Paragonimus westermani (Lung Fluke): Life Cycle, Pathogenesis, and Diagnosis

A young man in the hills has coughed for months. He brings up sputum streaked with blood, he has lost weight, and his chest X-ray is abnormal. Everything points to tuberculosis, and he is started on TB treatment. But three sputum smears show no acid-fast bacilli, and he does not get better.

Only when a technician examines the sputum again, this time looking for parasite eggs rather than bacteria, does the answer appear: golden-brown operculated eggs of a lung fluke. Months earlier he had eaten raw freshwater crab, prepared in the traditional way.

This is the reason the lung fluke matters far beyond its numbers. It produces an illness that looks like tuberculosis so closely that patients are treated for the wrong disease for months, while the real clue sits in the same sputum sample the laboratory is already examining. This article explains how the fluke reaches the lungs, why it imitates tuberculosis, and how its egg is finally recognized.

What Is Paragonimus westermani?

Paragonimus westermani, the oriental lung fluke, is a trematode whose adult worms live in the lungs rather than in the gut, liver, or blood vessels. It causes paragonimiasis, the only human fluke infection in which the adults settle in the lungs. The genus Paragonimus contains many species, and more than ten can infect people, but P. westermani is the most important and the one found across East and Southeast Asia.

Two features define this fluke and explain most of what follows.

1. First, humans are infected by eating raw or undercooked freshwater crab or crayfish, not fish or plants. This crustacean route is unique among the human flukes and is the fastest way to place the parasite from a food history. Traditional dishes of raw, pickled, or briefly marinated crab carry the infective stage, because these preparations do not kill it.
2. Second, the young fluke migrates out of the gut and up through the diaphragm into the lungs. Like Fasciola, it tunnels through tissue rather than taking a short duct route, but where Fasciola stops in the liver, Paragonimus keeps going through the diaphragm to reach the lungs. That destination is what makes the disease respiratory and what makes it imitate tuberculosis.

Life Cycle

The life cycle of Paragonimus uses two intermediate hosts, a snail and then a crustacean, and the human is infected by eating the crustacean.

1. Eggs leave the body unembryonated. Adult flukes in the lungs lay eggs that are coughed up in sputum, or swallowed and passed in the stool. Unlike the small liver fluke eggs, these are not yet developed when they leave the body.
2. Development in water. In fresh water the egg matures and releases a swimming miracidium.
3. First intermediate host: a snail. The miracidium penetrates a freshwater snail, where the parasite multiplies through sporocyst, redia, and cercaria stages.
4. Second intermediate host: a crab or crayfish. Cercariae leave the snail and invade a freshwater crustacean, where they encyst as metacercariae. This is the infective stage.
5. Human infection. A person is infected by eating the raw or undercooked crab or crayfish carrying metacercariae.
6. Migration to the lungs. The metacercaria excysts in the duodenum, bores through the intestinal wall into the abdominal cavity, then passes through the diaphragm into the lungs. There it becomes encapsulated in a cyst-like cavity, matures into an adult, and begins laying eggs. This journey takes roughly two to three months (about 65 to 90 days) before eggs appear.

Two features anchor the cycle. The infective stage is in a crustacean, not a fish or a plant, which is unique among the flukes. And the worm reaches the lungs by tunneling through the diaphragm, which is why the eggs come out in sputum. Adult worms are long-lived and infection can persist for many years.

Pathogenesis

The importance of Paragonimus lies in where it ends up and what that resembles. The adult flukes settle in the lung tissue, usually as a pair inside a cyst-like cavity connected to an airway. Their presence, their eggs, and the inflammation around them produce a chronic lung disease that mirrors tuberculosis point for point.

