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More than 30 species of trematodes (flukes) of the genus Paragonimus have been described. Paragonimus lung fluke species are distributed throughout the Americas, Africa, and Southeast Asia. Of the more than 10 species reported to infect humans, the most common is P. westermani, the Oriental Lung Fluke, which occurs in Southeast Asia and Japan. (Centers for Disease Control Parasites and Health Website)
Ninety percent of the estimated 22 million people infected with Paragonimus live in Southeast and East Asia; nearly all the rest live in Africa and Central and South America. Infection has nearly disappeared from Japan, Korea, and Thailand as a result of improved food hygiene and socioeconomic standards. However, infection still occurs in impoverished areas in Laos, Vietnam, and the Philippines and in some remote parts of China and northeastern India. Cases have also been reported in Asian immigrants to the United States and Australia. (Vidamaly et al. 2009 and references therein)
Paragonimus kellicotti is endemic to North America. Only a handful of cases of human infection with Paragonimus originating in North America are known, but the actual number of cases may be higher than previously recognized. Lane et al. (2009) suggests that physicians encountering patients with cough, fever, hemoptysis (i.e., coughing up blood), and eosinophilia (elevated levels of certain white blood cells known as eosinophils) should consider the possibility of paragonimiasis. Infection with Paragonimus lung flukes can cause severe lung disease and the symptoms of paragonimiasis often mimic those of tuberculosis and other non-helminthic diseases (Lane et al. 2009; Fried and Abruzzi 2010), including lung cancer (Song et al. 2011). Paragonimiasis can be avoided by not eating raw or improperly cooked shellfish.
The eggs of Paragonimus are excreted unembryonated in the host's sputum or are swallowed and passed with stool. In the external environment, the eggs become embryonated and miracidia hatch, seek out a first intermediate host (a snail), and penetrate its soft tissues. Miracidia pass through several developmental stages inside the snail: sporocyst, redia, and, from the rediae, many cercariae, 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. Human infection with P. westermani occurs by eating inadequately cooked or pickled crabs or crayfish that harbor metacercariae of the parasite. The metacercariae excyst in the duodenum (first portion of the small intestine), penetrate through the intestinal wall into the peritoneal cavity, and then pass through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (7.5 to 12 mm by 4 to 6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles (or, in at least one case, the tip of a little finger: Sim et al. 2010). However, when this occurs, the life cycle cannot be completed. Time from infection to oviposition is 65 to 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.