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A micrograph shows the scolex of a Taenia solium tapeworm. Image from the Centers for Disease Control and Prevention (CDC). [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Helminths

The scolex of a T solium tapeworm attaches to the intestinal wall of its host. Parasitic intestinal infection with helminths can cause significant morbidity and mortality if not recognized and treated appropriately. Although such infections are typically restricted to geographic regions based on host environments, global warming will disrupt these natural environments and cause shifts in population centers that may change the distribution of disease.[1]

The life cycle of Echinococcus is shown. Image from the CDC. [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Cestodes

Tapeworms are long, segmented worms of the class Cestoda, which includes the genera Taenia, Diphyllobothrium, Hymenolepis, Dipylidium, Echinococcus, and Spirometra.[2,3] Adult tapeworms have a head (scolex), a neck, and a segmented body with both male and female gonads,[2] but they lack an intestinal tract and absorb all nutrients through their integument.

Tapeworms require one or more intermediate hosts in their life cycle. Typically, the eggs are passed from the host into the environment, where they are ingested by an intermediate host; there, the eggs hatch, and the larvae enter the host tissues and encyst. The primary host then ingests the cysts by consuming the flesh of the intermediate host.[2]

Humans are the primary hosts for Taenia, Diphyllobothrium, and Hymenolepis,[2] but they may be intermediate hosts for Echinococcus[4] and Spirometra.[3] Infection is typically from either fecal-oral transmission or the ingestion of contaminated, undercooked meat.[3] Most cestode infections are uncommon in the United States, with Hymenolepis nana the most frequently diagnosed.[5]

A close-up view of the scolex of an Echinococcus granulosus, with suckers and a ring of rostellar hooks, is shown. Image from the CDC. [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Many cestode infestations are asymptomatic.[5] The most common symptoms are abdominal pain, anorexia, weight loss, and malaise. Diphyllobothrium absorbs large quantities of vitamin B-12, causing megaloblastic anemia in hosts.[4]T solium may deposit cysts in the central nervous system, leading to the development of seizures from neurocysticercosis.[6]

E granulosus may deposit cysts slowly over years, eventually precipitating a mass effect on the involved organ[7,8]; the alveolar form of the disease may not manifest until 5 to 15 years after infection.[8] Rupture of these cysts may cause fever, pruritus, urticaria, eosinophilia, and anaphylaxis.[7] Infections can be fatal if untreated.

This computed tomography (CT) scan image shows peritoneal, disseminated echinococcosis. Image courtesy of Medscape.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Tapeworm infections typically can be diagnosed by collecting two or three stool samples and checking for ova and parasites.[4] Enzyme-linked immunosorbent assay (ELISA), immunoblot analysis, and polymerase chain reaction (PCR) assay may help to confirm a diagnosis; sensitivities and specificities vary, depending on the species involved.[2,4,9]

Imaging studies usually are reserved for the evaluation of neurocysticercosis (CT scanning or magnetic resonance imaging [MRI]) and echinococcal cysts, usually located in the liver or lungs (CT scanning, MRI, or ultrasonography).[2,10]

Treatment for most infections involves antihelminthic medications, such as praziquantel, niclosamide, or albendazole. Procedural interventions typically are reserved for patients with symptomatic local infections, and include surgical excision and aspiration.[2,4,9]

Larvae of Enterobius vermicularis (human pinworm) are shown in the perianal folds. Image courtesy of Medscape | Joseph J Bocka, MD.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

E vermicularis

E vermicularis is the most common intestinal parasite in the United States.[11] Humans are the only host for this organism, found mostly in school-aged children. Infection can be contracted from the ingestion of eggs found on contaminated hands or in food or water.[12]

The pinworms live in the small intestine, but the gravid female parasites travel to the anus and deposit eggs in the perianal skin folds, typically at night. The hatched larvae then migrate back into the anus and lower intestine, causing retro-infection.[11]

Adult female worms of E vermicularis collected from a 2-year-old girl in a Korean orphanage after treatment with pyrantel pamoate 10 mg/kg. Image courtesy of Medscape.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

About one third of patients infected with E vermicularis are asymptomatic.[13] The most common complaint is anal pruritus, which may lead to erythema and even superficial bacterial infection from scratching.[11] Rarely, pinworm infections may cause appendicitis, urinary tract infections, and vaginitis.[12]

The diagnosis is made through microscopic identification of ova and female pinworms from perianal swabs (ideally collected in the morning via transparent adhesive tape).[12] Treatment is two doses of anthelmintic medications (mebendazole, pyrantel pamoate, or albendazole) spaced over 2 weeks to the entire household or classroom, as reinfection is common.[11,12]

The life cycle of Ascaris lumbricoides is shown. Image from the CDC. [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Ascaris

Ascariasis is the most common helminthic infection worldwide.[14]A lumbricoides is the largest of the roundworms that infect humans, growing as long as 35 cm, and it may live for up to 2 years in the small intestine. Its life cycle is complex and involves multiple human organs.

