Helminth Taxonomy - The Cestodes (Tapeworms)
These are long, extremely dorso-ventraly flattened hermaphroditic parasites. They do not have intestines, absorbing nutrients through their tegumental walls. There are two sub-classes in this group. The tegument is covered in microscopic, electron-dense microtriches. These parasites generally have larval forms encysted in an intermediate host, aiding geographical distribution, as the intermediate host moves around. Importantly, this type of lifecycle also aids the parasites temporal distribution. This is because to complete the lifecycle the intermediate host must often be eaten by a carnivorous definitive host and this may occur after different, sometimes very long, periods of time after infection of the intermediate host. Thus, the intermediate host may act as a reservoir of infection, even if all traces of the adult parasites have been vanished from the definitive host population. This may have important consequences for successful control of parasitic diseases caused by these organisms as, generally, it is relatively easy to chemotherapeutically cure hosts of the adult tapeworms, but the larval forms are much more resistant to antihelminthic drugs.
Subclass Cestodaria - These are non-segmented parasites, with only a single set of sexual organs. They are parasites of fish and other cold blooded vertebrates. There are two orders of cestodarians, the Amphilinidea and the Gyrocotylidea.
Subclass Eucestoda - The tapeworms. These are segmented parasites, each segment (proglottid), containing both male and female sexual organs, (though these may mature at different rates). They can be divided into a number of Orders on the basis of such features as the structure of the scolex and arrangement of sexual organs. The adult forms are found in the intestine (or more rarely bile duct) of, mainly, vertebrates. Only the Pseudophyllidean and Cyclophyllidean parasites are of major interest as tapeworm parasites of man.
Order Pseudophyllidea - The scolex (anterior attachment organ) has two bothria, (muscular grooves). Typically the lifecycle has two intermediate hosts, the first being a crustacean (such as Cyclops sp.), where the larval form is a procercoid. The second is a fish, where the larval form is a plerocercoid, with the definitive host being a fish eating vertebrate, (including in some cases humans). Carnivorous fish or other vertebrates ingesting these infected second intermediate hosts may act as paratenic hosts, which are also infective when eaten. The eggs are very similar to those of the Digeneans, with thin walls and an operculum.
Examples
Family Diphyllobothriidae
e.g. Diphylobothrium latum - The Fish tapeworm, adults in humans and other fish eating mammals, plerocercoids in fish, procercoids in Cyclops.
e.g. Spirometra sp. - Adults are found in cats and dogs, the second intermediate host being frogs. The plerocercoid (sparganum) may infect almost any vertebrate as paratenic hosts, (including occasionally man), if infected frogs are ingested.
e.g. Schistocephalus solidus - Definitive host is fish eating birds. The larval plerecercoid being found in the body cavity of the three-spined stickleback (marine and freshwater).
e.g. Ligula intestinalis- Definitive host is fish eating birds. The larval plerecercoid being found in the body cavity of fresh water fish.
Order Cyclophyllidea- The scolex (anterior attachment organ has hooks or suckers). The lifecycle usually has only one intermediate host, which may typically be a mammal, fish or arthropod. Pathology is usually associated with the larval forms of the parasite in the intermediate host (e.g. Echinococcus, causing hydatid disease, or Taenia solium, causing cysticercosis). There are many larval forms of these parasites, the simplest being the cysticercoid form, a solid form into which a scolex is depressed into a cavity, the scolex filling the cavity. These often with a small anterior tail. The eggs have very thick shells, and containing 'hexacanth' larvae, armed with hooks.
Examples
Family Taeniidae - Intermediate host vertebrate, the larvae being in a variety of cyst like forms. The most common of these include:
cysticercus - a bladder into which is invaginated a single scolex.
coenurus - similar to the cysticercus but with many scolices.
hydatid cyst - a large cyst containing free individual scolices, brood capsules from which these are generated, and daughter cysts.
(NB. In the case of the coenurus and hydatid cyst the larvae undergoes asexual reproduction).
The definitive host becomes infected by ingestion of the larvae in infected meat.
e.g. Taenia solium - Pork tapeworm - Humans are the definitive hosts. Intermediate hosts pigs (and sometimes man), with cysticercus larvae.
e.g. Taenia saginata - Beef tapeworm - Humans are the definitive hosts. Intermediate hosts cattle, with cysticercus larvae.
e.g. Taenia multiceps - Adults in the dog, intermediate hosts is the sheep, where a large coenurus forms in the brain.
e.g. Echinococcus granulosus - Dog tapeworm. Many mammals, including humans (usually as an accidental host), can act as intermediate host, the larval form here being hydatid cysts.Family Hymenolepididae - Intermediate host arthropods with cysticercoid larvae.
e.g. Hymenolepis nana - Dwarf tapeworm of man. Infection by accidental ingestion of arthropod intermediate host.
Family Dilepididae - Intermediate host insects with cysticercoid larvae.
e.g. Dipylidium caninum - Dog tapeworm, the more commonly found tapeworm of dogs. Infection by accidental ingestion of arthropod intermediate host (e.g. dog fleas).
Family Mesocestoididae - Usually two intermediate hosts, firstly arthropod, then vertebrate, with tetrathyridium larvae, similar to an elongated cysticercus, that is capable of asexually dividing in both intermediate hosts. The mammalian definitive host becomes infection on ingestion of the second intermediate host.
Order Tetraphyllidea - Parasitic in elasmobranchs. Scolex with four leaf-like outgrowths.
Order Proteocephalata - Parasitic in fish, amphibians and reptiles. Scolex with four suckers and prominent apical organ.
Order Spatheobothridea - Parasitic in teleost fish. Scolex weakly developed, or with funnel or cup-like apical organs.