Tapeworm

Descriptive text is not available for this imageBASICS

DESCRIPTION

  • Tapeworms cause two major types of zoonotic disease syndromes, depending on whether humans are the definitive or intermediate host.
  • When humans serve as definitive hosts, adult tapeworms infect the gastrointestinal (GI) tract and interfere with nutrition; patients may be asymptomatic.
  • When humans serve as intermediate hosts for the larval cestode, serious pathology results.
  • Causative organisms include the following:
    • Taenia saginata (beef tapeworm)
    • Taenia solium (pork tapeworm)
    • Diphyllobothrium latum (fish tapeworm)
    • Dipylidium caninum (dog tapeworm)
    • Echinococcus granulosus

EPIDEMIOLOGY

  • Beef tapeworm
    • Estimated >70 million people infected worldwide
    • Widespread in cattle-breeding areas of the world; endemic in Asia, Latin America, Eastern Europe
  • Pork tapeworm
    • Estimated >50 million people infected worldwide
    • Taeniasis: typically asymptomatic infection with adult tapeworm in the intestine (from undercooked pork)
    • Cysticercosis: infection with larval parasite (fecal–oral transmission from carriers)
    • Neurocysticercosis: leading cause of preventable epilepsy globally, 2.5 to 8.3 million cases per year
    • Estimated >2,000 new cases of neurocysticercosis in United States annually
  • Fish tapeworm
    • Infection is distributed worldwide because infected fish (usually from the Northern hemisphere) may be transported and consumed in any area of the world.
    • Persons who prepare or eat raw or undercooked freshwater fish are most at risk.
    • In the United States, infected salmon have been implicated in most cases.
  • Dog tapeworm
    • Found in dogs and cats worldwide
  • Echinococcosis
    • Estimated 2 to 3 million people infected worldwide
    • Endemic in Australia and New Zealand; also high prevalence in sheep- and cattle-raising areas of Europe, Asia, Africa, and the Americas (primarily in Latin America, western United States, Alaska, and Canada)

GENERAL PREVENTION

  • Adult tapeworms
    • Adequate freezing or cooking prevents transmission of beef, pork, and fish tapeworms. (See Centers for Disease Control and Prevention [CDC] guidelines for specific cooking/freezing recommendations.)
  • Pork tapeworm
    • Persons who are traveling to areas with high endemic rates of cysticercosis should avoid eating uncooked vegetables and fruit that can’t be peeled.
    • Hand hygiene is important for household contacts of positive cases.
  • Dog tapeworm
    • Periodic deworming of pets; flea control and prevention
    • Hand hygiene after contact with pet feces
  • Echinococcosis
    • Hand hygiene and avoiding exposure to dog and wild canid feces
    • Preventing dogs from feeding on sheep carcasses and treating dogs for parasitic infections can decrease the risk of subsequent human infection.

PATHOPHYSIOLOGY

  • Beef tapeworm
    • Bovines (intermediate host) ingest T. saginata eggs in contaminated feeds. The eggs hatch, releasing embryos which penetrate intestinal mucosa, enter the bloodstream, and settle in skeletal muscle, where they develop into larvae. Larvae in undercooked meat are consumed by humans and mature into adult tapeworms within the human (definitive host) GI tract.
  • Pork tapeworm: Humans are the only definitive host for the adult pork tapeworm. Both humans and pigs are intermediate hosts for its embryonic form, cysticercosis.
    • Pigs ingest T. solium eggs. In the intestine, the eggs release embryos that penetrate the mucosa, enter the bloodstream, and settle in various tissues to differentiate into cysticerci (infective larvae). Cysticerci are ingested by humans (definitive host) who consume undercooked pork.
    • Humans ingest food that is contaminated with human feces containing T. solium eggs. The eggs hatch, liberating embryos which penetrate the intestinal mucosa leading to blood-borne distribution to the brain, subcutaneous tissues, muscle, and eye, where they develop into cysticerci.
  • Fish tapeworm
    • When sewage containing D. latum eggs contaminates freshwater lakes and streams, hatched embryos are eaten by intermediate hosts (crustaceans and then freshwater fish). Humans (definitive host) are infected when they consume these undercooked fish. The larvae mature into adult tapeworms in the human intestine in 3 to 5 weeks and can survive up to 30 years.
  • Dog tapeworm
    • Larvae develop in fleas (intermediate host) after ingestion of the eggs; humans (definitive host) are infected through accidental ingestion of infected fleas.
  • Echinococcosis (hydatid disease)
    • Humans (intermediate host) ingest eggs of E. granulosus through contaminated dog feces and then embryos (oncospheres) penetrate intestinal mucosa, enter the bloodstream, and are distributed to the liver, lungs, and occasionally other organs, where larvae form cysts. Within the cysts, new larvae (scolices) develop, accumulate fluid, and encroach on surrounding structures.

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