Description

Tapeworms are also common and in the dog are usually Dipylidium caninum, which is spread by ingesting fleas and lice. Also common is Taenia pisiformis, spread by ingesting rabbits and rodents. Rare tapeworm infections are caused by species of the genera Echinococcus, Mesocestoides, and Spirometra. There are usually no symptoms.

Anatomy

Scolex

The worm's scolex ("head") attaches to the intestine of the definitive host. In some species, the scolex is dominated by bothria, which are sometimes called "sucking grooves", and function like suction cups. Other species have hooks and suckers that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex.

Once anchored to the host's intestinal wall, the tapeworm absorbs nutrients through its skin as the food being digested by the host flows past it and it begins to grow a long tail, with each segment containing an independent digestive system and reproductive tract. Older segments are pushed toward the tip of the tail as new segments are produced by the neckpiece. By the time a segment has reached the end of the tail, only the reproductive tract is left. It then drops off, carrying the tapeworm eggs to the next host.

While the scolex is often the most distinctive part of an adult tapeworm, it is often unnoticed in a clinical setting as it is inside the patient. Thus, identifying eggs and proglottids in feces is important.

Body systems

The main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number and complexity of the scolex. Smaller nerves emanate from the commissures to supply the general body muscular and sensory ending. The cirrus and vagina are innervated and sensory endings around the genital pore are more plentiful than other areas. Sensory function includes both tactoreception and chemoreception. Some nerves are only temporary. These are in the proglottids, and stop working with a detach.

Proglottids

The body is composed of successive segments (proglottids). The sum of the proglottids is called a strobila, which is thin, resembling a strip of tape, and is the source of the common name "tapeworm". Like some other flatworms, cestodes use flame cells (protonephridia), located in the proglottids, for excretion.

Mature proglottids are released from the tapeworm's posterior end and leave the host in feces.

Because each proglottid contains the male and female reproductive structures, they can reproduce independently. It has been suggested by some biologists that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids.

The layout of proglottids comes in two forms, craspedote, meaning proglottids are overlapped by the previous proglottid, and acraspedote which indicates a non-overlapping conjoined proglottid.

Reproduction and life cycle

True tapeworms are exclusively hermaphrodites having both male and female reproductive systems in their bodies. Reproductive system includes one or many testes, cirrus, vas efferens and seminal vesicle as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs. There is a common external opening for both male and female reproductive systems known as genital pore, which is situated at the surface opening of the cup-shaped atrium.  Even though they are sexually hermaphroditic, self-fertilization is a rare phenomenon. In order to permit hybridization, cross-fertilization between two individuals is often practiced for reproduction. During copulation, the cirrus one individual connects with that of the other through the genital pore, and then exchange their spermatozoa.

Life cycle of tapeworms is simple in the sense that there are no asexual phases as in other flatworms, but complicated in that at least one intermediate host is required in addition to the definitive host, and this life cycle pattern have been a crucial criteria for assessing evolution among Platyhelminthes.^[5] Many tapeworms have a two-phase life cycle with two types of host. The adult taenia saginata, for example lives in the gut of a primate such as a human. Proglottids leave the body through the anus and fall onto the ground, where they may be eaten with grass by animals such as cows. In the cow's body the juvenile forms migrate and establish themselves as cysts in body tissues such as muscles, rather than the gut; they cause more damage to this host than the intestinal form to its host. The parasite completes its life cycle when the grass-eater is eaten by a compatible carnivore—possibly a human with a preference for raw meat—in whose gut the adult taenia establishes itself.

Wikipedia and GNU

The content of this entry is from the Wikipedia article "Tapeworms" licensed under the GNU Free Documentation License.