Tapeworm Infections

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Tapeworm Infections

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

BIBLIOGRAPHY

Introduction

Tapeworms are parasitic animals also known as cestodes. The life cycle of the tapeworm involves humans as either a primary or intermediate host. Both of these situations cause infection in humans. Humans become infected with tapeworms when they either ingest meat containing encysted tapeworms or when they ingest tapeworm eggs. In the first case, humans act as primary hosts. In the second case, humans act as intermediate hosts.

While tapeworm infections tend to be asymptomatic, some symptoms may appear, including abdominal pain, nausea, diarrhea, stools containing mucus, and the passing of tapeworm segments, or proglottids. However, if a human is infected with eggs, more serious complications can arise. The cysts formed by the larvae in tissues can cause damage, including damage to vital organs, such as the brain.

Tapeworm infections occur worldwide, but are more prevalent in countries with low hygiene and sanitation conditions or in areas where humans live close to livestock. Tapeworms are passed on predominantly through ingesting meat containing encysted tapeworms or ingesting food and water contaminated with infected human feces. Treatment involves anti-parasitic medications, such as praziquantel, and is usually effective.

WORDS TO KNOW

CESTODE: A class of worms characterized by flat, segmented bodies, commonly known as tapeworms.

INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.

PRIMARY HOST: The primary host is an organism that provides food and shelter for a parasite while allowing it to become sexually mature, while a secondary host is one occupied by a parasite during the larval or asexual stages of its life cycle.

Disease History, Characteristics, and Transmission

Tapeworms are parasitic flatworms belonging to the class Cestoda. The life cycle of a tapeworm generally involves a primary host and an intermediate host. The life cycle begins when a tapeworm egg is passed from a primary host into soil or water. An intermediate host ingests the egg and the larvae hatch, enter tissues, and form cysts. A primary host then ingests cysts when they consume the flesh of the intermediate host. These cysts develop into adults, which sexually reproduce in the host's intestines.

In most cases, humans become infected with tapeworms after eating undercooked or raw animal flesh containing tapeworm cysts. These worms migrate to the intestines where they reproduce. The fecal matter of these infected individuals is infectious, since it contains tapeworm eggs. The most common tapeworms that infect humans in this manner are: Taenia solium, which is present in pigs; Taenia saginata, which is present in cattle; Diphyllobothrium species, which are present in freshwater fishes; Hymenolepis species, which are found in rodents and insects; and Diphyllobothrium caninum, which is present in cat and dog fleas.

In some cases, humans are intermediate hosts. In these cases, humans ingest the tapeworm eggs. These hatch, and the larvae migrate to tissues within the body and form cysts. This occurs with Taenia solium if humans swallow food or water that contains contaminated human fecal matter. It also occurs when humans accidentally swallow insects containing the larvae of Hymenolepis species.

Tapeworm infections tend to be asymptomatic. However, possible symptoms include abdominal pain, nausea, diarrhea, stools containing mucus, and the passing of tapeworm segments, or proglottids. A serious risk associated with an infestation of T. solium is the risk of developing cysticercosis. This occurs when the eggs are ingested and the larvae form cysts within tissues. The most serious form of this infection involves cysts that form in the central nervous system—a condition known as neurocysticercosis. This can cause neurological problems and seizures. In severe cases, permanent brain damage or death may occur.

Scope and Distribution

Tapeworm infections occur worldwide. However, certain species are only present, or are more prevalent, in certain regions. In the United States, only a few tapeworms commonly cause infection. Infection by T. saginata and T. solium is rare in the United States, with less than a 1% infection rate for T. saginata. This is a result of the almost complete absence of these parasites from the livestock industry. However, infection by Diphyllobothrium species, which is caused by ingesting raw or undercooked fish, occurs more commonly.

Elsewhere in the world, T. saginata and T. solium are more prevalent. T. saginata is endemic in Latin America, Africa, the Middle East, and central Asia. It also occurs in Europe, south Asia, Japan, and the Philippines. T. solium is common in Latin America, the Slavic countries, Africa, Southeast Asia, India, and China. It also occurs in Europe, but with lower prevalence. Cysticercosis, which occurs when humans are infected by the larval form of tapeworms, is endemic in almost all Latin American countries.

