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Eight species of Paragonimus lung flukes cause human disease. The most important is Paragonimus westermani. Paragonimus species are endemic in East Asia, Oceania, West Africa, and South America, where millions of persons are infected; rare infections caused by Paragonimus kellicotti have occurred in North America. Eggs are released into freshwater, where parasites infect snails, and then cercariae infect crabs and crayfish. Human infection follows consumption of raw, undercooked, or pickled freshwater shellfish. Metacercariae then excyst, penetrate into the peritoneum, and pass into the lungs, where they mature into adult worms over about 2 months (eFigure 35–22).

eFigure 35–22.

Life cycle of Paragonimus westermani (lung fluke). The eggs are excreted unembryonated in the sputum, or alternately, they are swallowed and passed with stool

image. In the external environment, the eggs become embryonated
image, and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues
image. Miracidia go through several developmental stages inside the snail
image: sporocysts
image, rediae
image, with the latter giving rise to many cercariae
image, which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, where they encyst and become metacercariae. This is the infective stage for the mammalian host
image. Human infection with P westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite
image. The metacercariae excyst in the duodenum
image, penetrate through the intestinal wall into the peritoneal cavity, then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults
image (7.5–12 mm by 4–6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this takes place completion of the life cycle is not achieved because the eggs laid cannot exit these sites. Time from infection to oviposition is 65–90 days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P westermani. (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)

Most persons have moderate worm burdens and are asymptomatic. In symptomatic cases, abdominal pain and diarrhea develop 2 days to 2 weeks after infection, followed by fever, cough, chest pain, urticaria, and eosinophilia. Acute symptoms may last for several weeks. Chronic infection can cause cough productive of brown sputum, hemoptysis, dyspnea, and chest pain, with progression to chronic bronchitis, bronchiectasis, bronchopneumonia, lung abscess, and pleural disease. Ectopic infections can cause disease in other organs, most commonly the CNS, where disease can present with seizures, headaches, and focal neurologic findings due to parasite meningitis and to intracerebral lesions.

The diagnosis of paragonimiasis is made by identifying characteristic eggs in sputum or stool or identifying worms in biopsied tissue. Multiple examinations and concentration techniques may be needed. Serologic tests ...

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