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Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
Peter F Weller, MD, MACP
Section Editor
Edward T Ryan, MD, DTMH
Deputy Editor
Elinor L Baron, MD, DTMH


Paragonimiasis is a trematode (fluke) infection transmitted via consumption of raw or undercooked crab or crayfish. Almost 50 species and subspecies of Paragonimus have been described, most of which are found in carnivorous animal hosts. Approximately 16 species have been reported to cause disease in humans, the most common of which is the oriental lung fluke, P. westermani. Paragonimiasis can have pulmonary and extrapulmonary manifestations, as discussed in the following sections.


Paragonimiasis occurs in several parts of the Far East, West Africa, and the Americas. It is estimated that over 20 million people are infected worldwide and more than 290 million are at risk [1,2]. The prevalence of infection increases in areas with numerous human and animal reservoir hosts, an abundance of first and second intermediate hosts (snails and crabs or crayfish, respectively), and social customs of eating raw or undercooked seafood. In addition, paragonimiasis can be acquired by ingesting raw meat from carnivorous animal hosts that contain young flukes (such as wild boars). Transmission has also been reported via contaminated utensils, such as knives or chopping boards [3,4].

Species distribution varies by geographic region (table 1) [3].

P. westermani is found in the Far East, particularly in China, Korea, Japan, the Philippines, Taiwan, and India. P. africanus occurs in West Africa; Paragonimus mexicanus occurs in Central and South America. In the United States, the disease is observed most frequently among immigrants from endemic countries, but local transmission has been described [5]. A few cases of P. kellicotti have been reported from North America [6-11]. In 2010, nine cases of human paragonimiasis were reported in Missouri among patients who had eaten raw or undercooked crayfish from rivers while canoeing or camping [11].

Life cycle — The life cycle of paragonimiasis begins with expectoration of unembryonated eggs in the sputum; alternatively, eggs may be swallowed and passed in the stool (figure 1). In fresh water, the eggs become embryonated; after a few weeks, the resulting miracidia hatch and penetrate the soft tissues of a snail (the first intermediate host). Miracidia go through several developmental stages inside the snail (sporocysts, rediae, and cercariae) over three to five months. Cercariae emerge from the snail and invade a crustacean such as a crab or crayfish (the second intermediate host) where they encyst and become metacercariae over six to eight weeks.

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Literature review current through: Oct 2017. | This topic last updated: May 11, 2017.
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