Roundworm (Ascaris lumbricoides) is the largest nematode (roundworm) parasitizing the human intestine.  Adult females are 20 to 35 cm and adult males 15 to 30 cm. It is the most common human helminthic infection, with worldwide distribution.  The highest prevalence is in tropical and subtropical regions, and areas with inadequate sanitation. It occurs in rural areas of the southeastern U.S.

Life Cycle ¹

Adult worms 

Fertile eggs embryonate and become infective after 18 days to several weeks

After infective eggs are swallowed 

The larvae mature further in the lungs in 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

Diagnosis

Although infections may cause stunted growth, adult worms usually cause no acute symptoms. High worm burdens may cause abdominal pain and intestinal obstruction. Migrating adult worms may cause symptomatic occlusion of the biliary tract or oral expulsion. During the lung phase of larval migration, pulmonary symptoms can occur. Pulmonary symptoms include cough, dyspnea, hemoptysis, and eosinophilic pneumonitis - Loeffler’s syndrome.

Microscopic identification of eggs in the stool is the most common method for diagnosing intestinal ascariasis.  The recommended procedure is as follows:

1. Collect a stool specimen.
2. Fix the specimen in 10% formalin.
3. Concentrate using the formalin – ethyl acetate sedimentation technique.
4. Examine a wet mount of the sediment.

Where concentration procedures are not available, a direct wet mount examination of the specimen is adequate for detecting moderate to heavy infections. For quantitative assessments of infection, various methods such as the Kato-Katz can be used. 

Larvae can be identified in sputum or gastric aspirate during the pulmonary migration phase (examine formalin-fixed organisms for morphology).  Adult worms are occasionally passed in the stool or through the mouth or nose and are recognizable by their macroscopic characteristics.

• Microscopy (Double click on the underlined words to see more information on a web page, including pictures.)
• Macroscopy

Treatment

The drugs of choice for treatment of ascariasis are albendazole*, mebendazole, and pyrantel pamoate*.  In the U.S., ascariasis is generally treated for 1-3 days with medication prescribed by a health care provider. 

The human hookworm is the second most common human helminthic infection, after ascariasis. It includes two nematode (roundworm) species, Ancylostoma duodenale and Necator americanus.  Adult females are 10 to 13 mm for A. duodenale and 9 to 11 mm for N. americanus. Adult males are 8 to 11 mm for A. duodenale and 7 to 9 mm for 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, Uncinaria stenocephala).

Hookworm has a 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.

Transmission

Hookworm larvae are in the soil or on feces. On contact with the human host, the larvae penetrate the skin and are carried through the veins and the heart to the lungs. 

Life Cycle ¹

Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant host blood loss.  Eggs are passed in the stool and under the favorable conditions of moisture, warmth and shade, hatch in 1 to 2 days. 

Larvae are released, grow in the feces and/or the soil, and after 5 to 10 days (and two molts) have become filariform (L-3) larvae that are infective.  These infective larvae can survive 3 to 4 weeks in favorable environments. 

On contact with the human host, the larvae penetrate the skin and are carried through the veins and the heart to the lungs.  They penetrate into the pulmonary alveolae, ascend the bronchial tree to the pharynx, and are swallowed. 

Upon reaching the small intestine, they undergo two more molts yielding fourth stage larvae (L4) and then adult worms. 

Five weeks or more are required from invasion by the L3 to oviposition by the adult female.  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.

Diagnosis

Iron deficiency anemia is the most common symptom of hookworm infection. It is caused by blood loss at the site of intestinal attachment of the adult worms. Cardiac complications, gastrointestinal and nutritional/metabolic symptoms can also occur. In addition, local skin manifestations, called “ground itch,” can occur during penetration by the filariform (L3) larvae, and respiratory symptoms can be observed during pulmonary migration of the larvae.

Microscopic identification of eggs in the stool is the most common method for diagnosing hookworm infection.  The recommended procedure is as follows:

1. Collect a stool specimen.
2. Fix the specimen in 10% formalin.
3. Concentrate using the formalin – ethyl acetate sedimentation technique.
4. Examine a wet mount of the sediment.

