Thoracoscopy Reveals Living Lung Fluke Moving on the Pleura in Rare Case

Diagnosing paragonimiasis, a parasitic lung infection, is notoriously difficult. Clinicians typically search for microscopic eggs in sputum or pleural fluid, yet these tests frequently yield false negatives, leaving patients to endure months of incorrect treatments.

In a remarkable clinical encounter, a thoracoscope inserted into a 38-year-old woman’s chest to investigate recurrent pleural effusion revealed something extraordinary. Direct visualization showed a living, moving object on the right posterior pleura near the costal angle. The organism exhibited a distinct, peristaltic wave-like motion, confirming the presence of an adult living Paragonimus fluke.

Published in the journal *Infection, Genetics and Evolution*, the researchers behind this case describe it as the first recorded instance of directly visualizing a living adult Paragonimus fluke via thoracoscopy.

Two Admissions, One Missed Diagnosis

The patient experienced significant morbidity, being hospitalized twice within a six-month period due to pleural effusion, which is the accumulation of fluid in the pleural space between the lung and chest wall.

During her first hospitalization, serological testing using enzyme-linked immunosorbent assay (ELISA) returned positive results for antibodies against *Schistosoma japonicum* and sparganum. Based on these serological markers, a parasitic infection was assumed, and treatment with praziquantel was initiated.

However, the pleural effusion recurred. Upon her second admission, clinicians performed a medical thoracoscopy to directly inspect the thoracic cavity, leading to the discovery of the actual parasitic agent.

Microscopic examination confirmed the parasite as belonging to the genus *Paragonimus*. Additionally, metagenomic next-generation sequencing of the pleural fluid independently detected *Paragonimus* DNA, providing dual confirmation of the diagnosis. Following a second course of praziquantel, the patient remained free of pleural effusion at her four-month follow-up.

This clinical trajectory highlights a critical diagnostic challenge. Cross-reactivity of antibodies between different parasitic species is a well-documented phenomenon; a positive serological result for one type of fluke cannot definitively identify the active pathogen. Thus, while the initial treatment regimen was appropriate, the underlying diagnosis was ultimately incorrect.

The Parasite Gets In Through Undercooked Crab

*Paragonimus* is a trematode, or flatworm, transmitted to humans through the consumption of raw or undercooked freshwater crabs and crayfish harboring the infectious larval stage. Among the various species, *Paragonimus westermani* is the most prevalent cause of human infection.

According to clinical guidelines from the Centers for Disease Control and Prevention (CDC), larvae excyst within the small intestine and penetrate the duodenal wall. They subsequently migrate through the peritoneal cavity for approximately one week before traversing the diaphragm to enter the pleural cavity. From there, they migrate through lung tissue to the bronchi, where they establish cystic cavities and mature into adult worms over a period of five to six weeks. Adult flukes are reddish-brown and typically measure between 7.5 and 12 millimeters in length.

A critical diagnostic window exists because adult flukes do not begin producing eggs until they have fully matured. Consequently, for over a month post-infection, sputum examinations may yield no detectable eggs. Even after this period, egg detection remains unreliable in cases of light infections. This limitation explains why clinicians rely on antibody testing, and why cross-reactive serological results can mislead clinical decisions, as seen in this patient’s initial presentation.

While often associated with endemic regions in Asia, paragonimiasis is not geographically confined. A published case series in the United States documented four infections, three of which were linked to consuming live crabs at sushi bars—including one served in a martini glass. In North America, the primary species involved is typically *Paragonimus kellicotti*. Public health records note that several cases have followed outdoor river rafting trips in Missouri, where participants consumed uncooked crawfish.

It Gets Mistaken for Tuberculosis and Cancer

Clinically, paragonimiasis presents with cough, chest pain, blood-tinged sputum, and frequently eosinophilia—an elevated level of a specific type of white blood cell. Radiographic imaging typically reveals pulmonary cavities, nodules, patchy consolidation, and pleural effusions.

This clinical presentation closely mimics tuberculosis (TB), leading to empirical anti-tuberculous therapy in endemic areas before parasitic infection is reconsidered. In a documented South Korean case, a 38-year-old man completed a two-month course of tuberculosis treatment before surgical intervention revealed *Paragonimus* eggs in his pleural tissue.

Complications of paragonimiasis can be severe. A retrospective study of 179 patients at a Korean university hospital reported pneumothorax (collapsed lung) in 10.6% of cases, with asthma occurring significantly more frequently among those who developed this complication.

The therapeutic intervention itself is relatively straightforward. Praziquantel, administered at 25 mg/kg three times daily for two days, is highly effective, and patients typically show rapid clinical improvement. The principal barrier to successful treatment remains achieving an accurate and timely diagnosis.

What This Means for Anyone Eating Freshwater Crustaceans

It is important to emphasize that this isolated case report does not suggest that lung fluke infection is a common consequence of eating seafood. Paragonimiasis remains a neglected tropical disease that most clinicians in the United States will never encounter in their careers.

Nevertheless, the transmission route is specific and entirely preventable. Consuming raw or undercooked freshwater crabs or crayfish carries a significant risk. Traditional preparation methods such as pickling in vinegar, brine, or wine, or using light smoking and salting, do not reliably eliminate the larvae. Thorough cooking, however, is highly effective. The CDC recommends heating crabs and crayfish to an internal temperature of at least 145 degrees Fahrenheit to ensure safety.

For patients, the key to an accurate diagnosis lies in obtaining a thorough dietary and travel history alongside the clinical presentation. An individual presenting with a persistent cough, chest pain, hemoptysis, and eosinophilia who has a history of consuming raw freshwater crustaceans or visiting an endemic area must explicitly communicate this exposure. This detail is often the pivotal factor that redirects the clinical workup toward the correct etiology.

Patients experiencing these symptoms should seek clinical evaluation rather than self-diagnosing a parasitic infection, as the symptoms overlap with many other far more common respiratory conditions.

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.

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