The tapeworms belong to another branch of the platyhelminths known as the cestodes. Like the flukes, they are flat in cross-section, they are entirely parasitic and are hermaphroditic. However, the 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. The common ancestor which the tapeworms had with the other platyhelminths has been lost to time.

Tapeworms consist of an anchoring organ or 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 or proglottids - repeating organs which 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 the faeces.

Tapeworm life-cycles tend to follow a set pattern which depends on the presence of a predator-prey relationship.

Tapeworm Myths Being the biggest and among the most well known of the parasites, tapeworms have a certain mythology built up around them. One method for curing tapeworms is still told to me by university students as the way "Grandma and Grandad" use to treat worms : stick a biscuit (or a lump of meat) up the anus each day for a week. Then on the 8th day, stop doing it. When the tapeworm comes out looking for its biscuit, grab it an pull it out. Naturally this is a joke - tapeworms wouldn't recognise a biscuit and lack the gullet to digest one anyway. Another legend which regularly makes the rounds is "The Hollywood Tapeworm Diet", whereby young starlets and supermodels are allegedly intentionally infecting themselves with large tapeworms in an effort to lose weight. While this comes under the category of "anything can happen in Hollywood", Jan Brundvand, Urban Folklorist extraordinaire had classified it as a bona fide urban legend. Another variation mentions the doctor who purposefully infects himself with a tapeworm to stop himself from getting food poisoning while travelling through India. Plenty of the locals have tapeworms and food poisoning concurrently to make this tale reliable As for the dietary benefits of tapeworms, large parasites like the beef tapeworm and Ascaris certainly do compete for us for nutrients. However, like any method of starvation, they tend to rob us of micronutrients (like vitamins) before they get to the stuff we don't really need. Signs of infection with large tapeworms (especially with the broad fish tapeworm, Diphyllobothrium latum) include conditions associated with vitamin B12 deficiency (eg. megaloblastic anaemia). Just to make matters worse, one of the symptoms of have a large lump of tapeworm protein in one's guts is to generate an immune response and the resultant ascites - a collection of fluid in the abdomen resulting in a pot belly - not exactly the look that the dieters are looking for.

The adult tapeworms live in the predator (definitive host), while the prey plays host to the intermediate stages, most commonly called cysts or cysticerci (SISS-ih-SIR-KIE). Eggs laid by the adult pass out in the faeces of the definitive host and are eaten by the intermediate host. These develop into the intermediate stage or cyst, which generates the adult when eaten by the definitive host. 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,

Things may get a bit more serious if we play host to the intermediate stage of the parasite. Tapeworm cysts may grow to a reasonable size and, because they are found deep in the tissue, they may start taking up valuable space. If the cyst settles in the brain, eye or liver, normal function of these organs may be severely restricted. The host's body may also play a role - allergic reactions to the presence of the parasite tissues may end up exacerbating the problem, and calcification of large cysts may lead to a worse scenario than the live parasite.

Intestinal Tapeworms

Taenia saginata : The Beef Tapeworm

Taenia saginata (tay-EE-nee-ah SAJ-in-ARE-tah) is a giant among the human parasites. A complete specimen may grow to 8m in length - nearly the length of the digestive tract of an adult human. The worms attach high up in the small intestine and grow downwards. The segments of this worm may reach 1.5x1cm. 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 will look for these cysticerci in the cattle slaughtered at abattoirs. The gravid (ie. full of eggs) proglottids passed in the faeces are the diagnostic stage of the worm, and these may be differentiated from those of Taenia solium (SO-lee-um) by counting the lateral branches of the darkly coloured uterus (T. saginata has approximately 20, T. solium has 9-12). Taenia saginata is endemic in Australia, although 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).

There is a probably apocryphal tale of a false pregnancy in a Brisbane hospital caused by a beef tapeworm - the presence of the worm stopped the patient from menstruating, caused the collection of fluid in the abdomen (ascites) and the nausea was associated with morning sickness. As nice as the story is, it is probably a parasitologist's version of an urban legend (see Tapeworm Myths box).

Taenia solium : The Pork Tapeworm

Diphyllobothrium latum (DIE-fill-oh-BOH-three-um LAR-tum) belongs to a simple group of tapeworms known as the pseudophyllideans (SUE-doe-fill-ID-ee-uns - see also Spargana). 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 (PLAIR-oh-SIR-coyd) 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 associated with vitamin B12 deficiency.

Images of Diphyllobothrium latum are contained in Wormlearn.

Hymenolepis nana and Hymenolepis diminuta

Hymenolepis nana (HI-men-oh-LEAP-iss NAH-nah) and Hymenolepis diminuta (DIE-min-YOU-tah) are tapeworms normally found in the mouse and the rat, respectively. The intermediate host 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 a centimetre 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. H. nana is one of the more commonly identified tapeworms in certain regions of Australia.

Images of Hymenolepis nana and Hymenolepis diminuta are contained in Wormlearn.

Dipylidium caninum - The Dog Tapeworm

One of the more common parasites of domestic dogs is the tapeworm Dipylidium caninum (DIE-pill-ID-ee-um kay-NINE-um). This parasite grows to around 10-15cm, with individual segments resembling cucumber seeds in size and shape. The intermediate host for dog tapeworms are the dog and cat fleas Ctenocephalides canis (TEN-oh-KEFF-ah-LIE-dees KAY-niss) and Ctenocephalides felis (FEEL-iss). The eggs passed out in the dog's faeces 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 harbour 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 (ie. 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.

Images of Dipylidium caninum are contained in Wormlearn.