Tapeworm

(Date of publication 23 January 2004)

Let us begin this tour with some rather startling statistics about these flatworms which make up the class Cestoda. The broad fish tapeworm Diphyllobothrium latum is the longest human tapeworm, capable of attaining 15 metres and producing 1,000,000 eggs every day, but its presence often goes unnoticed by the host. By contrast, Echinococcus multilocularis is only 1-4 mm long but may give rise to parasitic tumours in the liver, lungs, brain and other organs, which can prove fatal. Furthermore, in some parts of the world infestation with the larval form of Taenia solium is the predominant cause of epilepsy.

All these flatworms spend the adult phase of their life in the gut of a vertebrate - the primary or definitive host. However, another part of the life cycle is usually spent in the tissues of one or more other animals - the intermediate hosts - which may be vertebrates or arthropods. There is a good overview of the tapeworms which affect man at an Australian site, pointing out that infestation is most common where sanitation is poor, people live in close proximity to livestock and regularly eat raw or undercooked beef, pork or fish.

The beef tapeworm (Taenia saginata) and the pork tapeworm (Taenia solium) have similar life cycles, with man as the definitive host. Mature segments, or proglottids, break off from the adult worm and are passed with the faeces. Once in the open air they dry out, releasing the eggs, which are then ingested by grazing cattle or foraging pigs. In the animal's duodenum the eggs hatch, releasing hexacanth (six-hooked) larvae which penetrate the gut wall and are carried by the circulation to all parts of the body, especially the muscles. Here they metamorphose into cysticerci, or bladderworms, in which the head (or scolex) and neck are invaginated into a large fluid-filled cyst. This structure can be distinguished (just!) in this photomicrograph. If the animal's flesh is subsequently eaten, the action of the unlucky human diner's bile causes the scolex to evaginate, hook onto the gut lining, and develop into an adult tapeworm. This life cycle is outlined here. The photograph here shows a mature proglottid and scolex from Taenia solium, the scolex bearing four suckers and a circular row of hooks, or rostellum, for attachment. The scolex of Taenia saginata is different, having four suckers but no rostellum.

The pork tapeworm is particularly dangerous because humans can become infected with cysticerci as a result of ingesting eggs, by eating food contaminated with faeces from a human carrier. Autoinfection may occur if an adult tapeworm is already present in an individual's gut and hygiene standards are poor. The eggs hatch in the intestine and larvae pass through the gut wall, not only to striated muscle but also to the brain, eye, liver and other organs, where they develop into cysticerci. A highly detailed emedicine article states that neurocysticercosis is the most frequent parasitic disease of the nervous system and the main cause of acquired epilepsy in developing countries. The devastating effect of a serious infection of the brain can be seen in this CT scan.

The broad fish tapeworm , which can live in humans for up to 20 years, has a more complex lifestyle with two intermediate hosts, a crustacean and a fish. Infestation can result from eating raw or lightly-cooked freshwater or migratory fish, and the popularity of dishes such as sushi and sashimi helps to spread the disease. This tapeworm has the ability to absorb most of the vitamin B12 in the host's intestine, which may give rise to severe anaemia with neurological symptoms. The photographs of an egg and proglottids at the bottom of this page are very clear, but for a macroscopic view, click here.

Echinococcus granulosus and Echinococcus multilocularis are minute tapeworms for which the definitive hosts are canines such as dogs, foxes and wolves. The eggs of these worms are passed in the faeces and are very robust, capable of surviving temperatures between -40°C and 50°C. If these are ingested by a human, rather than a natural intermediate host such as a sheep, goat, pig, cow, horse or camel, they hatch in the small bowel, penetrate the gut wall and migrate to various organs, especially the liver and lungs. Here they produce gradually enlarging cysts which, if ruptured, can cause fever, urticaria and even anaphylactic shock. However, symptoms may not develop for many years. Treatment is by total surgical resection combined with chemotherapy using benzimidazole drugs, which may need to be continued for life.

Hymenolepis nana, the dwarf tapeworm, occurs worldwide and is the commonest cestode infestation. It is also the only cestode that can parasitise humans without requiring an intermediate host. This is because ingested eggs develop into cysticerci in the intestinal villi, which subsequently rupture and return the cysticerci to the intestinal lumen, where they develop into adult worms. Internal autoinfection also occurs, when the eggs release their larvae without passage through the external environment. This mechanism enables infestation to persist for many years, even though adult worms live for only 4-6 weeks. Hymenolepis diminuta or the rat tapeworm, which has an arthropod as an intermediate host, may also infect man: for example, by consuming insects in cereals. This site has some good photographs.

For many years scientists believed that humans first contracted tapeworms while domesticating cattle and pigs some 10,000 years ago. If this were the case then the three Taenia species which only parasitise man would resemble those in wild pigs and oxen. However, Dr Eric Hoberg and his colleagues at the US Agricultural Research Service analysed the body structure of 35 tapeworm species and found that the closest relatives to human parasites were species which infect lions and hyenas. His conclusion was that hominids first became hosts when they shared food with large carnivores on the savannahs of Africa, before evolving into humans.

Most of this tour has been concerned with disease caused by tapeworms, but we can strike a positive note. Researchers recently isolated a signal molecule from Hymenolepis diminuta, cyclic GMP, that slows the passage of gut contents. This compound, cheap and easy to produce, holds great promise for drug therapy in the future. By prolonging the residence time of oral medications in the small intestine, absorption could be enhanced and higher serum levels achieved. Knock-on effects could include lower dosages, reduced side effects and fewer chemicals released into the environment, all of which are highly desirable.

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A Tour around Tapeworms

A Tour around Tapeworms