The tapeworms belong to a branch of the platyhelminths known as the cestodes. Flat in cross-section, tapeworms are highly specialised parasites: many of their organ systems have disappeared in their millions of years of parasitic existence, and there is no free-living organism which even closely resembles them. Tapeworms consist of an anchoring organ ("scolex") which attaches them to the intestinal wall (adult tapeworms are invariably intestinal). The scolex may be armed with suckers, hooks, both or neither to help it hold fast. Growing out behind the scolex are the segments ("proglottids"), which are repeating organs that are complete reproductive organisms in themselves (they have both male and female sexual organs and self-fertilise). These continue to grow out from the scolex in a chain, maturing as they go, until the last segments break off and are passed out with feces.
Tapeworm life-cycles tend to follow a set pattern which depends on the presence of a predator-prey relationship. The adult tapeworms live in the predator (definitive host, for example a dog), while the prey (intermediate host, for example a sheep) plays host to the intermediate stages, most commonly called cysts or cysticerci. Eggs laid by the adult pass out in the feces of the dog (in this example) and are eaten by the sheep (via contaminated grass). These develop into the intermediate stage or cyst, which generates the adult when sheep meat containing a cyst is eaten by the dog. A tapeworm may pass through several intermediate hosts before it finds the right definitive host.
Adult tapeworms live reasonably at peace with their hosts. They do not feed off us, but rather rob us of our digested food – tapeworms lack a digestive tract and absorb nutrients directly across the skin or cuticle. Problems arise when the tapeworm becomes too large and starts blocking the bowel or robbing us of vital nutrients – the large tapeworms may cause deficiencies of vitamins such as B12 if left for too long.
Intestinal tapeworms are those which reach the adult stage in the human intestine. Note that some tapeworms (e.g. Taenia solium) may use humans as both intermediate and definitive host.
The Beef Tapeworm (Taenia saginata)
This is a giant among the human parasites. A complete specimen may grow to 8 meters (27 feet) in length, which is nearly the length of an adult human's digestive tract. The worms attach high up in the small intestine and grow downwards. The segments of this worm may reach 1.5cm x 1cm (approximately 6 by 4 tenths of an inch). With such a large body, these worms are prime candidates for causing nutritional deficiencies in the host. As the name suggests, the intermediate host for this worm is the cow, and the cysticerci may be observed as 0.5cm diameter fluid filled bladders in the muscle of these animals. Meat inspectors should look for these cysticerci in slaughtered cattle. Taenia saginata is endemic in many countries, but modern methods of meat processing mean that it is rarely seen now. Aside from nutritional problems, the presence of this tapeworm generally causes mild to moderate abdominal symptoms (nausea, pain, etc).
The Pork Tapeworm (Taenia solium)
This is a close relative of Taenia saginata, although the intermediate host of this parasite is the pig. Humans are infected by eating the cysticercus in undercooked pork. Taenia solium is slightly smaller than Taenia saginata ("only" 3-4 meters long), but is considered more dangerous. Unlike T. saginata, humans are susceptible to developing the cysticercus of T. solium if the eggs are ingested. Therefore, if someone harbors a pork tapeworm, they pose a risk to themselves and others around them of developing cysticercosis. These cycticerci may lodge in the brain, eye or muscle, causing serious problems. Furthermore, if the body kills the parasites, calcium salts are laid down in their place, creating tiny pebbles in the soft tissue.
The Broad Fish Tapeworm (Diphyllobothrium latum)
This tapeworm belongs to a simple group known as the pseudophyllideans. These tapeworms, while still featuring the basic tapeworm body plan of holdfast and segments, share a number of features in common with the flukes. Pseudophyllidean eggs are similar in appearance to fluke eggs and the organism that hatches out (the coracidium) is similar to the miracidium in flukes. Pseudophyllidean tapeworms also need more than one intermediate host to complete their life-cycles (normally a waterflea and some small vertebrate). The infective stage to the definitive host is known as the plerocercoid larva.
In Diphyllobothrium the plerocercoid is found in large marine fish. If the flesh of these fish containing the parasite is served raw or undercooked, the adult tapeworm may develop in humans. Diphyllobothrium is large tapeworm and infection with this parasite is particularly associated with vitamin B12 deficiency.
Hymenolepis nana and Hymenolepis diminuta
These are tapeworms normally found in the mouse and the rat, respectively. The intermediate hosts for these tapeworms are grain beetles, so humans can become infected should they accidentally swallow the beetles while eating grain products. H. nana is a minute tapeworm, less than 0.5mm wide and less than 1cm long. H. diminuta is slightly larger (4-5cm in length). H. nana has the unusual characteristic of being able to complete its life-cycle without the aid of the intermediate host – the intermediate stage is capable of developing within the tissues of the definitive host. This makes H. nana the only tapeworm in which an adult will grow after the definitive host ingests the eggs. Infection with these tapeworms normally causes mild to moderate intestinal symptoms.
