INTRODUCTION TO THE TREMATODES Trematodes are solid bodied organisms that are part of the Phylum Platyhelminthes along with the Cestodes. The Trematoda are related to a group of free living worms called the Turbellaria, which contain the earth dwelling Planaria and various marine forms. The Trematodes are divided into three different groups, the Monogenea, the Aspidogastrea, and the Digenea. The Monogenea are free living forms. The Aspidogastrea are parasitic in some fish and primitive vertebrates, such as turtles. The Digenea, which are all parasitic, is the group containing all the parasites of human beings. The Digenea are also characterized by all requiring a molluscan intermediate host in which there is a form of asexual multiplication. In general the Digenea are all solid bodied organisms with parenchymal cells that fill in any spaces that might exist between any cavities produced in the body by the various organs. The typical digenian has a mouth but no anus and the mouth is usually surrounded by an oral sucker. The intestinal tract is a blind tube, and after food is digested any undigested material is expelled from the mouth. On the ventral surface of the body, there is typically another sucker that is solely an attachment organ. The ventral sucker is called an acetabulum and it is usually located in the anterior part of the body some distance posterior to the oral sucker. However, there are some trematodes (flukes) in which the sucker is posteriorly located. Most of the trematodes are hermaphroditic and have a complete complement of male and female organs and are capable of reproducing without the need of a mate. The typical fluke has a single ovary and a pair of testes, but this pattern can vary greatly between different genera and families. The ovary typically leads to a uterus that is filled with eggs. The eggs of a fluke typically have brown shells and a small cap, called an operculum, from which the enclosed larval larva escapes. The male reproductive system typically ends in a penile organ that is inserted into the other fluke at the time of mating. The male and female reproductive openings typically exit from a common genital pore. The genital pore may have its own sucker that is used in reproduction that is called a gonotyl. The life cycle of digenetic flukes involves an alternation of generations. In almost all fluke infections, the eggs leave the host in the feces in an undifferentiated state. After they reach water, they undergo a period of development that often takes several days. The stage that develops in the egg is called a miracidium. The developed miracidium exits the egg through a hole, which develops when the operculum is caused to separate from the eggshell as a result of its own secretions. In most cases, the ciliated miracidium swims about until it finds a snail host. In other cases, the miracidium will not hatch until a snail ingests the egg. Whether the miracidium enters the snail by penetration or by ingestion, the next stages of development occur within the snail host, which are important for asexual multiplication. The first stage that is usually formed is a simple bag that is called a sporocyst. The sporocyst will typically produce a second generation of sporocysts that are called daughter sporocysts that often will leave the mother sporocyst to take up residence of their own in the tissues. In the sporocyst, a redia (plural rediae) develops. These stages have a mouth and a pharynx that may or may not be surrounded by a sucker. The redia will feed on snail tissue or sometimes are predatory on other trematode stages present in snails. Occasionally there is a second generation of rediae. In all cases the final stage produced in the snail is a stage called a cercaria (plural cercariae). This is a stage that has a body that looks something like the adult and typically has a tail. The tails may be long or short, and may be forked. The cercariae is a motile stage that typically seeks out the next intermediate host in which it will encyst after losing its tail to produce the stage called a metacercaria. The metacercarial stage can occur in many different types of hosts and sometimes do not require a host. In some trematodes, the cercaria will turn into a metacercaria on the surface of a plant or free within the water of a pond. In other cases, the metacercaria will form in the tissues of a second snail or develop in the tissues of a fish. In another scenario, the metacercaria will be localized in an arthropod, such as an ant, a crab, or a crayfish. With the genus Alaria, the stage after the metacercaria, called a mesocercaria, never encysts; this is a stage that can pass through a series of paratenic hosts. In all cases, except in the case of the schistosomes, the final host becomes infected through the ingestion of the metacercarial stage. With the schistosomes, the cercaria penetrates the skin of the vertebrate host and there is no metacercarial stage. After entering the vertebrate final host by ingestion, the metacercarial cyst wall is digested away in the intestinal tract. The young adult fluke will either develop in the intestine of the final host or migrate to its final site of maturation and persistence. Schistosomes differ from the other flukes in that after the penetration of the skin, the young fluke migrates to the lungs and then migrates to the site of development within a blood vessel without ever having entered the digestive tract. Trematodes tend to be highly selective in their use of snail hosts, but tend to be less selective in their use of final vertebrate hosts. Thus, there are a number of various trematodes reported from people who have accidentally ingested the second intermediate host without cooking it first. This chapter does not discuss many of the trematodes that have been reported only rarely.
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