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What are Tapeworms?[edit]

Tapeworms (Cestoda) are flatworms (Platyhelminthes) related to the direct cycle flukes (Monogenea), the complex cycle flukes (Digenea), and the free-living flatworms (planaria and turbellarians on the gill books of horseshoe crabs), with a few other representatives. Tapeworms consist of an attachment end called the scolex, a neck or budding part, and then a series of segments. The structure of the scolex is important in determining relationships.

The adult tapeworm lives in the small intestine of most vertebrates, here they reproduce, so this vertebrate is called the definitive host. In sharks, rays, and skates, tapeworms occur in its equivalent portion of the gut called the spiral valve.

The principal tapeworm groups of elasmobranchs are the Tetraphyllidea, with a leaf-like or flower-like scolex, and the Trypanorhyncha, with thorny, eversible tubes in the scolex. The principal tapeworm group in fishes is the Proteocephala, and a unique additional group (with but a single member) occurs in the primitive freshwater fish, Amia calva. Other groups occur in fishes, and in birds, mammals, and herptiles.

Tapeworm Reproduction and How they Spread[edit]

The adult tapeworm consists of the scolex, a growing or generative region immediately behind (away from the intestine), and a series of incareasingly older segments containing sex organs (both male and female), yolk-producing glands, and a sac for fertilized eggs. A tapeworm cannot fertilize itself within the same segment, but it can fertilize segments that fold back onto itself.

Because the male and female organs develop at different rates, that's not a difficult feat. In many higher animals, usually a single tapeworm or just a few occupy the intestine, but in fishes it's normal for large numbers of tapeworms (of the same species) to occur in a single host. Depending on species, tapeworms either shed old, gravid segments or simply release fully developed eggs from the older segments. The eggs (or segments) are swept down the gut and evacuated with the faeces into (in the case of fishes) water.

Again, depending on the species, the eggs either hatch into a ciliated larva that seeks a host or are ingested by that host, which is (in many cases) a copepod. The copepod serves as the first intermediate host, but there may be others, and in some cases there may be paratenic hosts that only serve as unnecessary (but useful) transfer hosts. In paratenic hosts, no new development occurs, but the parasite survives.

Typically the copepod is eaten by a small fish in which the larval tapeworm develops further, and there may be an additional step, such as a larger fish (either a paratenic or a developmental host). In some intermediate fish hosts, the late larva is large, but not sexually developed nor does it make segments. In some fish, it may encyst as the host walls it off with scar tissue. At this stage, the scolex is sometimes formed sufficiently to make an identification. Finally, if this last intermediate host is eaten by the correct host (the same as the original parent), it is digested, releasing the late larval tapeworm (usually called a plerocercus or pleroceroid), where it passes the stomach (if there is one) and arrives and attaches in the small intestine (or its equivalent), where it matures, produces segments, and reproduces more egg-laden segments. The segments of a tapeworm are often called proglottids, but this is incorrect. A proglottid is all the equipment for reproduction and, while typically one segment contains one proglottid, that's not always the case.

Tapeworms and other flatworms have a unique water-balance system based on ciliated vase-like sacs that drive water and wastes out through a network of tubes most readily seen as in tapeworms as a pair of ladder-like rungs on either side of the segment, and continuous through the segments. This flame-cell system has no equivalent, one reason the platyhelminthes are a single phylum. Humans are hosts for several kinds of tapeworm associated with eating the intermediate stages in mammals and fishes. The broad fish tapeworm of humans is linked to eating fishes of the great lakes and elsewhere. Tapeworms absorb nutrients from the intestinal villi of the host, and are biggest and most abundant in big, fat, healthy hosts.

A starved host cannot provide nutrition for tapeworms, and they weaken and are lost to the outside where they die. Because hunger is based on blood sugar and stomach filling, there is nothing to the old wives tale about tapeworms causing hunger.


  • Panacur (Fenbendazole)