The Dog Tapeworm (Dipylidium caninum)
One of the more common parasites of domestic dogs is the tapeworm Dipylidium caninum. This parasite grows to around 10-15cm in length, with individual segments resembling cucumber seeds in size and shape. The intermediate host for dog tapeworms are the dog and cat fleas Ctenocephalides canis and Ctenocephalides felis. The eggs passed out in a dog's feces are eaten by the larval stage of the flea, and the immature tapeworm stays with the flea through its metamorphosis to the blood-sucking adult. When the flea bites, the dog may bite at the flea and swallow the larval tapeworm. In heavily infested dogs, their mouths may harbor many of the parasites in the bodies of fleas removed during grooming.
The segments of Dipylidium are capable of limited movement, and if this happens in the dog's rectum or anus, it causes intense itching. Afflicted dogs may be observed to "scoot" or drag their backsides along the ground to relive the itching. This, of course helps to crush the segments and release the eggs into the environment.
If humans swallow infected fleas, they too may become infected. This is easier than it sounds. Allowing the dog to lick your mouth may transfer the larval tapeworms to your mouth, while crushing the fleas between your fingernails spreads the parasites across your hands. Children are particularly prone to infection with Dipylidium. Like other flatworms, this tapeworm is not affected by routine dog worming treatments (i.e. those directed against the intestinal nematodes like roundworm, whipworm or hookworm). Instead, dogs should be routinely wormed with a broad spectrum anthelmintic – one which has been shown to be effective against tapeworms.
Tissue tapeworms are those which do not reach an adult stage in the human host. Instead, they infect humans with their intermediate or cyst stage. Such infections are frequently more serious than those caused by the adult tapeworm, as they involve more intimate connection with the host's tissue.
The Hydatid Tapeworm (Echinococcus granulosus)
This is a dangerous tapeworm found in dogs. This tapeworm normally cycles between carnivores such as dogs, wolves or foxes (as definitive hosts) and herbivores such as sheep (as intermediate hosts). The adult tapeworms are small – rarely more than three segments long and cause minor symptoms in the dog. However, upon eating the eggs from the dog faeces, the intermediate host develops the hydatid cyst. The cysticerci of tapeworms such as Taenia are rarely larger than 0.5cm in diameter, and one cycticercus upon ingestion generates one adult tapeworm. However, in the case of hydatid disease, the cysts may grow to the size of football and generate many parasites. From the inside surface of the fluid-filled cyst grow hundreds of tiny protoscolices which bud off and fall to the bottom of the cysts, forming what is known as hydatid sand. Each one of these protoscolices is capable of generating a new adult tapeworm upon ingestion by a dog.
If a human should ingest eggs from the dog's feces, the hydatid cysts will grow inside them. The cysts are typically found in the liver and lungs, but may also be found in the brain. As they grow larger, they gradually replace the tissue in which they grow. Worse still, should a cyst rupture, the release of foreign parasite proteins into the body can trigger a massive and sometimes fatal allergic reaction. The only means of treating hydatid cysts is through surgical removal, and the surgeons must be very careful not to rupture the cyst during removal, due to the risk of allergic reaction and the fact that protoscolices may be capable of starting "daughter" cysts.
Naturally, it is easier to prevent hydatid infection than to cure it. Sheep farming areas normally have intensive anti-hydatid programs, regularly worming dogs and ensuring farm dogs are not fed uncooked offal (the New Zealand Government went so far as to supply farmers with guaranteed "hydatid-free" dog biscuits to feed their sheep dogs).
There are a number of variations on the life-cycle, with hydatids cycling between dingos and kangaroos in Australia for example, and between feral pigs and pig hunting dogs.
The Pork Tapeworm (Taenia solium)
See reference above under Intestinal Tapeworms.
Closely related to Diphyllobothrium is Spirometra. The definitive hosts of this tapeworm are small carnivores such as cats and dogs, while the intermediate hosts are waterfleas and small cold blooded vertebrates such as frogs and snakes. Should a human accidentally ingest the infected waterfleas, or the plerocercoid larva in undercooked snake or frog flesh, the worm will not mature in its new host, but remain at that stage. This is known as the sparganum. Spargana are long flat unsegmented white ribbons around 5cm long that creep about through the flesh by peristaltic movements of their bodies. They cause localised inflammation and irritation wherever they go, and may end up in sensitive areas such as the eye or brain. They can be best treated by surgical removal. Feral pigs in tropical regions frequently show spargana in their flesh.
Large parasites like the beef tapeworm compete with us for nutrients by robbing us of micronutrients (such as vitamins) before they get to the things we don't really need.