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Image from Wikimedia Commons | Centers for Disease Control and Prevention.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

The above micrograph shows the scolex of a Taenia solium tapeworm, which attaches to the intestinal wall of its host.

Parasitic intestinal infection with helminths and/or protozoa can lead to significant morbidity and mortality if not recognized and treated appropriately. Although such infections are typically thought of as being associated more with the developing world than with industrialized nations, rising worldwide travel and immigration will increase the number of cases diagnosed within the United States.

Image from the Centers for Disease Control and Prevention (CDC).

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Helminths

Cestodes

The life cycle of Echinococcus is shown above.

Tapeworms are long, segmented worms of the class Cestoda, which includes the genera Taenia, Diphyllobothrium, Hymenolepis, Dipylidium, Echinococcus, and Spirometra.[1,2] Adult tapeworms lack an intestinal tract and absorb all nutrients through their integument. The adults have a head (termed a scolex), a neck, and a segmented body with both male and female gonads.[1]

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. In the intermediate host, 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.[1]

Humans are the primary hosts for Taenia, Diphyllobothrium, and Hymenolepis,[1] but they may be intermediate hosts for Echinococcus[3] and Spirometra.[2] Infection is typically from either fecal-oral transmission or the ingestion of contaminated, undercooked meat.[1] Most cestode infections are uncommon in the United States, with H nana being the most commonly diagnosed.[3]

Image from the CDC | Dr Peter M. Schantz.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Echinococcus granulosus is shown above.

Many cestode infestations are asymptomatic.[4] The most common symptoms are abdominal pain, anorexia, weight loss, and malaise. Diphyllobothrium absorbs large quantities of vitamin B12, causing megaloblastic anemia in hosts.[3]Taenia solium may deposit cysts in the central nervous system, leading to the development of seizures from neurocysticercosis.[5]

Echinococcus granulosus may deposit cysts slowly over years, eventually precipitating a mass effect on the involved organ;[6,7] the alveolar form of the disease may not manifest until 5-15 years after infection.[7] Rupture of these cysts may cause fever, pruritus, urticaria, eosinophilia, and anaphylaxis.[6] Infections can be fatal if untreated.

Image from Wikipedia | Fabio Crippa and Enrico Brunetti.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

The above liver ultrasonogram shows a cystic lesion that is consistent with an echinococcal cyst.

Tapeworm infections can typically be diagnosed by collecting two or three stool samples and checking for ova and parasites.[3] 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.[1,3,8]

Imaging studies are usually reserved for the evaluation of neurocysticercosis (computed tomography [CT] scanning or magnetic resonance imaging [MRI]) and echinococcal cysts (CT scanning, MRI, or ultrasonography).[1]

Treatment for most infections involves anthelmintic medications, such as praziquantel, niclosamide, or albendazole. Procedural interventions are typically reserved for patients with symptomatic local infections and include surgical excision and aspiration.[1,3,8]

Image courtesy of Medscape | Joseph J Bocka, MD.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Enterobius vermicularis

In the above image, larvae of Enterobius vermicularis (human pinworm) are seen in the perianal folds.

E vermicularis is the most common intestinal parasite in the United States.[9] Humans are the only host for this parasite, which is found most commonly in school-aged children. Infection can be contracted from the ingestion of eggs found on contaminated hands or in food or water.[10] The pinworms live in the small intestine, but the gravid female parasites will travel to the anus and deposit eggs in the perianal skin folds, typically at night. The hatched larvae will then migrate back into the anus and lower intestine, causing retro-infection.[9]

Most infected patients are asymptomatic. The most common complaint is anal pruritus.[9] However, pinworm infection can also cause appendicitis, urinary tract infections, and vaginitis.[10]

The diagnosis is made through microscopic identification of ova and female pinworms from perianal swabs (ideally collected in the morning via transparent adhesive tape).[10] Treatment involves the administration of anthelmintic medications (mebendazole, pyrantel pamoate, or albendazole) to the entire household,[9] as well as cleaning all clothing and bedding.