The pulmonary picture. The settled adults and the eggs they release provoke inflammation and cavity formation in the lung. The result is a chronic cough, the bringing up of discolored, rusty-brown or blood-stained sputum, frank coughing of blood (hemoptysis), chest pain, and abnormal shadows on the chest X-ray. To a clinician in a region where tuberculosis is common, this is tuberculosis until proven otherwise, and that is exactly the trap. The discolored sputum is not just a symptom: it is the diagnostic specimen, because it carries the fluke's eggs.

The acute (migratory) phase. While the young flukes are still migrating from the gut to the lungs, some patients have an earlier illness with abdominal pain, diarrhea, fever, hives, and eosinophilia, sometimes with an enlarged liver and spleen. This phase reflects the larvae tunneling through tissue and the allergic response to them, and it precedes any egg production.

Extrapulmonary and cerebral disease. The migrating flukes do not always reach the lungs. They can end up in other sites, and the most serious is the brain. In cerebral paragonimiasis, flukes or their eggs lodge in brain tissue and cause severe disease, because the inflammation sits in a location where any mass effect or scarring is dangerous. Involvement of the abdomen, the tissues under the skin, and other organs also occurs. These ectopic forms follow the same rule seen across the flukes: wherever the parasite or its eggs lodge, inflammation follows, and the organ decides the severity.

Other Paragonimus Species

P. westermani is the main species and the focus of this article, but a few others are also human pathogen. Paragonimus kellicotti is the North American lung fluke, acquired from raw crayfish, and is the species behind most human cases in the United States.

Paragonimus africanus occurs in West and Central Africa, and Paragonimus mexicanus in Central and South America. All cause a broadly similar lung disease, and their eggs are hard to separate from one another, so species identification usually rests on geography and, when available, the adult worm or molecular testing.

Laboratory Diagnosis

The diagnosis of paragonimiasis turns on finding the egg, and the distinctive point is that the primary specimen is sputum, not stool. Two features make the diagnosis easy to miss: the eggs appear late, and the routine test for tuberculosis can destroy them.

Which specimen. The main specimen is sputum, examined for eggs. Because eggs are also swallowed, they can appear in the stool as well, and stool examination is a useful second sample, particularly in children who swallow their sputum. The eggs do not appear until roughly two to three months after infection, when the adults mature, so an early test can be negative even when the clinical picture fits.

What the egg looks like. The Paragonimus westermani egg is yellow-brown, oval, and fairly large (about 80 to 120 µm long by 45 to 70 µm wide), with a thick shell. It is operculated, with a clearly visible lid sitting on flattened shoulders at one end, and the opposite (abopercular) end is thickened. It is passed unembryonated. In a sputum sample the eggs are often seen in clusters among the brownish, blood-tinged material.

The staining trap that matters. When sputum is examined for tuberculosis, it is treated with an acid-fast (Ziehl-Neelsen) staining method. That process can dissolve or distort Paragonimus eggs, so a laboratory looking only for acid-fast bacilli may destroy the very eggs that would give the correct diagnosis. Awareness of this is important in exactly the setting where the two diseases are confused. A wet mount of fresh sputum, not an acid-fast smear, is the way to see the eggs.

When eggs are absent. Because eggs appear late and can be scanty, and because the eggs may be destroyed by TB staining, antibody blood tests are valuable.

In the United States, an immunoblot for P. westermani was developed specifically to help tell paragonimiasis apart from tuberculosis in immigrants from Asia, and it is both sensitive and specific.

Serology is also useful for extrapulmonary disease, where no eggs reach the sputum or stool, and for confirming cure after treatment. One limitation is that a test built for P. westermani may not reliably detect antibodies to other Paragonimus species. Imaging of the chest, and biopsy when an adult or developing fluke is recovered, provide further confirmation, and eosinophilia is a supportive clue.

For how this operculated sputum egg sits alongside the operculated stool eggs of the other flukes and the spined schistosome eggs, see the comparison in the trematodes overview article.

Treatment

The drug of choice for paragonimiasis is praziquantel, which is effective against the adult lung flukes. This places Paragonimus with the small liver flukes and the schistosomes, which also respond to praziquantel, and apart from Fasciola, the one fluke that does not.

