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Salem Health: Infectious Diseases & Conditions, 2nd Edition

Echinococcus

by Cynthia L. Mills, , D.V.M.

Category: Pathogen

Transmission route: Ingestion

Definition

Echinococcus species are tapeworms of carnivores that cause serious disease in humans and herbivores as intermediate hosts. This disease is known as echinococcosis.

Natural Habitat and Features

Echinococcus species are cestodes (tapeworms) with complex life cycles requiring two mammalian hosts. Their definitive hosts (where sexual reproduction occurs) are carnivores, commonly dogs and foxes. In definitive hosts, the worms remain small, usually less than 6 millimeters in size, and consist of a head and three egg packets or proglottids. Proglottids pass with feces and contaminate the environment, where they are ingested by the intermediate species, typically herbivores and sometimes humans.

After ingestion by herbivores, Echinococcus species eggs develop into oncospheres that can penetrate intestinal epithelium. Once there, they pass into blood or lymph vessels and then travel passively to target organs, usually the liver or lung but occasionally the spleen, brain, heart, and kidneys. Once there, they form metacestodes, reproductive structures called hydatid cysts. These cysts grow to between 5 and 10 centimeters in size within one year and can persist and remain viable for years, growing, on occasion, so large they contain several liters of fluid. The cysts contain protoscoleces (called hydatid sand) and are ingested by carnivores, where they develop into adult Echinococcus cestodes.

Pathogenicity and Clinical Significance

The different species of Echinococcus cause different syndromes of disease in humans. E. granulosus causes the oldest known disease, cystic echinococcosis (CE). CE cysts are large, unilocular, and fluid-filled. Typically, CE occurs most commonly in humans living pastoral lives in contact with sheep, goats, and dogs. Signs of CE often take years to develop and include abdominal pain occurring when cysts have grown large enough to displace organs. Allergic signs and anaphylaxis can occur with cyst rupture. Treatment is usually a combination of chemotherapy and the surgical removal of cysts. CE is a disease of the Northern Hemisphere.

E. multicularis was recognized as a separate parasite in the nineteenth century that caused a syndrome known as alveolar echinococcosis (AE). E. multicularis is most often found in foxes and rodents. AE cysts differ from CE in that they are multilocular, solid, and frequently pulmonary. Signs are similar to CE, but pulmonary AE also causes coughing. AE is more likely to be fatal. Treatment of AE is usually a combination of chemotherapy and the surgical removal of cysts. AE is a disease of the Northern Hemisphere and of suburban regions where humans and wildlife are close.

E. vogeli and E. oligarthus cause polycystic echinococcosis (PE). PE cysts tend to be small. The disease is not well characterized because it has only recently been recognized; also, only a few cases have been described. Treatment is usually chemotherapy. PE occurs in Central America and South America.

Drug Susceptibility

Albendazole, mebendazole, and praziquantal are drugs used to treat echinococcosis. High doses are used, and treatment may last for years or even be lifelong.

Further Reading

1 

Deplazes, Peter, et al. “Wilderness in the City: The Urbanization of Echinococcus multicularis.” Trends in Parasitology 20 (2004): 77-84.

2 

Eckert, J., et al. WHO/OIE Manual on Echinococcosis in Humans and Animals: A Public Health Problem of Global Concern. Geneva: World Organization for Animal Health and World Health Organization, 2001.

3 

Krauss, Hartmut, et al. Zoonoses: Infectious Diseases Transmissible from Animals to Humans. 3d ed. Washington, D.C.: ASM Press, 2003. Explores the myriad infections introduced by human-animal contact.

4 

Zhang, Wenbao, et al. “Concepts in Immunology and Diagnosis of Hydatid Disease.” Clinical Microbiology Reviews 16 (2003): 18-36.

Web Sites of Interest

American Veterinary Medicine Association

http://www.avma.org

National Center for Emerging and Zoonotic Infectious Diseases

http://www.cdc.gov/ncezid

See also: Dogs and infectious disease; Parasites: Classification and types; Parasitic diseases; Tapeworms; Worm infections; Zoonotic diseases.

Citation Types

Type
Format
MLA 9th
Mills, Cynthia L. "Echinococcus." Salem Health: Infectious Diseases & Conditions, 2nd Edition, edited by H. Bradford Hawley, Salem Press, 2020. Salem Online, online.salempress.com/articleDetails.do?articleName=Infect2e_0186.
APA 7th
Mills, C. L. (2020). Echinococcus. In H. B. Hawley (Ed.), Salem Health: Infectious Diseases & Conditions, 2nd Edition. Salem Press. online.salempress.com.
CMOS 17th
Mills, Cynthia L. "Echinococcus." Edited by H. Bradford Hawley. Salem Health: Infectious Diseases & Conditions, 2nd Edition. Hackensack: Salem Press, 2020. Accessed September 16, 2025. online.salempress.com.