3 larvae would normally develop into young adults, but human brains are dead ends for the worms, which eventually die there without fully maturing.

Some of the luckiest rat lungworm victims don't need treatment and make full recoveries after the larvae die off on their own. But the woman in this case was not so lucky. Her doctors noted in their report that the worms typically stay in the pia mater of the brain, a delicate innermost layer of tissue that surrounds the brain and spinal cord. They rarely cause lesions in the white matter, a deeper tissue containing nerve fibers that transmit signals between different parts of the brain.

In her case, the worms appeared to be directly and actively invading her white matter, which was captured by the FLAIR MRIs. The lesions progressed "rapidly" in the three days after eating the uncooked crayfish and the two weeks when she was misdiagnosed and treated for a bacterial infection (labeled as "after treatment" in the second image). The authors noted that the lesions looked significantly different from the lesions seen when white matter is damaged from a lack of blood flow, such as in a stroke. This "may be related to direct invasion of A. cantonensis or inflammation of small vessels," they speculated.

While there isn't a clearly defined standard treatment for such infections, the doctors started the woman on an anti-parasitic drug called albendazole, which is often used. After two weeks, her symptoms improved, and she was able to communicate again. After four months, a third FLAIR MRI showed a "significant reduction" in white matter lesions. It's unclear, however, if she made a full recovery.

Beth Mole Senior Health Reporter

Beth is Ars Technica’s Senior Health Reporter. Beth has a Ph.D. in microbiology from the University of North Carolina at Chapel Hill and attended the Science Communication program at the University of California, Santa Cruz. She specializes in covering infectious diseases, public health, and microbes.

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