When the brain is involved, treatment is more complex and is managed with specialist care, because the inflammatory response to dying worms in the brain needs careful handling.

The practical public health message is prevention: thorough cooking of freshwater crab and crayfish prevents infection, since the traditional raw and pickled preparations do not kill the metacercariae.

How to Remember

• Crab to lung, through the diaphragm. The lung fluke rides in on raw crab or crayfish and tunnels up through the diaphragm to the lungs. Crab and lungs, the way fish means the liver flukes and plants means Fasciola.
• The tuberculosis mimic. Chronic cough, bloody sputum, weight loss, abnormal chest X-ray, and repeatedly negative acid-fast smears in someone who ate raw crab: think lung fluke, not TB. This is the single most important pattern on the page.
• The answer is in the sputum. Unlike every other fluke, this one is diagnosed mainly from sputum, and the bloody sputum that looks like a TB symptom is actually the specimen that carries the eggs.
• The TB stain destroys the clue. Acid-fast (Ziehl-Neelsen) staining for TB can dissolve the Paragonimus eggs. In a suspected case, ask for a wet mount of fresh sputum, not just an acid-fast smear.
• Praziquantel works here. Like the small liver flukes and schistosomes, the lung fluke responds to praziquantel. Fasciola remains the praziquantel exception.
• The brain is the dangerous detour. When the fluke wanders instead of reaching the lungs, the brain is the worst place it can go.

Key Exam Facts

Where Students Get Confused

This is a lung disease that mimics tuberculosis, not a gut disease. Because most flukes involve the liver, bile ducts, or intestine, students expect the same here. Paragonimus is the exception: the adults live in the lungs, the main symptom is a chronic cough with bloody sputum, and the whole picture imitates tuberculosis. The food history (raw crab) and repeatedly negative acid-fast smears are what point away from TB.

Sputum, not stool, is the main specimen. For almost every other fluke the answer is in stool or urine; the lung fluke is the exception. Stool can show swallowed eggs as a secondary sample, but leading with stool alone can miss the diagnosis.

The TB stain can destroy the evidence. Acid-fast staining for tuberculosis can dissolve the eggs, so the very test used to chase the wrong diagnosis can erase the right one. When paragonimiasis is suspected, a wet mount of fresh sputum is what to request (see Laboratory Diagnosis).

Eggs appear late. As with Fasciola, there is a gap between infection and egg production, here about two to three months. Early in the illness, or in the migratory phase, stool and sputum can be negative, and serology is more useful. A negative smear does not exclude the disease.

Praziquantel works, unlike for Fasciola. Students who have just learned that Fasciola resists praziquantel may overgeneralize. The lung fluke responds to praziquantel; Fasciola is the single fluke exception.

Cooking, not pickling, makes crab safe. As with the fish-borne flukes, light preservation is not enough. Pickled, marinated, or briefly salted crab can still carry live metacercariae. Only thorough cooking reliably prevents infection.

References

1. Centers for Disease Control and Prevention. DPDx: Paragonimiasis. Atlanta: CDC Division of Parasitic Diseases and Malaria; last reviewed June 6, 2024. Available from: https://www.cdc.gov/dpdx/paragonimiasis/index.html
2. Esteban JG, Muñoz-Antolí C, Toledo R, Ash LR. Diagnosis of human trematode infections. In: Toledo R, Fried B, editors. Digenetic Trematodes. Advances in Experimental Medicine and Biology, vol 1154. Cham: Springer; 2019. p. 437–471. Available from: https://doi.org/10.1007/978-3-030-18616-6_14
3. Garcia LS. Diagnostic Medical Parasitology. 6th ed. Washington, DC: ASM Press; 2016.
4. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
5. World Health Organization. Foodborne trematode infections. Geneva: WHO Health Topics. Available from: https://www.who.int/health-topics/foodborne-trematode-infections