Paragonimus westermani & Paragonimus kellicotti

Paragonimus westermani is the causative agent of Oriental lung fluke infection while Paragonimus kellicotti is the causative agent of North/South American lung fluke infections. The disease name given for an infection with any Paragonimus species is paragonimiasis. There are various other species of Paragonimus that can infect man. Pergonimus kellicotti, or the North American lung fluke, is an important cause of Paragonimiasis in the United States and is therefore mentioned here. Below is a table describing some of the other species to consider in a case of paragonimiasis.

Species of Paragonimus Geographic Location of Paragonimus speciesMode of transmission
P. heterotremusChina-Laos-Thailand crabs-raw shrimp and salad in Thailand
P. mexicanusMexico-Central & South Americacrabs
P. africanus Cameroon-Liberia-Nigeria-Congo-Guinea-Cote d’Ivoire-The Gambiacrabs
P. miyazakiiJapancrabs-raw juice of crabs-crayfish
P. philippinensisPhilippinescrabs
P. skrjabiniChinacrabs
P. hueitungensisChinacrabs
P. uterobilateralisCameroon-Liberia-Nigeria-Congo-Guinea-Cote d’Ivoire-The Gambia-South Africacrabs
Some Less Common Paragonimus species

Disease / Pathology

Paragonimus westermani as well as other species of Paragonimus produce pulmonary type symptoms that can mimic those of tuberculosis. Hemoptysis is a key symptom. This is due to the presence/location of the adult flukes in the lungs. The organism can also cause symptoms in the host due to the migration of larva through the the intestinal wall into the abdominal cavity where under most circumstances there is little to no pathologic changes but can in some cases produce abdominal tenderness, fever, diarrhea, vomiting, intra-abdominal masses, and most importantly an eosinophilia. Once in the larvae enter the peritoneal cavity there can also be symptoms related to their entry such as localized hemorrhage and leukocytic infiltrates. Most important are the symptoms associated with pulmonary involvement where the adult worms will induce a significant tissue reactions associated with the formation of cyst formations containing eggs. These eggs are released when the cysts are perforated and release along with the eggs necrotic debris, and metabolic by-products that produce an inflammatory/allergic reaction.

The reaction in the host is dependent on the worm burden. Initial infections and those that are low have insidious onsets. Common to all infections is the presence of a peripheral eosinophilia. X-ray examinations may reveal lesions. And in the presence of hemoptysis the presentation may mimic a case of tuberculosis. Chronic infections without hemoptysis may be misdiagnosed as bronchial asthma.

Other symptoms may include chest pain, dyspnea, and a cough that may produce rusty sputum with a characteristic fish smell.

The most ominous pathology of a Paragonimus infection is the presence of ectopic infections from the larval migration. Cyst formations can occur in various parts of the body (liver – intestinal walls – peritoneum – brain – etc.). The involvement of the brain is most concerning as cerebral complications can be serious.

Location in the Host

Paragonimus westermani is found encapsulated in the parenchyma of the lung. But can also be found in other tissues in the host as well and the tissue involved will many times determine the symptoms that present in the infected host.

Geographic Distribution

Paragonimus westermani is found throughout Asia. Paragonimus kellicotti is known as the North American lung fluke but is also found in Southeast Asia and China.

Life Cycle

Paragonimus westermani infections are acquired by the ingestion of undercooked or raw, crabs and crayfish and various products derived from them. An example is the ingestion of raw juice from crushed crayfish used as a home remedy for measles causing cerebral paragonimiasis in children. Infections of other Paragonimus species as well as westermani can also occur through the ingestion of raw or uncooked meats (containing migrating larva) from wild animals.

Morphology & Diagnosis

Paragonimus westermani eggs detected in the feces or sputum during an ova & parasite examination are diagnostic. Eggs are shed in low numbers and therefore detection is difficult without repeated specimen collected for analysis. Eggs are also sometimes confused with those of Diphylobothrium latum (the broad fish tapeworm). Diphylobothrium latum eggs are smaller in size and have a distinctive abopercular knob at the posterior end of the egg.

Paragonimus westermani eggs are large, measuring 80-120u in length and 45-70u wide. Those of Diphylobothrium latum are 58-75u in length by 40-50u wide.


Adult Paragonimus westermani fluke
Photo by W. Vientos
Paragonimus westermani egg