Adult A lumbricoides roundworms feed on digestive products, which can lead to protein, calorie, or vitamin A deficiency in children at risk for malnutrition.[15] Because these worms do not multiply in the host, infection is limited to 2 years unless reexposure occurs. Most infected individuals are asymptomatic, but patients may develop growth restriction, pneumonitis, pneumonia, eosinophilia, intestinal or pancreatic obstruction, and hepatobiliary injury.[14]

An Ascaris worm being surgically removed from a human bile duct is shown. Image from SuSanA Secretariat| Larry Hadley, via Flickr. [Creative Commons 2.0 generic license (CC by 2.0).]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

The diagnosis of ascariasis typically is made by means of stool ova and parasite examination.[2,16,17] The World Health Organization (WHO) recommends using a Kato-Katz thick smear to detect Ascaris infection in endemic areas. This test is inexpensive, specific, and can identify multiple different parasitic infections.[14]

Abdominal radiographs may show signs of bowel obstruction, and ultrasonography may reveal a single worm or a mass of the worms with segmented sections and curling movements.[18]

Treatment with a single dose of albendazole, mebendazole, or ivermectin is usually effective for eradication. Medical therapy during active pulmonary infection is not indicated, not only because pulmonary ascariasis is a self-limited disease but also owing to the high risk of developing pneumonitis from the dying larvae.[18] Endoscopic retrograde cholangiopancreatography may be used to diagnose and treat infection of the biliary system.[18]

The life cycle of the hookworm is shown; the inset image depicts a magnified Necator americanus hookworm. Life cycle image from the CDC [public domain]; inset image from Wikimedia Commons | Jasper Lawrence [CC ShareAlike 4.0 International (CC by-SA 4.0.)].

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Hookworms

Human hookworms, predominantly Ancylostoma duodenale and N americanus, infect hundreds of millions of people (576-740 million) worldwide.[18,19] They are the second most common cause of helminthic infections after ascariasis and a leading cause of iron deficiency anemia in the developing world.[18]

Hookworm larvae rapidly penetrate the skin of humans who are exposed to soil contaminated by human feces. The larvae then burrow into venules and embolize into the lungs, where they penetrate the alveoli and cause an asymptomatic alveolitis with eosinophilia.[3] Coughing and then swallowing transports the larvae into the intestines, where they mature into adults. Adult worms feed on blood from the mucosal capillaries.

Cutaneous larva migrans. Image from Wikimedia Commons | WeisSagung. [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

The lifespan of Necator is 3 to 10 years, and that of Ancylostoma is 1 to 3 years.[2]

Patients may report diarrhea, vague abdominal pain, colic, or nausea.

The diagnosis is made by means of stool ova and parasite examination.[2,18,19] A single dose of albendazole or mebendazole or one dose daily of pyrantel pamoate for 3 days typically is sufficient to eradicate the infection.[20]

An image of an unstained Fasciolopsis buski egg at 500 × magnification is shown on the left; an adult F buski fluke is displayed on the right. The left image is from the CDC; the right image is from the CDC | Georgia Division of Public Health. [Both images: public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

Intestinal trematodes

Intestinal flukes are flat hermaphroditic worms (class Trematoda) that range in length from a few millimeters to several centimeters.[21] Although they are endemic worldwide, they are most common in East and South Asia.[22] The species that most often infect humans are F buski (shown), which is the largest and most common human intestinal fluke; Heterophyes heterophyes; and Metagonimus yokogawai, along with Echinostoma species.[22]

The life cycle of trematodes is shown. Adapted image from the CDC. [Public domain.]

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | November 15, 2021 | Contributor Information

The life cycle of each species is complex and can involve a number of intermediate hosts, such as snails, fish, tadpoles, and vegetables, which are usually the source of human infection.[21] Adult flukes cause inflammation, ulceration, and mucus secretion at the site of small intestinal wall attachment.

Most infected persons are asymptomatic, but some may develop loose stools, weight loss, malaise, and nonspecific abdominal pain.[21]

The diagnosis is made by means of stool ova and parasite examination. Treatment with three doses of praziquantel over 1 day is typically sufficient to clear infection.[22]

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