Image from the CDC.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Ascaris

Ascariasis is the most common helmintic infection worldwide.[11] Ascaris 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 (shown) is complex and involves multiple human organs.

Adult A lumbricoides roundworms will feed on digestive products, which can lead to protein, calorie, or vitamin A deficiency in children at risk for malnutrition.[12] 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 retardation, pneumonitis, pneumonia, eosinophilia, intestinal or pancreatic obstruction, and hepatobiliary injury.[11]

Image courtesy of Creative Commons | Larry Hadley.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

The above image shows an ascariasis worm being surgically removed from a human bile duct. The diagnosis of ascariasis is typically made by means of stool ova and parasite examination.[1,13,14] The World Health Organization 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.[11]

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.[15]

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 due to the high risk of developing pneumonitis from the dying larvae.[15] Endoscopic retrograde cholangiopancreatography may be used to diagnose and treat infection of the biliary system.[16]

Adapted intestinal hookworm lifecycle image from the CDC; inset image of Necatur americanus from Wikimedia Commons | Jasper Lawrence.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Hookworms

The life cycle of the hookworm is shown above; the inset shows a magnified Necator americanus hookworm.

Human hookworms, predominantly Ancylostoma duodenale and Necator americanus, infect hundreds of millions of people (576-740 million) worldwide.[17,18] They are the second most common cause of helmintic infections after ascariasis and are a leading cause of iron deficiency anemia in the developing world.[17]

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

Patients may report diarrhea, vague abdominal pain, colic, or nausea. The diagnosis is made by means of stool ova and parasite examination.[1,18,19] A single dose of albendazole or mebendazole or one dose daily of pyrantel pamoate for 3 days is typically sufficient to eradicate the infection.[19]

Image of an unstained Fasciolopsis buski egg at 500× magnification from the CDC (left); image an adult F buski fluke from the CDC | Georgia Division of Public Health (right).

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Intestinal trematodes

Intestinal flukes are flat hermaphroditic worms (class Trematoda) that range in length from a few millimeters to several centimeters.[20] Although they are endemic worldwide, they are most common in East and South Asia.[21] The species that most commonly 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.[20]

The life cycle of each species is complex and can involve a number of intermediate hosts, such as snails, fish, tadpoles, and vegetables.[20] Humans are typically infected after ingesting a raw or undercooked aquatic plant or intermediate host. Adult flukes cause inflammation, ulceration, and mucus secretion at the site of small intestinal wall attachment.

Most infected persons are asymptomatic, but some may suffer from loose stools, weight loss, malaise, and nonspecific abdominal pain.[20]

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.[21]

Image from the CDC.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Microsporidia

An electron micrograph of Encephalitozoon hellem spores rupturing from a eukaryotic cell is shown above.

Microsporidia are obligate, intracellular, spore-forming parasites.[1] The phylum Microsporidia contains over 1200 fungal species, a large number of which cause infection in humans. Two of the most important species are Encephalitozoon hellem and E intestinalis.[23] Most infections are found in persons infected with human immunodeficiency virus (HIV), organ transplant recipients, people with diabetes, children, and the elderly.[23]

Microsporidiosis occurs through fecal-oral ingestion or inhalation of microsporidial spores via human-to-human or waterborne transmission. Patients with intestinal microsporidiosis may develop chronic, nonbloody diarrhea; weight loss; abdominal pain; nausea; vomiting; and malnourishment.[24] With dissemination, cholecystitis and renal failure, as well as infections of the muscles, brain, and respiratory tract, may occur. In rare cases, patients with HIV/acquired immunodeficiency syndrome (AIDS) may develop microsporidial keratoconjunctivitis.[25]

The diagnosis is made via stool microscopy, but this does not allow species identification.[22] Cytologic and histologic examination may also be helpful; additionally, immunofluorescence assays (IFA) and PCR assays are available. Typically, treatment with albendazole for 2-4 weeks is effective for most ocular, intestinal, and disseminated microsporidiosis.[22,25]