Where concentration procedures are not available, a direct wet mount examination of the specimen is adequate for detecting moderate to heavy infections.  For quantitative assessments of infection, various methods such as the Kato-Katz can be used.

• Microscopy (Double click on the underlined words to see more information on a web page, including pictures.)

Examination of the eggs cannot distinguish between N. americanus and A. duodenale.  Larvae can be used to differentiate between N. americanus and A. duodenale, by rearing filariform larvae in a fecal smear on a moist filter paper strip for 5 to 7 days (Harada-Mori).  Occasionally, it may be necessary to distinguish between the rhabditiform larvae (L2) of hookworms and those of Strongyloides stercoralis.

Treatment

In countries where hookworm is common and reinfection is likely, light infections are often not treated.  In the U.S., hookworm infections are generally treated for 1-3 days with albendazole. This drug is approved by the FDA, but considered investigational for this purpose.

The nematode (roundworm) Trichuris trichiura, also called the human whipworm. It is the third most common round worm of humans. Whipworm occurs worldwide, with infections more frequent in areas with tropical weather, poor sanitation practices and among children.  It is estimated that 800 million people are infected worldwide. Trichuriasis occurs in the southern U.S.

Life Cycle ¹

The adult worms are approximately 4 cm in length. They live in the cecum and ascending colon. Female worms in the cecum shed between 3,000 and 20,000 eggs per day. The unembryonated eggs are passed with the stool. 

In the soil they embryonate and 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 are fixed in that location, with the anterior portions threaded into the mucosa.  The females begin to oviposit 60 to 70 days after infection. The life span of the adults is about 1 year.

Diagnosis

Whipworm is usually asymptomatic. Heavy infections, especially in small children, can cause gastrointestinal problems including abdominal pain, diarrhea and rectal prolapse. It can cause growth retardation in children.

Microscopic identification of whipworm eggs in feces is evidence of infection.  Because eggs may be difficult to find in light infections, a concentration procedure is recommended.  Because the severity of symptoms depend on the worm burden, quantification of the latter (e.g. with the Kato-Katz technique) can prove useful.

• Microscopy (Double click on the underlined words to see more information on a web page, including pictures.)
• Examination of the rectal mucosa by proctoscopy (or directly in case of prolapses) can occasionally demonstrate adult worms.

Treatment

Mebendazole is the drug of choice, with albendazole as an alternative. 

Pinworm

The human pinworm is the nematode (roundworm) Enterobius vermicularis (previously known as Oxyuris vermicularis).  Adult females are 8 to 13 mm and adult males are 2 to 5 mm.  Humans are practically the only hosts of E. vermicularis.

Pinworm has a worldwide distribution. Infections occur more frequent in school aged or preschool children and in crowded conditions. It is the most common helminthic infection in the U.S. where there is an estimated 40 million persons infected. ¹

Transmission

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. 

Life Cycle ¹

Adult worms live in the lumen of the human colon. 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 in 4 hours under optimal conditions. 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. 

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.

Diagnosis

Pinworm infestation (enterobiasis) is frequently asymptomatic.  The most typical symptom is perianal pruritus, especially at night, which may lead to excoriations and bacterial superinfection.  Occasionally, invasion of the female genital tract with vulvovaginitis and pelvic or peritoneal granulomas can occur.  Other symptoms include anorexia, irritability, and abdominal pain.

Microscopic identification of eggs collected in the perianal area is the method of choice for diagnosing enterobiasis. This must be done in the morning, before defecation and washing, by pressing transparent adhesive tape ("Scotch test", cellulose-tape slide test) on the perianal skin and then examining the tape placed on a slide. 

Alternatively, anal swabs or "Swube tubes" (a paddle coated with adhesive material) can also be used.  Eggs can also be found, but less frequently, in the stool, and occasionally are encountered in the urine or vaginal smears.  Adult worms are also diagnostic, when found in the perianal area, or during ano-rectal or vaginal examinations.

• Microscopy (Double click on the underlined words to see more information on a web page, including pictures.)

Treatment

The drug of choice is pyrantel pamoate. Measures to prevent re-infection, such as personal hygiene and laundering of bedding, should be discussed and implemented in cases where infection affects other household members. 

