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Paragonimus westermani 01.jpg
Egg of Paragonimus westermani
Scientific classification e
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Trematoda
Order: Plagiorchiida
Family: Troglotrematidae
Genus: Paragonimus
Braun, 1899 [1]

Paragonimus is in the genus of flatworms, or platyhelminths, which includes Paragonimus westermani, an infectious lung fluke endemic to Asia causing a human disease called paragonimiasis.

The first intermediate hosts of Paragonnimus includes at least 54 species of freshwater snails from superfamilies Cerithioidea and Rissooidea.[2] Worldwide, approximately nine species of Paragonimus are known to cause paragonimiasis in which many of the species reside in East Asia, West Africa, and in North and South America.[3]


Species of Paragonimus vary in size in the adult stage with a length up to 15 millimetres (0.59 in) and a width up to 8 mm (0.31 in).[4] The adult flatworm has an oval shape body with spines covering its thick tegument. Both the oral sucker and acetabulum are round and muscular where the acetabulum is slightly bigger than the oral sucker – 0.19 mm and 0.12 mm, respectively.[4] Ovaries are located behind the acetabulum and posterior to the ovary are the testes. Between the acetabulum and the ovary are the uterus, seminal receptacle, and metraterm.[4]

Life cycle[edit]

The parasite uses two intermediate hosts, an aquatic snail and a crustacean in order to get into its mammalian definitive host which includes dogs, cats, and humans. Contracting paragonimiasis occurs usually when humans ingest freshwater crustaceans, such as freshwater crabs or crayfish, that are undercooked and contain metacercariae. Once ingested and in the intestine, the parasite will move into the abdomen and into the lungs. In the lung, the parasites transform into a cyst where they will cross fertilize with one another. The cyst will rupture in the lungs where they can be either be coughed up or swallowed and excreted in the feces. Eggs landing in the freshwater will hatch into a ciliated miracidum infecting their first intermediate host usually an aquatic snail. A crustacean may also become infected by eating the infected snail. The cycle starts again when the definitive host ingests the infected crustacean.


There are approximately 20 million infected with Paragonimus worldwide. Paragonimus infections are more common in areas with the following characteristic: 1) a large number of human and animal reservoir hosts, 2) an abundance of intermediate hosts, such as snails, crabs, or crayfish, and 3) common consumption of raw or undercooked seafood. Infection can also occur following consumption of raw meat from land animal hosts, such as wild boar, if they contain Paragonimus flukes. [5]


Some symptoms of paragonimiasis include abdominal pain, diarrhea, fever, and hives. These symptoms can last for months and sometimes even up to 20 years.[6] Paragonimiasis is caused by the body's natural immune response to the worms and eggs that are present and also migrating from the intestines to the lungs. On average, it takes about eight weeks for the parasites to begin making eggs in the lungs and three weeks after ingestion to obtain the symptoms. In some cases, patients will develop cerebral paragonimaisis where eggs are produced in the brain causing headache, vomiting, and seizures.[3] If untreated, death occurs due to the increased intracrainal pressure.


Praziquantel has been used to effectively treat paragonimiasis by separating the tegument with a high cure rate of 100% after three days of treatment.[7] Other medications can also be used such as bithionol, niclofan, and triclabendazole with high cure rates.


Cooking the crustacean thoroughly kills the parasite. Crab meat should not be eaten raw and pickled crab increase the chance of contracting the infection since the solution may not kill the parasite. Utensils and cutlery boards should be cleaned thoroughly prior to food preparation.[3]


Species within the genus Paragonimus include:


  1. ^ M. Braun (1899). "Über Clinostomum Leidy". Zoologischer Anzeiger 22 (603): 489–493. 
  2. ^ a b G. M. Davis, C. E. Chen, Z. B. Kang & Y. Y. Liu (1994). "Snail hosts of Paragonimus in Asia and the Americas". Biomedical and Environmental Sciences 7 (4): 369–382. PMID 7535537. 
  3. ^ a b c Gary W. Procop (2009). "North American paragonimiasis (caused by Paragonimus kellicotti) in the context of global paragonimiasis". Clinical Microbiology Reviews 22 (3): 415–446. doi:10.1128/CMR.00005-08. PMC 2708389. PMID 19597007. 
  4. ^ a b c Imelda Vélez, Luz E. Velásquez and Iván D. Vélez (2003). "Morphological description and life cycle of Paragonimus sp. (Trematoda: Troglotrematidae): causal agent of human paragonimiasis in Colombia". Journal of Parasitology 89 (4): 749–755. doi:10.1645/ge-2858. JSTOR 3285872. PMID 14533686. 
  5. ^ Karin Leder; Peter F Weller. "Paragonimiasis". UpToDate. Retrieved 15 May 2014. 
  6. ^ "Paragonimiasis (lung fluke)". August 2006. Retrieved 8 December 2011. 
  7. ^ Nawa Yukifumi (2000). "Re-emergence of paragonimiasis". Internal Medicine 39 (5): 353–354. doi:10.2169/internalmedicine.39.353. PMID 10830172.