Infection from Diphyllobothrium species also commonly occurs in Europe, Canada, Africa, some Asian countries, South America, and Australia. However, the most common tapeworm infection in humans is caused by Hymenolepis nana. Infection arises when humans accidentally ingest infected insects, or ingest food or water contaminated by infected insects. In addition, the eggs are transmissible from human to human through contaminated feces. Children, the developmentally disabled, and psychiatric patients are most commonly infected. In addition, these parasites are commonly found in regions with poor hygiene and sanitation methods.

Treatment and Prevention

Tapeworm infections are usually treated with anti-parasitic drugs. One of the most common and effective medications is praziquantel. This drug effectively kills adult tapeworms. There are a few mild side effects of praziquantel, but these are generally short-lived. Albendazole is an alternative to praziquantel, with similar effects. Another alternative to praziquantal is niclosamide, which is used to treat infections by Taenia and Diphyllobothrium tapeworms. The side effects of this drug include nausea, abdominal pain, vomiting, diarrhea, light-headedness, and skin rash. In the case of neurocysticercosis, in which tapeworm larvae form cysts in the central nervous system, early treatment of this infection can minimize damage to the system, and thus decrease the risk of neurological complications.

These treatments kill the adult tapeworms, not the eggs, so it is possible for a patient to remain infected following treatment. Therefore, a visit to the doctor three months after treatment is necessary to check for continued infection and to determine whether further treatment is needed.

There are no vaccinations to prevent tapeworm infections. Therefore, the best method of prevention is to avoid becoming contaminated. For tapeworms found in meats, cooking the meat above a temperature of 150°F (65.5°C) or freezing it for 12–24 hours kills the tapeworms. In addition, ensuring that livestock are dewormed decreases the risk that they are infected and likely to pass on an infection to humans.

Food and water may be contaminated with infected fecal matter, particularly in regions with poor hygiene and sanitation. Therefore, washing raw food or thoroughly cooking it helps to ensure parasites are removed. In addition, boiling or filtering drinking water decreases the chance of ingesting parasites from the water. Good personal hygiene, such as washing hands with soap and water prior to handling food, and rigorous sanitation practices, especially where human waste is involved, also decrease the likelihood that parasites will be transmitted among a human population.

Impacts and Issues

Tapeworms are a major health issue for a number of countries. Since tapeworms are usually originate in livestock or contaminated human feces, regions in which humans live near their livestock and areas with poor hygiene and sanitation standards have a higher prevalence of tapeworm infections. Tapeworm infections tend to be endemic in developing countries, where sanitation is poor due to a lack of funding and medical treatment is often unavailable.

Tapeworm infections do still occur in developed countries for a number of reasons. Travel and immigration have become a source of tapeworm infection as infected people interact with non-infected people, potentially spreading the tapeworms. In addition, since tapeworm infections are usually asymptomatic, the tapeworm may go undetected for years before treatment is given and the tapeworm is removed from the body.

Since tapeworms form cysts in animal flesh, eating raw or undercooked meats increases the likelihood of becoming infected. This is not a significant problem for countries such as the United States in which livestock are almost totally free of tapeworm infestations due to deworming practices. However, for countries where the practices associated with livestock farming are more relaxed, there is a higher chance that eating meat may cause infection.

Although many cases of tapeworm infection go undetected for years due to the asymptomatic nature of infection, the most severe form of infection—cysticercosis—can cause serious health issues. If a human ingests tapeworm eggs, they hatch and the larvae form cysts in tissues within the body. This can cause damage to vital body organs, and, in the worst-case scenario, can damage the nervous system. This may result in death or permanent brain injury.

See AlsoEndemicity; Food-borne Disease and Food Safety; Handwashing; Parasitic Diseases; Sanitation; Taeniasis (Taenia Infection).

BIBLIOGRAPHY

Books

Beers, M. H. The Merck Manual of Medical Information. New York: Pocket Books, 2003.

Bush, A.O., et al. Parasitism: The Diversity and Ecology of Animal Parasites. New York: Cambridge University Press, 2001.

Mandell, G.L., J.E. Bennett, and R. Dolin. Principles and Practice of Infectious Diseases. 6th ed. Philadelphia: Elsevier, 2004.

Web Sites

Centers for Disease Control and Prevention. “Hymenolepis Infection.” September 21, 2004. <http://www.cdc.gov/ncidod/dpd/parasites/hymenolepis/factsht_hymenolepis.htm> (accessed March 9, 2007).

WebMD. “Tapeworm Infestation.” August 8, 2005. <http://www.emedicine.com/emerg/topic567.htm> (accessed March 9, 2007).