Doctors almost never see the parasite. In paragonimiasis, diagnosis usually comes from finding microscopic eggs in sputum or pleural fluid, and even that fails often enough that patients spend months on the wrong treatment.
So when a thoracoscope went into a 38-year-old woman’s chest to work out why fluid kept collecting there, and the camera found something moving on the right posterior pleura near the costal angle, it settled the question immediately. The object was contracting in a peristaltic, wave-like motion. It was a living adult Paragonimus.
The case was reported in Infection, Genetics and Evolution, and the authors describe it as the first visualization of a living adult Paragonimus under thoracoscopy.
Two Admissions, One Missed Diagnosis
The woman was hospitalized twice within six months for pleural effusion, fluid accumulating in the space between the lung and the chest wall.
During the first admission, serological testing by ELISA came back positive for antibodies against Schistosoma japonicum and sparganum, two other parasites. Parasitic infection was suspected on that basis, and praziquantel was started.
The fluid came back. On the second admission, doctors performed a medical thoracoscopy to establish what was actually going on and found the worm.
The parasite was identified as Paragonimus under the microscope, and metagenomic next-generation sequencing of the pleural fluid also detected Paragonimus, providing two independent confirmations. She received another course of praziquantel, and at four months of follow-up, the effusion had not returned.
That sequence is the useful part. Antibody cross-reactivity between parasite species is a known problem, and a positive serology for one fluke does not confirm which fluke you are dealing with. The initial treatment was reasonable, and the diagnosis was still wrong.
The Parasite Gets In Through Undercooked Crab
Paragonimus is a trematode, a flatworm, acquired by eating raw or undercooked freshwater crabs and crayfish carrying the larval stage. Several species infect humans, with Paragonimus westermani the most widespread.
According to the CDC’s clinical overview, the larvae excyst in the small intestine, penetrate the duodenal wall, and migrate through the peritoneal cavity for roughly a week. They then push through the diaphragm, enter the pleural cavity, and travel through lung tissue to reach the bronchi, where they form cystic cavities and mature into adult worms over five to six weeks. Adult flukes are reddish-brown and measure about 7.5 to 12 millimeters long.
Because eggs are not produced until the flukes reach adulthood, there is a window of more than a month during which sputum testing has nothing to find. Even after that, detection is unreliable in light infections. That is why antibody testing exists at all, and why this patient’s serology sent her doctors toward the wrong parasite.
Infection is not confined to Asia. A published US case series described four American patients, three of whom had eaten live crabs at sushi bars, including one served in a martini. In North America, the species involved is usually Paragonimus kellicotti, and CDC guidance for the public notes that several cases followed river raft float trips in Missouri where uncooked crawfish were eaten.
It Gets Mistaken for Tuberculosis and Cancer
The clinical picture is cough, chest pain, blood-tinged sputum, and often eosinophilia, an elevated count of a particular white blood cell. Chest imaging shows cavities, nodules, patchy consolidation, and effusions.
That combination looks a great deal like tuberculosis, which is why patients in TB-endemic regions have been treated with anti-tuberculous drugs before anyone reconsidered. In one published Korean case, a 38-year-old man went through two months of empirical TB treatment before surgery, during which Paragonimus eggs were found in his pleura.
Complications can be serious. A study of 179 paragonimiasis patients at a Korean university hospital found pneumothorax, a collapsed lung, in 10.6% of them. Asthma was significantly more common in the patients who developed it.
The treatment itself is not the hard part. Praziquantel at 25 milligrams per kilogram three times daily for two days is the drug of choice, and patients typically improve quickly. The difficulty is arriving at the diagnosis at all.
What This Means for Anyone Eating Freshwater Crustaceans
This is a single case report and does not mean that a lung fluke is a routine consequence of eating seafood. Paragonimiasis is a neglected disease that most US clinicians will never encounter.
But the exposure route is specific and avoidable. Raw or undercooked freshwater crab and crayfish carry the risk. Preparing crab in vinegar, brine, or wine without cooking does not reliably kill the larvae. Neither does salting, pickling, or light smoking. Cooking does, and the CDC advises heating crabs and crayfish to at least 145 degrees Fahrenheit.
The practical signal for a patient is a history plus a symptom pattern. Someone with a persistent cough, chest pain, blood in the sputum, and an elevated eosinophil count who ate raw freshwater crustaceans or spent time in an endemic region should explicitly mention that history, because it is the detail that reframes the whole workup.
Anyone with those symptoms should be evaluated by a clinician rather than assuming a parasitic cause, since the same clinical picture has many explanations, most of which are more common.
Key Questions Answered
What is paragonimiasis?
An infection caused by parasitic lung flukes of the genus Paragonimus, acquired by eating raw or undercooked freshwater crabs and crayfish. It usually settles in the lungs.
What made this case unusual?
Doctors directly observed a living adult worm moving on the pleural lining during thoracoscopy. Diagnosis normally relies on finding microscopic eggs, and the authors describe this as the first step.
How is it usually diagnosed?
By identifying eggs in sputum or, less often, in stool or pleural fluid. Antibody testing helps, but the flukes do not produce eggs until they mature, roughly five to six weeks after infection.
Why is it so often missed?
Because the symptoms and chest imaging closely resemble tuberculosis and lung cancer. Patients have been treated for TB for months before the parasite was found.
Can this happen in the United States?
Rarely. Cases have been reported, usually involving Paragonimus kellicotti and raw crayfish or live crab eaten at restaurants.
How is it treated?
Praziquantel is the drug of choice, given at 25 milligrams per kilogram three times daily for two days. Most patients improve quickly once the diagnosis is made.