Image from the CDC | Oregon Public Health Laboratory.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Protozoa

Balantidium coli

Balantidiasis is an intestinal infection caused by the ciliated protozoan Balantidium coli (shown).[26] The primary reservoir is thought to be pigs. Thus, those who handle pigs or pig byproducts are at increased risk for infection. People who are malnourished or in areas of poor hygiene are also at risk.[27]

After human ingestion of infective cysts via contaminated food or water, B coli organisms migrate to the large intestine.[27] Once in the intestine, they develop into trophozoites, which replicate by binary fission and conjugation while consuming bacteria.[26] They reside primarily within the intestinal lumen but may penetrate the mucosa and cause ulcers.[26] Although most immunocompetent individuals will be asymptomatic, patients may develop bloody, mucoid diarrhea; nausea; vomiting; abdominal pain; anorexia; weight loss; fever; colitis; and dehydration.

The diagnosis is made on the basis of wet smears of stool specimens.[26] Treatment includes volume and electrolyte replacement, as well as antimicrobial coverage with tetracycline, metronidazole, or iodoquinol.[26] Infections are easily treated if the correct diagnosis is made.[27]

Image from the CDC.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Dientamoeba fragilis

Dientamoeba fragilis is a nonflagellated protozoan (shown) that infects the large intestine.[28] Transmission is thought to occur via human-to-human fecal-oral spread or through coinfection with the eggs of E vermicularis (human pinworm);[28,29] however, the life cycle is incompletely understood.

Trophozoites infect the mucosal crypt cells of the large intestine,[28] invoking an eosinophilic inflammatory response. Abdominal pain and nonbloody diarrhea are the most common symptoms, but anorexia, weight loss, nausea, vomiting, flatulence, headaches, fever, malaise, and fatigue may also develop.[28,29]

The diagnosis is typically made through microscopic evaluation of permanently stained fresh fecal smears (trichrome).[28,29] Treatment typically involves the use of an anthelmintic medication such as iodoquinol (drug of choice) or paromomycin.[28-30] Clinical improvement has also been observed following treatment with tetracycline.[30]

Image from Pixnio | Dr. Stan Erlandsen, USCDCP.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Giardia

Giardiasis is a major diarrheal illness found worldwide; in the United States, it is the most frequently diagnosed intestinal parasitic disease.[31] It is most commonly caused by the flagellate protozoan Giardia lamblia (shown; also known as G intestinalis).[28,32,33] Infection occurs via the ingestion of Giardia cysts, typically by way of contaminated water.

Most infections are asymptomatic, and asymptomatic carriage is quite common.[28,31,32] Person-to-person transmission from poor hygiene and sanitation is a primary means of infection. Symptomatic individuals may report explosive, watery diarrhea; abdominal cramps; foul flatus; vomiting; fever; malaise; anorexia; lactose intolerance; and weight loss.[28,31,32] Symptoms may last for up to 3 weeks. The illness is self-limited in most people, but chronic giardia infections have been reported and can cause chronic fatigue syndrome, irritable bowel syndrome, and food allergies.[33]

The diagnosis is made by means of stool examination for cysts or trophozoites; stool antigen detection with ELISA or IFA; or, rarely, duodenal sampling.[31,32] Treatment typically involves aggressive fluid and electrolyte replacement plus administration of an antimicrobial agent such as metronidazole.[33]

Composite micrograph of Entamoeba histolytica cyst (iodine stain) and trophozoites (Giemsa stain) from the CDC (left); trichrome-stained micrograph of E histolytica trophozoite from the CDC | Drs Mae Melvin and Greene (right inset).

