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Mass of Ascaris lumbricoides worms passed by infected child. Image courtesy of CDC.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Helminths

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 on the basis of host environments, global warming will disrupt these natural environments and cause shifts in population centers that may change the distribution of disease.[1] Efforts to develop vaccines for soil-transmitted helminths are still very preliminary.[2]

Life cycle of Echinococcus. Image courtesy of CDC.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Cestodes

Tapeworms are long, segmented worms of the class Cestoda, which includes the genera Taenia, Diphyllobothrium, Hymenolepis, Dipylidium, Echinococcus, and Spirometra.[3,4] Adult tapeworms have a head (scolex), a neck, and a segmented body with both male and female gonads,[3] but they lack an intestinal tract and absorb all nutrients through their integument.

Tapeworms require one or more intermediate hosts in their life cycle (shown). 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.[3]

Humans are the primary hosts for Taenia, Diphyllobothrium, and Hymenolepis,[2] but they may be intermediate hosts for Echinococcus[5] and Spirometra.[4] Infection is typically from either fecal-oral transmission or ingestion of contaminated, undercooked meat.[4] Most cestode infections are uncommon in the United States, with Hymenolepis nana the most frequently diagnosed.[6] There is growing recognition of the negative impact of tapeworm infections on noncommunicable diseases (eg, cardiovascular and chronic respiratory disease).[7]

FDG-PET scan demonstrating active alveolar echinococcosis in liver. Necrotic liver is due to inactive parasitic lesion, and high FDG uptake is due to active disease. Image courtesy of Medscape | Solange Bresson-Hadni, MD, PhD, and Oleg Blagosklonov, MD, PhD.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Many cestode infestations are asymptomatic.[6] 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.[5]Taenia solium may deposit cysts in the central nervous system (CNS), leading to the development of seizures from neurocysticercosis.[8]

Echinococcus granulosus may deposit cysts slowly over years, eventually precipitating a mass effect on the involved organ[9,10]; the alveolar form of echinococcosis (shown) may not manifest until 5-15 years after infection.[10] Rupture of these cysts may cause fever, pruritus, urticaria, eosinophilia, and anaphylaxis.[9] Infections can be fatal if untreated.

Adult Dipylidium caninum tapeworm. Image courtesy of CDC.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Tapeworm infections typically can be diagnosed by collecting two or three stool samples and checking for ova and parasites.[5] 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.[3,5,10]

Imaging studies usually are reserved for the evaluation of neurocysticercosis (computed tomography [CT] or magnetic resonance imaging [MRI]) and echinococcal cysts, usually located in the liver or lungs (CT, MRI, or ultrasonography [US]).[3,11]

Treatment for most infections involves anthelmintic pharmacotherapy (eg, praziquantel, niclosamide, or albendazole). Procedural interventions typically are reserved for patients with symptomatic local infections and include surgical excision and aspiration.[3,5,10]

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

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Enterobius vermicularis

E vermicularis is the most common intestinal parasite in the United States.[12] Humans are the only host for this organism, found mostly in school-aged children.[13] Infection can be contracted from the ingestion of eggs found on contaminated hands or in food or water.[14]

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 (shown) then migrate back into the anus and lower intestine, causing retroinfection.[12]

Adult female worms of Enterobius vermicularis collected from 2-year-old girl in 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 | January 27, 2023 | Contributor Information

About one third of patients infected with E vermicularis are asymptomatic.[15] The most common complaint is anal pruritus, which may lead to erythema and even superficial bacterial infection from scratching.[12,16] Rarely, pinworm infections may cause appendicitis, urinary tract infections, and vaginitis.[14]

The diagnosis is made through microscopic identification of ova and female pinworms from perianal swabs (ideally collected in the morning via transparent adhesive tape).[14] Treatment is two doses of anthelmintic medications (mebendazole, pyrantel pamoate, or albendazole) spaced over 2 weeks to the entire household or classroom; reinfection is common.[12,14]

Ascaris lumbricoides egg in feces. Image courtesy of Medscape.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Ascaris lumbricoides

Ascariasis is the most common helminthic infection worldwide, with an estimated 700 million people infected.[17,18]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.[19] 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.[18]

Ascaris worm being surgically removed from human bile duct. Image courtesy of SuSanA Secretariat| Larry Hadley, via Flickr.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

The diagnosis of ascariasis typically is made by means of stool ova and parasite examination.[3,20,21] The World Health Organization (WHO) has recommended using a Kato-Katz thick smear to detect Ascaris infection in endemic areas. This test is inexpensive, specific, and capable of identifying multiple different parasitic infections.[18]

Abdominal radiographs may show signs of bowel obstruction, and US may reveal a single worm or a mass of the worms with segmented sections and curling movements.[22]

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 because there is a high risk of developing pneumonitis from the dying larvae.[22] Endoscopic retrograde cholangiopancreatography (ERCP) may be used to diagnose and treat infection of the biliary system.[22]

Anterior end of adult Ancylostoma duodenale worm, with mouth parts visible. Image courtesy of Medscape | Patrick W Hickey, MD.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Hookworms

Human hookworms, predominantly Ancylostoma duodenale and Necator americanus, infect hundreds of millions of people (576-740 million) worldwide.[22,23] They are the second most common cause of helminthic infections after ascariasis and a leading cause of iron deficiency anemia in the developing world; however, because of the variability in morbidity from specific hookworm species, the true extent of the disease is not fully understood.[22,24]

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.[4] Coughing and then swallowing transport the larvae into the intestines, where they mature into adults. Adult worms feed on blood from the mucosal capillaries.

Cutaneous larva migrans. Image courtesy of Wikimedia Commons | WeisSagung.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

The lifespan of N americanus is 3-10 years, and that of A duodenale is 1-3 years.[3]

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

The diagnosis is made by means of stool ova and parasite examination.[3,22,23] A single dose of albendazole or mebendazole or one daily dose of pyrantel pamoate for 3 days typically is sufficient to eradicate the infection.[25]

Unstained Fasciolopsis buski egg at 500× magnification (left); adult F buski fluke (right). Left image courtesy of CDC; right image courtesy of CDC | Georgia Division of Public Health.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

Intestinal trematodes (flukes)

Intestinal flukes are flat hermaphroditic worms (class Trematoda) that range in length from a few millimeters to several centimeters.[26] Although they are endemic worldwide, they are most common in East and South Asia.[26] The species that most often infect humans are Fasciolopsis buski (shown), which is the largest and most common human intestinal fluke; Heterophyes heterophyes; and Metagonimus yokogawai, along with Echinostoma species.[26]

Life cycle of Fasciolopsis buski. Image courtesy of CDC.

You've Got Worms! Common Intestinal Parasites

Lars Grimm, MD, MHS | January 27, 2023 | Contributor Information

The life cycle of each species is complex and can involve a number of intermediate hosts (eg, snails, fish, tadpoles, and vegetables), which are usually the source of human infection.[26] Adult flukes cause inflammation, ulceration, and mucus secretion at the site of attachment to the wall of the small intestine.

Most infected persons are asymptomatic, but some may experience loose stools, weight loss, malaise, and nonspecific abdominal pain.[26]

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.[26]

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