Out of nowhere, a woman’s right arm and leg started shaking uncontrollably. In the moments leading up to the experience, she seemed well; she had no headache, confusion, or weakness. But for about a minute, half of her body juddered.

The woman, a rice farmer in the Philippines, headed to the emergency department. There, doctors gave her a physical exam and found nothing out of the ordinary. Then they took a look at her brain. Magnetic resonance imaging revealed a bright cluster of nodules surrounded by fluid in her left frontal lobe. The doctors had an immediate guess as to what they were: worm eggs.

In a case report published in the latest issue of the New England Journal of Medicine, doctors write that they suspected the woman had neuroschistosomiasis, an uncommon complication of an infection with a Schistosoma parasitic worm, aka a blood fluke. In the Philippines, the most common fluke is S. japonicum. It’s also found in China and part of Indonesia; despite its name, it has been eliminated from Japan.

An unfortunate fluke

S. japonicum, like its relatives, has a diabolical life cycle, which starts with its eggs being shed in the urine or feces of various animals, including humans. When conditions are right, the eggs hatch into a free-swimming larval form. These seek out specific types of aquatic snails, which they bore into and inhabit to go through two life stages. Late-stage larvae then burst out, grow a forked tail, and swim to find an animal host. These tailed larvae, called cercariae, have the ability to tunnel through intact skin and take a trip through the vascular system—visiting the lungs, heart, and then liver, where they develop into adults.

The adult male and female worms live paired together with the slender female worm tucked into a groove within the thicker male. Adult worms typically move to large blood vessels in the abdomen connecting the liver to the intestines. From there, the cuddly couples spew out eggs that make their way into feces and urine.