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The cestode (tapeworm) Taenia solium (pork tapeworm) is the main cause of human cysticercosis.  In addition, the larval stage of other Taenia species (e.g., multiceps, serialis, brauni, taeniaeformis, crassiceps) can infect humans in various sites of localization including the brain, subcutaneous tissue, eye, or liver.

Life Cycle:

Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium.  This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier.  Pigs and humans become infected by ingesting eggs or gravid proglottids.  Humans are infected either by ingestion of food contaminated with feces, or by autoinfection.  In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis.  Once eggs are ingested, oncospheres hatch in the intestine, invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci.  In humans, cysts can cause serious sequellae if they localize in the brain, resulting in neurocysticercosis.  The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci.  Cysts evaginate and attach to the small intestine by their scolex.  Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years.

Geographic Distribution:

Taenia solium is found worldwide.  Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork.  Taeniasis and cysticercosis are very rare in Muslim countries.  It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier, and thus can occur in populations that neither eat pork nor share environments with pigs.


Ascaris lumbricoides is the largest nematode (roundworm) parasitizing the human intestine. (Adult females: 20 to 35 cm; adult male: 15 to 30 cm.)

Life Cycle:

Adult worms live in the lumen of the small intestine. A female may produce approximately 200,000 eggs per day, which are passed with the feces. Unfertilized eggs may be ingested but are not infective. Fertile eggs embryonate and become infective after 18 days to several weeks, depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed, the larvae hatch, invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs. The larvae mature further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed. Upon reaching the small intestine, they develop into adult worms. Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female. Adult worms can live 1 to 2 years.

Geographic Distribution:

The most common human helminthic infection. Worldwide distribution. Highest prevalence in tropical and subtropical regions, and areas with inadequate sanitation. Occurs in rural areas of the southeastern United States.


Hookworm Close-UpThe human hookworms include two nematode (roundworm) species, Ancylostoma duodenale and Necator americanus.  (Adult females: 10 to 13 mm (A. duodenale), 9 to 11 mm (N. americanus); adult males: 8 to 11 mm (A. duodenale), 7 to 9 mm (N. americanus).  A smaller group of hookworms infecting animals can invade and parasitize humans (A. ceylanicum) or can penetrate the human skin (causing cutaneous larva migrans), but do not develop any further (A. braziliense, A. caninum, Uncinaria stenocephala).  Occasionally A. caninum larva may migrate to the human intestine causing eosinophilic enteritis; this may happen when larva is ingested rather than through skin invasion.

Life Cycle:

Eggs are passed in the stool, and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days.  The released rhabditiform larvae grow in the feces and/or the soil, and after 5 to 10 days (and two molts) they become become filariform (third-stage) larvae that are infective. These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.  On contact with the human host, the larvae penetrate the skin and are carried through the veins to the heart and then to the lungs.  They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed.  The larvae reach the small intestine, where they reside and mature into adults.  Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host.  Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years.

Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle).  In addition, infection by A. duodenale may probably also occur by the oral and transmammary route.  N. americanus, however, requires a transpulmonary migration phase.

Geographic Distribution:

The second most common human helminthic infection (after ascariasis).  Worldwide distribution, mostly in areas with moist, warm climate.  Both N. americanus and A. duodenale are found in Africa, Asia and the Americas.  Necator americanus predominates in the Americas and Australia, while only A. duodenale is found in the Middle East, North Africa and southern Europe.


The nematode (roundworm) Trichuris trichiura, also called the human whipworm.

Life Cycle:

The unembryonated eggs are passed with the stool.  In the soil, the eggs develop into a 2-cell stage, an advanced cleavage stage, and then they embryonate; eggs become infective in 15 to 30 days.  After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae that mature and establish themselves as adults in the colon.  The adult worms (approximately 4 cm in length) live in the cecum and ascending colon.  The adult worms are fixed in that location, with the anterior portions threaded into the mucosa.  The females begin to oviposit 60 to 70 days after infection.  Female worms in the cecum shed between 3,000 and 20,000 eggs per day.  The life span of the adults is about 1 year.

Geographic Distribution:

The third most common round worm of humans.  Worldwide, with infections more frequent in areas with tropical weather and poor sanitation practices, and among children.  It is estimated that 800 million people are infected worldwide.  Trichuriasis occurs in the southern United States.


The nematode (roundworm) Enterobius vermicularis (previously Oxyuris vermicularis) also called human pinworm.  (Adult females: 8 to 13 mm, adult male: 2 to 5 mm.)  Humans are considered to be the only hosts of E. vermicularis.  A second species, Enterobius gregorii, has been described and reported from Europe, Africa, and Asia.  For all practical purposes, the morphology, life cycle, clinical presentation, and treatment of E. gregorii is identical to E. vermicularis.

Life Cycle:

Eggs are deposited on perianal folds.  Self-infection occurs by transferring infective eggs to the mouth with hands that have scratched the perianal area.  Person-to-person transmission can also occur through handling of contaminated clothes or bed linens.  Enterobiasis may also be acquired through surfaces in the environment that are contaminated with pinworm eggs (e.g., curtains, carpeting).  Some small number of eggs may become airborne and inhaled.  These would be swallowed and follow the same development as ingested eggs.  Following ingestion of infective eggs, the larvae hatch in the small intestine and the adults establish themselves in the colon.  The time interval from ingestion of infective eggs to oviposition by the adult females is about one month.  The life span of the adults is about two months.  Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area.  The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions. Retroinfection, or the migration of newly hatched larvae from the anal skin back into the rectum, may occur but the frequency with which this happens is unknown.

Geographic Distribution:

Worldwide, with infections more frequent in school- or preschool- children and in crowded conditions.  Enterobiasis appears to be more common in temperate than tropical countries.  The most common helminthic infection in the United States (an estimated 40 million persons infected).

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