Trematoda

Trematoda
Botulus microporus, a giant digenean parasite from the intestine of a lancetfish
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Platyzoa
(unranked):	Bilateria
Phylum:	Platyhelminthes
Class:	Trematoda Rudolphi, 1808
Subclasses
Aspidogastrea Digenea

The Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic worms, commonly referred to as flukes.

Taxonomy and biodiversity

The Trematoda are estimated to include 18 000 to 24 000 species, and are divided into two subclasses. Nearly all trematodes are parasites of molluscs and vertebrates. The smaller Aspidogastrea, comprising about 100 species, are obligate parasites of molluscs and may also infect turtles and fishes, including cartilaginous fishes. The Digenea, which constitute the majority of trematode diversity, are obligate parasites of both molluscs and vertebrates, but rarely occur in cartilaginous fishes.

Formerly the Monogenea were included in the Trematoda on the basis that these worms are also vermiform parasites, but modern phylogenetic studies have raised this group to the status of a sister class within the Platyhelminthes, along with the Cestoda.

Life cycles

Almost all trematodes infect molluscs as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are monoecious and alternately reproduce sexually and asexually. The two main exceptions to this are the Aspidogastrea, which have no asexual reproduction, and the schistosomes, which are dioecious.

In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimming larval forms that are infective to the intermediate host, in which asexual reproduction occurs.

Infections

Human infections are most common in the Orient, Africa, South America, or the Middle East. However, trematodes can be found anywhere that human waste is used as fertilizer. Trematodes normally inhabit the digestive system and/or liver. Some trematodes seek out the lungs, or may wander to the heart, brain or skin. In the digestive system or liver, they disrupt digestion and nutrient absorption. Symptoms include chronic diarrhea and abdominal pain. Other symptoms occur from long-standing infections. These symptoms include ulcers, hemorrage and abscessess of the intestional wall, and liver damage. Sometimes severe toxemia results when the host's body absorbs the worm's metabolites. The most common medications used to kill trematodes are praziquantel, niclosamide, or tetrachloroethylene. However, currently, only limited information about treating these infections is available.

Trematode infections are usually acquired after ingesting one of the infective forms. Most fluke infections are acquired by (1) drinking infected water, (2) swallowing infected water while swimming; transmission from wet hands to mouth or nose, etc., (3) eating infected aquatic vegetation, or (4) eating infected raw meat or raw intermediate hosts (snails, shellfish, crustaceans, fish). One can kill trematodes by thoroughly cooking meats and vegetable taken from suspect waters. Salting, pickling, drying, and smoking does not always kill the parasites in meat.

Etymology

Trematodes are commonly referred to as flukes. This term can be traced back to the Saxon 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.