Trematodes are flukes of the class Trematoda; phylum Platyhelminthes. Important ones affecting man belong to the genera Schistosoma (blood fluke), Echinostoma (intestinal fluke), Fasciolopsis (liver fluke), Gastrodiscoides (intestinal fluke), Heterophyes (intestinal fluke), Metagonimus (intestinal fluke), Clonorchis (Asiatic liver fluke), Fasciola (liver fluke), Dicrocoelium (liver fluke), Opisthorchis (liver fluke), and Paragonimus (lung fluke). Man usually becomes infected after ingesting insufficiently cooked fish, crustaceans, or vegetables that contain their larvae. The cycle begins when larvae are released into freshwater by infected snails. The free-swimming larvae can then directly penetrate the skin of humans while swimming or be ingested after encycsting in or on various edible vegetation, fish, or crustaceans.
The oval-shaped fluke (sometimes called a flatworm) has a tough outer body layer called a tegument that covers layers of circular, longitudinal, and diagonal muscles that protects it from the human digestive tract. Some can inhabit the liver, bile duct, or lymph vessels. They can be several inches long, an inch or so wide, and only thick enough to hold themselves together. Below are just a very few examples of the thousands known.
- Blood flukes:
- Schistosoma japonicum, S. mansoni, S. haematobium are three species of blood flukes (schistosomes) that cause the disease schistosomiasis, which infects about 200 million people worldwide. One of the three types of disease, S. japonicum is found in Asia; S. mansoni occurs in Africa, the Eastern Mediterranean, the Caribbean, and South America; the third, S. haematobium is found in Egypt. Freshwater snails play intermediate host in the life cycle development of these blood flukes. The snails release larvae into the water, where they can penetrate the skin of swimmers or bathers. The parasites burrow into the skin, and then are carried into the bloodstream to be taken to the liver, intestines, or bladder. There are two forms of schistosomiasis. With one, inflammation begins when the worms lodge in the lining of the intestine or liver. With the other form, the bladder and urinary tract can become fatally infected by worms as they lodge in the walls. Travellers to Africa, especially, are warned not to bath, wade, or swim in fresh water because of possible infestations blood flukes. Infection causes fever and chills, but also elevates the number of white blood cells (eosinophils), as well as producing abdominal pain resulting from enlargements of the liver and spleen. Often, these symptoms do not show up for four to eight weeks after exposure, and, therefore, may not be associated with the possibility of parasite infestation while on vacation.
- Liver fluke:
- Clonorchis sinensis is common in the Orient and Hawaii, and is transmitted through the ingestion of raw, dried, salted, pickled, or undercooked fish. Snails, carp, and forty additional fish species have been known to play the intermediate host to this fluke. In humans, it inhabits the bile ducts of the liver, causing it to enlarge and become tender, as well as producing chills, fever, jaundice, and a type of hepatitis.
- Oriental lung fluke:
- Paragonimus westermani is found mainly in the Far East, where it enters the body, producing the disease called paragonimiasis. Humans acquire the fluke ingesting infected crabs and crayfish that have not been sufficiently cooked or are served raw. The adult worms go to the lungs, and, sometimes, the brain, where seizures similar to epilepsy can occur. Symptoms include an occasional mild cough, producing a peculiar rusty brown sputum. The lung fluke can perforate lung tissue and deplete oxygen supplies to the entire bloodstream. Symptoms often resemble those of pulmonary tuberculosis.
- Sheep liver fluke:
- Fasciola hepatica is more common in Central and South America, parts of Africa, Asia, and Australia. Infection is usually acquired from eating the larva worms encysted on such aquatic vegetation as watercress. Worms migrate to the liver and bile ducts, where they produce upper right quadrant abdominal pain, liver abscesses, and fibrosis.
- Intestinal fluke:
- Fasciolopsis buski is more common in Southeast Asia, Australia, and Latin America. Transmission occurs when individuals bite into the unpeeled outer skin of plants that harbor encycsted larvae. Such plants can be water chestnuts, bamboo shoots, and lotus plant roots because they are often cultivated in ponds and streams infected by animal waste. Adult flukes live in the duodenum (the shortest and widest part of the small intestine) and jejunum (connects the duodenum and the ileum, which opens into the large intestine), where they cause ulceration. Symptoms include the following: diarrhea, nausea, vomiting, abdominal pain, as well as facial and abdominal edema.