The surgical team expected pus. A 54-year-old man had spent more than a month with chest tightness, breathlessness and pain on his left side, and in the final two days his breathing had gotten much worse. Imaging showed air and fluid trapped around a partly collapsed left lung. He was booked for keyhole surgery to clean out what everyone assumed was an infected chest cavity.
They drained about 500 milliliters of yellow pus. Then, exploring further with the camera, they found something moving inside his chest wall.
It was a worm, alive and wriggling. They removed it and sent it to pathology, which identified it as an adult Paragonimus, a lung fluke, roughly 8 millimeters long and 4 millimeters wide, reddish-brown and oval, with the oral and ventral suckers visible.
How a Flatworm Ends Up Loose in a Chest Cavity
The case was published Aug. 5 in the peer-reviewed journal Frontiers in Medicine by thoracic surgeons in Guizhou Province, China. The patient had long lived in a rural county of Qiandongnan Prefecture and had a habit of eating raw beef, sashimi, and freshwater crabs.
That last item is the route. According to the Centers for Disease Control and Prevention, people acquire the infection by eating raw or undercooked freshwater crabs and crayfish carrying immature flukes. Once swallowed, the larvae break out in the small intestine, bore through the duodenal wall into the abdominal cavity, migrate for about a week, then cross the diaphragm into the space around the lungs before moving through lung tissue itself. Adult worms mature there over five to six weeks.
The man was caught in the middle of that journey. His disease had lasted about a month, meaning the parasites had reached his chest but had not yet matured and started laying eggs. That timing is why his stool test for parasite eggs came back negative, and why the usual diagnostic route failed.
The Clues That Were Sitting in Plain Sight
The authors identify three findings that pointed toward a parasite rather than ordinary empyema.
His eosinophils, the white cells that expand during parasitic infection and allergic reactions, made up 14.1% of his white count, above the normal ceiling of 8%. He had two small painless lumps under the skin over his left ribs, no more than 0.8 centimeters across, which in this disease represent larvae that wandered off course. He lived in a region where the parasite is endemic and ate raw crustaceans.
The pleural fluid told a confusing story. It looked purulent and had very low glucose and a raised adenosine deaminase, a pattern that overlaps with tuberculous pleurisy. But acid-fast smears, a tuberculosis PCR, and a T-SPOT.TB test were all negative, Gram stain found no bacteria, and both aerobic and anaerobic cultures grew nothing. The pus resulted from inflammation triggered by the parasite rather than a bacterial infection.
This diagnostic trap is well documented. Paragonimiasis is routinely mistaken for tuberculosis, bacterial pneumonia, or lung cancer, and a large series analyzing 685 cases over 22 years documented the wide range of ways it can present.
Americans Get This Too, From Crawfish
This is not purely an overseas problem, which most U.S. readers will not know.
North America has its own native lung fluke, Paragonimus kellicotti, carried by crayfish in freshwater river systems. Physicians at Washington University in St. Louis described three patients infected after eating raw crayfish from rivers in Missouri, seen at a single medical center over 18 months, and noted that only seven homegrown North American cases had been reported before that.
The circumstances are consistent enough to be worth naming. A clinical review of North American cases found that of 19 patients who clearly ate raw crayfish, 15 did so while intoxicated on river float trips.
Cases remain sporadic and rare. But the infection is not imported-only, and it does not require foreign travel. Prevention is simple: the CDC advises never eating raw freshwater crabs or crayfish and cooking them to at least 145 degrees Fahrenheit.
What Happened Next
The man had the pus drained and the fibrous rind cleaned off his lung, and he was discharged seven days after surgery with his left lung re-expanded. Infectious disease specialists then treated him with praziquantel, the standard drug, given in three courses.
A brain MRI showed scattered small white matter spots. The authors were careful here: the findings were nonspecific, and while wandering flukes reaching the brain could not be entirely excluded, ordinary chronic ischemic changes were equally plausible. He had no neurological symptoms, declined a spinal tap, and was scheduled for imaging follow-up instead.
Three months after treatment, the lumps under his skin were gone, and his eosinophil count had dropped substantially. He remained well.
Two cautions about how to read this. The species was never confirmed by molecular testing, so the authors' identification rests on regional patterns and the worm's appearance. And direct sighting of a live worm is a striking way to make a diagnosis, but the authors stress it is not a recommended first step. Blood antibody tests and egg detection remain the standard confirmatory methods. Surgery earned its place here only because the noninvasive workup had failed and the patient needed his chest drained anyway.
Key Questions Answered
What is paragonimiasis?
An infection caused by parasitic flatworms called lung flukes. People catch it by eating raw or undercooked freshwater crabs and crayfish carrying immature parasites.
Can Americans get it without traveling?
Yes, though it is rare. North America has a native species carried by crayfish in freshwater rivers, and people have been documented with cases after eating raw crayfish from Missouri rivers.
How do you avoid it?
The CDC advises never eating raw freshwater crabs or crayfish and cooking them to at least 145 degrees Fahrenheit.
Why was it initially mistaken for something else?
The chest fluid looked bacterial and shared features with tuberculosis. Stool testing for eggs was negative because the parasites had not yet matured.
Is it treatable?
Yes. Praziquantel is the standard drug. This patient recovered after drainage surgery and three courses of treatment.