There are two species of tapeworms: Taenia saginata (beef tapeworm) and T. solium (pork tapeworm). Taenia solium can also cause cysticercosis. Both species are worldwide in distribution. Taenia solium is more prevalent in poorer communities where humans live in close contact with pigs and eat undercooked pork, and is very rare in Muslim countries.

Life cycle of Taenia saginata (beef tapeworm) ¹

Humans are the only definitive hosts for Taenia saginata. The adult tapeworm’s length is usually 5 m or less, but up to 25 m. It resides in the small intestine, where they attach by their scolex. They produce proglottids. Each worm has 1,000 to 2,000 proglottids which mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (approximately 6 per day).  The eggs contained in the gravid proglottids with 80,000 to 100,000 eggs per proglottid. These are released after the proglottid becomes free and are passed with the feces. 

The eggs can survive for months to years in the environment. Cattle and other herbivores become infected by ingesting vegetation contaminated with eggs (or proglottids). In the animal's intestine, the eggs release the oncosphere, which evaginates, invades the intestinal wall and migrates to the striated muscles, where it develops into a cysticercus. Cysticercus is a bladder-like cyst that contains a larval form of the tapeworm. The cysticercus can survive for several years in the animal. Humans become infected by ingesting raw or undercooked infected meat. In the human intestine, the cysticercus develops over 2 months into an adult tapeworm, which can survive for more than 30 years.

Life cycle of Taenia solium (swine tapeworm) ¹

The life cycle of Taenia solium is similar to that of T. saginata. The adults worm’s length is 2 to 7 m; it has less than 1,000 proglottids, which are less active than in T. saginata, and each has 50,000 eggs; and it’s longevity is up to 25 years) develop not only in humans but also some other animal species (monkeys, hamsters). 

The cysticercus develops not only in striated muscle, but also in the brain, liver, and other tissues of pigs and other animals, including humans. Humans develop taeniasis when they ingest undercooked pork meat containing cysticerci. They develop cysticercosis by ingesting T. solium eggs, either by ingestion of fecally contaminated food, or by autoinfection.  In the latter case, a human infected with adult T. solium ingests eggs produced by that tapeworm, either through fecal contamination or, more arguably, from proglottids carried into the stomach by reverse peristalsis.

Diagnosis

Taenia saginata taeniasis produces only mild abdominal symptoms. The most striking feature consists of the passage (active and passive) of proglottids.  Occasionally, appendicitis or cholangitis can result from migrating proglottids.  

Taenia solium taeniasis is less frequently symptomatic than Taenia saginata taeniasis. The main symptom is often the passive passage of proglottids.  The most important feature of Taenia solium taeniasis is the risk of development of cysticercosis.

Microscopic identification of eggs and proglottids in feces is diagnostic for taeniasis, but is not possible during the first 3 months following infection, prior to development of adult tapeworms. 

Microscopic examination of eggs does not distinguish between the two species. In addition, taeniid eggs are also morphologically undistinguishable from those of Echinococcus and Multiceps; however, these parasites do not result in eggs in human stools. Repeated examination and concentration techniques will increase the likelihood of detecting light infections. Microscopic identification of gravid proglottids or, more rarely, examination of the scolex allows species determination.

• Microscopy (Double click on the underlined words to see more information on a web page, including pictures.)
• Antibody detection may prove useful especially in the early invasive stages, when the eggs and proglottids are not yet apparent in the stools.

Treatment

Treatment is simple and very effective. Praziquantel is the drug of choice. This drug is approved by the FDA, but considered investigational for this purpose.

(Double click on the underlined words to see more information on a web page, including pictures.)

1. Parasites and Parasitological Resources. Ohio State University, Columbus, Ohio
2. Collection of Images of Medically Important Parasites
3. Parasitological URLS. London, England
4. World of Parasites. McGill University, Montreal-Quebec, Canada
5. Atlas of Medical Parasitology. Carlo Denegri Foundation, Turin, Italy
6. The Medical Letter Home Page

1. Center for Disease Control, November 2001 http://www.cdc.gov/ncidod/dpd/parasiticpathways/insects.htm