Trematodes are the flattened, oval or worm like parasites commonly known as flukes. Trematodae is a subphylum of Platyhelminthes. Mollusks and vertebrates are the internal parasites of trematode.

By Sadia Ghazanfer1*, Muhammad Sohail Sajid1, Muhammad Zeeshan1


Most trematodes have a complicated life cycle that involves at least two hosts. The major host, where the flukes reproduce sexually, is a vertebrate. A snail is typically used as an intermediate host for asexual reproduction. Paragonimiasis is a food-borne parasitic infection caused by the lung fluke. The most common species of lung fluke that infects people and causes paragonimiasis is Paragonimus westermani. The species is also known as the Japanese lung fluke or the Oriental lung fluke. In size, shape, and color, Paragonimus westermani resembles a coffee bean when alive. Adult worms measure 7.5 mm to 12 mm in length and 4 mm to 6 mm in width. The thickness varies between 3.5 mm and 5 mm. The worm’s skin (tegument) is densely coated with scale like spines. Reservoir hosts of Paragonimus spp. include numerous species of carnivores including felids, canids, viverrids, mustelids, some rodents, and pigs. Humans become infected after eating raw freshwater crabs or cray fishes that have been encysted with the metacercaria. Southeast Asia is more predominately more infected because of lifestyles. Raw seafood is popular in these countries.

Crab collectors string raw crabs together and bring them miles inland to sell in Taiwan markets. These raw crabs are then marinated or pickled in vinegar or wine to coagulate the crustacean muscle. This method of preparation does not kill the metacercaria, consequently infecting the host. Smashing rice-eating crabs in rice paddies, splashing juices containing metacercaria, can also transmit the parasite, or using juices strained from fresh crabs for medicinal uses. This parasite is easily spread because it can infect other animals (zoonosis). An assortment of mammals and birds can be infected and act as paratenic hosts. Ingestion of the paratenic host can lead to infection of this parasite.

Southeast Asia and Japan are home to Paragonimus westermani. Other Paragonimus species are found in regions of Asia, Africa, and South and Central America. P. westermani has been increasingly recognized in the United States during the past 15 years because of the increase of immigrants from endemic areas such as Southeast Asia and is estimated to infect 22 million people worldwide. Paragonimus has a quite complex life cycle that involves two intermediate hosts as well as humans. After being evacuated by coughing (unembryonated) or passing in human feces, eggs first grow in water.

The eggs get embryonated in the external environment. The parasite miracidia then hatch and infects the first intermediate host, which is a type of freshwater snail. Miracidia enter the snail’s soft tissues and proceed through numerous developmental stages before maturing into cercariae in 3 to 5 months. Cercariae then attack the second intermediate host, such as crabs or crayfish, and grow into metacercaria within two months. Consumption of raw or undercooked crabs infects humans or other animals (definitive hosts). Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercaria of the parasite. The metacercaria excyst in the duodenum, 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. 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 cycles is not achieved, because the eggs laid cannot exit these sites.

The parasite P. westermani is typically transmitted to humans and animals by the ingestion of raw or undercooked seafood. P. westermani is found in an estimated 80% of freshwater crabs in Asia. Live crabs are crushed during preparation, and metacercaria may contaminate the person preparing the meal’s fingers/utensils. Accidental transmission of infective cysts can occur when food handlers touch raw fish and contaminate cooking equipment and other items. Consumption of animals that feed on crustaceans can potentially spread the parasite; for example, raw boar meat has been linked to human infection in Japan. Food preparation procedures like pickling and salting do not kill the causal factor. In Chinese research, for example, eating “drunken crabs” was shown to be very dangerous since the infection rate was 100 percent when crabs were submerged in wine for 3–5 minutes and offered to cats/dogs.

The diagnosis is predicated on microscopic evidence of eggs in stool or sputum, which do not appear until 2 to 3 months after infection. Eggs, on the other hand, are occasionally found in effusion fluid or biopsy material. Furthermore, morphologic comparisons with different intestinal parasites can be used to identify potential causal agents. Finally, antibody detection is effective in the identification of minor infections as well as extrapulmonary paragonimiasis. Antibodies to Paragonimus westermani have helped clinicians distinguish paragonimiasis from TB among Indochinese immigrants in the United States. Radiological procedures can also be used to X-ray the chest cavity and check for worms. Because lung infections resemble TB, pneumonia, or spirochetosis, this approach is readily misinterpreted. This parasite can also be identified by a lung biopsy. Infection with Paragonimus in humans can result in either acute or persistent symptoms.

Diarrhea, stomach discomfort, fever, cough, urticaria, hepatosplenomegaly, pulmonary abnormalities, and eosinophilia may occur during the acute phase (invasion and migration).The acute stage, which coincides to the period of fluke invasion and migration, comprises of stomach discomfort, diarrhea, and rashes, followed by fever, chest discomfort, cough, and/or dyspnea 1 to 2 weeks later. Chronic Symptoms: Cough, expectoration of discolored sputum, hemoptysis, and chest radiography abnormalities are common during the chronic phase. Praziquantel is the preferred treatment for paragonimiasis. The suggested dosage of 75 mg/kg per day, given into three doses over three days, has been shown to kill P. westermani. Bithionol is an alternate therapy for this illness, however it has been linked to skin rashes and asthma. Prevention efforts can encourage safer cooking procedures and more sanitary handling of possibly contaminated seafood to promote more hygienic food preparation.