Trematoda (flukes)

Class: Trematoda (flukes)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Lophotrochozoa > Platyhelminthes

Several thousand species of flukes have been described; all are parasites. Adults are typically armed with a pair of suckers. The suckers were originally thought to be holes, hence the name Trematoda: in Latin, a trema is a hole. Most trematodes have an anterior mouth that opens into an oral sucker connected to a forked digestive tract. Since adult trematodes live in the host's body, they have a tough covering called a tegument which, in contrast to the skins of turbellarians, lacks cilia.

Because of the small chance of transmission of parasite stages from host to host, internal parasites put much energy into reproduction, producing many, many eggs and/or larvae because so few ever reach the next host in the life cycle. In consequence, a great proportion of a parasite’s body is given over to reproductive organs. Indeed, on maturity, some parasites become virtually nothing but a bag of eggs.

Liver and blood flukes are of great medical and veterinary importance but both in their own way are atypical of the Trematoda.

All trematodes have at least two hosts in the life cycle, one of which is a mollusc and the other a vertebrate; some species may have a third and even a fourth host, too. Eggs of the adult fluke, living in the body of a vertebrate host, escape to the environment where they hatch, releasing a ciliated, free-swimming larva called a miracidium. This minute larva, a fraction of a millimetre in length, seeks a suitable snail host. It bores into the tissue of the mollusc, loses its cilia and migrates to a suitable site, where it develops into a simple bag-like mother sporocyst. By an asexual process the mother sporocyst produces many similar daughter sporocysts, which find their way to the gonad or digestive gland of the snail. Depending on the fluke species, some mother sporocysts produce daughter sporocysts and others produce more muscular, motile offspring called rediae which, in contrast to daughter sporocysts, have a well-developed digestive system. Both rediae and daughter sporocysts produce balls of cells that develop into the tadpole-like larval stage called a cercaria. Many cercariae have tails and are able to swim. Their sensory organs enable them to detect a host, whose skin they penetrate using chemical means. If this is the final host, the cercariae shed their tails and migrate in the blood to their adult resting place, where they mature and begin laying eggs. In contrast, the cercariae of some trematodes enclose themselves in a tough coat and then encyst on vegetation, or inside the next intermediate host if there is one in the life cycle, or even in the snail host. The cyst stage contains a metacercaria (Greek meta = after or later). If it is ingested by the final host, the wall of the cyst is digested away and the freed metacercaria develops into an adult fluke.

Adults of sheep liver flukes (Fasciola hepatica) can reach 25 millimetres in length. They are hermaphroditic, as are most trematodes, and the reproductive and digestive systems are much branched. The life cycle follows that described above except that the cercaria encysts on vegetation in wetlands and waits to be eaten by the final host (sheep, cow or human). Once in the body of the final host, it escapes from the cyst and migrates from the stomach to the liver via the bile duct where it develops into an adult. The adult flukes damage the walls of the bile duct, leading to blood loss and reduction in bile flow. A bizarre and aberrant human infection called halzoun has been reported from some areas in the Middle East where raw liver is customarily eaten. Should this contain liver flukes they are able to attach themselves temporarily to the pharynx causing pain and distress.

Blood flukes (Schistosoma spp.) are the causative organisms of the disease bilharzia. The name Schistosoma is derived from the Greek schistos = split and soma = the body. The split is the groove down the ventral surface of the male in which the female permanently resides. Separate sexes as seen in Schistosoma are atypical of the trematodes: hermaphroditism is more common because transmission from host to host is uncertain and so the likelihood of invidual flukes of the opposite sex meeting in the host is low. Schistosoma compensates at least in part by male and female meeting and pairing up in the liver before migrating, joined together, to the blood vessels of the bladder or rectum. The female lays a large numbers of eggs each day. The shell of each egg has a spine, and releases enzymes that soften the host tissue. The pulsing of the host’s blood system works the spine through the vessel wall and the eggs pass into the bladder. This causes blood loss to the urine and the host frequently suffers from anaemia, with consequent fatigue and lethargy. The eggs may also lodge in the capillaries of the liver and lungs, or even in the nervous system, where they kill tissues and cause fibrous growths. The infection also has a more sinister aspect, the irritant effect of the spine and enzymes sometimes inducing development of cancer of the urinary system. Eggs that manage to escape from the human host into water hatch into miracidia, which seek and penetrate water snails. Here they go through sporocyst stages to release cercariae, which penetrate the skin of the final host - a human. The cercaria loses its tail and the infective stage, called a schistosomula, eventually arrives in the blood vessels of the liver where it matures and finds a mate to complete the cycle.