Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes". The trematodes or flukes are estimated to include 18,000 to 24,000 species. This term Fluke can be traced back to the Old English name for flounder, and refers to the flattened, rhomboidal shape of the worms.
The flukes can be classified into two groups, on the basis of the system which they infect in the vertebrate host.
Tissue flukes infect the bile ducts, lungs, or other biological tissues. This group includes the lung fluke, Paragonimus westermani, and the liver flukes, Clonorchis sinensis and Fasciola hepatica.
Blood flukes inhabit the blood in some stages of their life cycle. Blood flukes include species of the genus Schistosoma.
They may also be classified according to the environment in which they are found. For instance, pond flukes infect fish in ponds.
Varied trematodes, are flattened oval or worm-like animals, usually no more than a few centimetres in length, although species as small as 1 millimetre (0.039 in) and as large as 7 metres (23 ft) are known. Their most distinctive external feature is the presence of two suckers, one close to the mouth, and the other on the underside of the animal.
The mouth is located at the forward end of the animal. As in other flatworms, there is no anus, and waste material must be egested through the mouth. Most trematodes are simultaneous hermaphrodites, having both male and female organs.
The blood fluke is found in Africa and the Far East and currently infects approximately 250 million people. The trematode enters through an opening in the skin and then lives in the human's lungs, heart and circulation system.
Giant Intestinal Flukes
The giant intestinal fluke is common to southeast and central Asia and enters the body by eating infected water chestnuts. This group of flukes lives in the human small intestine.
Liver flukes are found in all parts of the world and are contracted by eating undercooked watercress. The liver is the primary organ damaged by this group of trematodes.
Spindloid flukes are ingested by eating raw or undercooked fish. The bile ducts are damaged by the spindloid fluke infestation.
The lung fluke is most commonly found in Asia, South America and Africa where infested crab meat is eaten. The fluke creates a tissue capsule in the lungs creating a dry cough and occasionally pleurisy.
Fasciola hepatica the common name of the "sheep liver fluke" and is somewhat misleading since this parasite is found in animals other than sheep (including cattle and humans), and the parasite resides in the bile ducts inside the liver rather than the liver itself. This species is a common parasite of sheep and cattle and, therefore, relatively easy to obtain.
The adult parasites reside in the intrahepatic bile ducts, produce eggs, and the eggs are passed in the host's feces. After passing through the first intermediate host (a snail), cercariae encyst on vegetation. The definitive host is infected when it eats the contaminated vegetation. The metacercaria excysts in the definitive host's small intestine, and the immature worm penetrates the small intestine and migrates through the abdominal cavity to the host's liver. The juvenile worm penetrates and migrates through the host's liver and finally ends up in the bile ducts.
Flukes normally inhabit the digestive system and/or liver. Some flukes seek out the lungs, or may wander to the heart, brain or skin.
The majority of infections caused are asymptomatic. When present, initial symptoms are usually abdominal pain and diarrhea. Severe infection, characterized by a heavy worm load, may be associated with severe non-bloody diarrhea, anemia, facial edema, peripheral edema, ascites, and intestinal obstruction. Erosion into the intestinal wall can lead to mucosal ulceration, GI hemorrhage, and/or abscess formation. Absorption of toxic metabolites may lead to the development of systemic symptoms.
Symptoms appear in three distinct phases. In the initial phase, symptoms include: fever, skin rash, abdominal pain, bronchitis, enlargement of the liver and spleen, and diarrhea. In the intermediate phase, symptoms include pathological changes in the intestinal and urinary tracts, and eggs in the urine and feces. The final phase results in complications involving major bodily systems.
Fasciola hepatica is found in parts of the United States (view distribution), as well as in Great Britian, Ireland, Europe, the Middle East, the Far East, Africa, and Australia. Fascioliasis in sheep and cattle results in animals that show low productivity (low weight gain, low milk production, etc.). The migration of the worms through the host's liver, and the presence of the worms in the bile ducts, are responsible for the pathology associated with fascioliasis.
Swimmers itch is caused by a schistosome parasite of another species that tries to invade a human host. When the parasite tries to penetrate and is unable to complete the migration, it dies, but leaves a small, inflammed, pus-filled pimple. The condition is not a serious health threat, but more of an annoyance.
Flukes are commonly diagnosed by locating eggs in the human urine or stool
Human infections are most common in the Orient, Africa, South America, or the Middle East. However, flukes can be found anywhere that human waste is used as fertilizer.
The main lines of precaution are those dictated by sanitary science:
- Use a water filter
- If you eat meat or fish or seafood, make sure it is well done. Salting, pickling, drying, and smoking does not always kill the parasites in meat.
- Thoroughly washing food and cooking food
- Don't swim in infested waterways. Also watch transmission from wet hands to mouth or nose. Wash your body with clean water.
- Do not walk barefoot in known infected areas
- Washing hands after going to the toilet, and before handling or eating food.
- Using safe food preparation practices.
- Cleaning of animal living areas regularly, and hygienically collecting and disposing of faeces.
- Do not use human excrement or raw sewage or untreated 'night soil' as manure/fertilizer in agriculture
- Checking sewage systems often to ensure they are not broken or faulty.