Tapeworms
Tapeworms are flatworms belonging to the class Cestoda, with over a thousand known species, seven of which can infect humans. These parasites primarily inhabit the intestinal tract of their final hosts, where they absorb nutrients directly through their skin. Tapeworm infections often occur when humans ingest raw or undercooked meat containing encysted larvae, which then develop into adult worms within the intestine. Symptoms of infestation can be minimal, with many individuals unaware of their infection for years, although symptoms may include nausea, abdominal pain, or weight loss in some cases.
Certain species, such as the Taenia saginata (beef tapeworm) and T. solium (pork tapeworm), can lead to more severe conditions if larvae migrate to other body tissues, causing diseases like cysticercosis. Treatment typically involves anthelmintic medications such as praziquantel, which can effectively rid the body of the parasite. In some instances, particularly with larval-stage infestations, additional treatments or surgical intervention may be necessary. Understanding tapeworms and their transmission routes is crucial for prevention and effective management of infections.
Tapeworms
- TRANSMISSION ROUTE: Ingestion
Definition
Tapeworms are flatworms of the class Cestoda. Seven species are known to infect humans with intestinal worms, and other species can cause diseases by forming cysts in body tissues.
Natural Habitat and Features
Tapeworms are parasitic flatworms (also known as cestodes), with more than one thousand known species in worldwide distribution. The worms are located in the intestinal tract of the final host, although a larval-stage disease is also encountered in other body tissues. Tapeworms have a complicated life cycle of intermediate and final hosts. In intermediate hosts, the cestode travels through the circulation system, forming a larval cyst within the host's body tissues. When the final host eats the intermediate host, the cestode develops into the mature worm in the intestine, where it absorbs nutrients through its skin as it passes through the host’s intestine. Depending on the species, the worms can range in size from less than one inch to more than 50 feet long.
The head of a tapeworm is called a scolex, and it usually has suckers that enable the worm to attach to the intestine; some species also have hooks. The worm's body is referred to as the strobila, which comprises segments called proglottids. Proglottids can produce thousands of eggs, which are released to pass from the host in the feces. Most tapeworms are hermaphroditic, and a single tapeworm can produce many thousands of viable eggs. Eventually, proglottids break off from the tapeworm's body and pass in the feces. Some tapeworms, such as the Taenia species, may live for as long as twenty years, while Hymenolepis nana (dwarf tapeworm) lives for one year only.
Tapeworms causing intestinal infestation in humans are usually known as the intermediate host. The worms that infest humans are T. saginata (beef tapeworm), T. solium (pork tapeworm), T. asiatica (previously confused with T. saginata), Diphyllobothrium latum (fish tapeworm), Dipylidium caninum (dog tapeworm), H. diminuta (rat tapeworm), and H. nana (dwarf tapeworm). Echinococcus granulosus and Dipylidium caninum may also infect humans. The dwarf tapeworm is so named not only because of its small size, but also because the worm lacks an intermediate host.
Pathogenicity and Clinical Significance
Most tapeworm infestations in humans involve mature worms in the intestines and involve few symptoms, if any. When symptoms are noticed, they usually manifest as intestinal distress, such as nausea, loss of appetite, abdominal pain, weight loss, or diarrhea. Because of a lack of symptoms, the host can carry the tapeworm for many years without being aware they are doing so. Although most tapeworm infestations do not cause a visible problem for the host, a worm that grows very large may block the bile duct, the pancreatic duct, or the intestine.
Tapeworms are generally acquired from intermediate hosts by eating poorly cooked or raw beef, pork, or fish containing encysted larvae. A second transmission route also involves intermediate hosts, whereby a person accidentally ingests the host by eating insects in dried foods such as grains and cereals (H. diminuta). Another route is more direct: children accidentally ingest fleas or lice from dogs (D. caninum). Children are also most susceptible to acquiring H. nana.
A larval-stage infestation, which can be a much more serious disease, comes from directly ingesting tapeworm eggs, either from contaminated food and water or self-infection with T. solium eggs by hosts carrying pork tapeworm. In addition to T. solium, such larval-stage disease is caused by Spirometra species, T. multiceps, Echinoccocus granulosus, E. multilocularis, and E. vogeli.
When eggs from these species are swallowed, they form larvae that move through the blood to body tissues and then form cysts, using the human as an intermediate host. This form of infestation has various names, depending on the species and type of cyst involved. The disease is known as cysticercosis when the larvae come from T. solium. Such larval-stage diseases can have serious health consequences, particularly if cysts lodge in the heart, eye, central nervous system, and brain. If the cysts migrate to other organs within the body, they can disrupt the functioning of the organ; in the eye, they may lead to blindness. When cysts form in the central nervous system, including the brain (neurocysticercosis), they can result in headaches, seizures, meningitis, hydrocephalus, dementia, and death.
Drug Susceptibility
Tapeworms are treated based on the species of tapeworm, the infection location, and patient demographics. Most intestinal infestations are treated with praziquantel in a single oral dose. With treatment, there is a good chance of ridding the body of the parasite altogether. After treatment, only part of the worm may be seen in the feces, as the host may break it down by digestion before it is excreted. Other possible anthelmintic drugs include albendazole and niclosamide. Nitazoxanide is a new medication that is most often used to treat tapeworms in children.
Cysticercosis and other larval-stage infestations are not necessarily treated unless they involve the brain. When they are treated, praziquantel may also be used, though, in persons with neurocysticercosis, side effects may require the administration of corticosteroids and praziquantel. Anti-seizure medications may also be recommended. In some cases, surgery is necessary to remove the cysts of larval disease.
Bibliography
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