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Entamoeba histolytica

Amebiasis, caused by infection with the protozoan Entamoeba histolytica, is the second leading cause of death by parasitic infection.[34] Infection occurs via ingestion of cystic protozoa, typically in fecally contaminated soil, food, or water.[35] Excystation into trophozoites occurs in the cecum, terminal ileum, or colon.[1] The trophozoites then penetrate the colonic mucosal barrier and, with severe invasive disease, produce tissue destruction, secretory bloody diarrhea, and colitis.[1] Hematogenous spread can cause trophozoite deposition into the liver, brain, and lungs, leading to abscess formation.[1,35] Liver abscesses are the most common extraintestinal infection.[35]

The diagnosis can be made by means of stool microscopy for cysts and trophozoites, ELISA, IFA, indirect hemagglutination, or PCR assay.[1,36] Iodoquinol or paromomycin are first-line therapies for Entamoeba histolytica infection, but these medications have poor gastrointestinal absorption and are therefore most effective at treating asymptomatic intestinal colonization.[35] Symptomatic intestinal and extraintestinal disease is treated with metronidazole or tinidazole, followed by administration of iodoquinol or paromomycin. Extraintestinal abscesses generally require both drainage and metronidazole or tinidazole administration.[35]

Adapted image from the CDC | Alexander J da Silva, PhD, and Melanie Moser.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Cryptosporidium

The life cycle of Cryptosporidium is shown above.

Cryptosporidiosis is caused by infection with protozoa from the genus Cryptosporidium, most commonly C hominis or C parvum.[37] Infection generally occurs via human-to-human fecal-oral transmission but may also result from animal-to-person or waterborne transmission.

Cryptosporidium infection causes severe, explosive diarrhea through a combination of increased intestinal permeability, chloride secretion, and malabsorption within the small intestine.[38]

In healthy adults, cryptosporidiosis typically produces a self-limited diarrheal illness that lasts for up to 4 weeks. In children or immunocompromised patients, including those with AIDS or who have undergone organ transplantation, the disorder may cause persistent and more severe diarrhea, which can last for months.[37,38] The estimated prevalence of Cryptosporidium infection in patients with HIV/AIDs is 9% globally.[39]

Image of Cryptosporidium species oocysts with Ziehl-Neelsen modified acid-fast stain from the CDC | Oregon State Public Health Laboratory.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Cryptosporidiosis is diagnosed by means of stool microscopy (with modified acid-fast stain, demonstrating red-stained, round oocytes, as shown), antigen-detection assay, or PCR assay.[37-39]

Treatment with nitazoxanide is modestly effective and will shorten the duration of diarrhea in immunocompetent hosts,[37,39,40] but it has no proven efficacy in patients with AIDS.[37-39] Symptomatic therapy with fluids, nutritional support, and antimotility agents will help to prevent subsequent morbidity. Reducing immunosuppression with anti-retroviral therapy is paramount in treating the diarrhea.[41]

Adapted image from the CDC.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

Cystoisospora belli

The life cycle of Cystoisospora belli is shown above.

Cystoisosporiasis (formerly known as isosporiasis) is a diarrheal illness caused by infection with the protozoan Cystoisospora (Isospora) belli.[42,43] Infection occurs via human-to-human fecal-oral transmission; no animal reservoir has been identified. Oocysts can remain infectious in the environment outside of a host for months.[43] Infection is most common in South America, Africa, and Southeast Asia.[43]

The signs and symptoms of cystoisosporiasis include profuse, watery, nonbloody diarrhea; crampy abdominal pain; malaise; anorexia; vomiting; and fevers. In infants/children and immunocompromised patients, the disorder may cause persistent and more severe diarrhea.[42,43]

Images from the CDC.

What's Eating You: 12 Common Intestinal Parasites

Lars Grimm, MD, MHS | November 25, 2019 | Contributor Information

The diagnosis of cystoisosporiasis is made by means of microscopic examination of stool specimens in wet mounts to identify oocysts or with modified acid-fast stains of fecal smears (left).[43] Ultraviolet autofluorescence microscopy (right) is also helpful, because oocysts will autofluoresce when exposed to an ultraviolet filter. Diagnostic PCR testing has high sensitivity and specificity but is not frequently used for diagnosis.[43]

Treatment is supportive, with fluid and electrolyte replacement. Oral trimethoprim-sulfamethoxazole is used for curative therapy, with ciprofloxacin employed as a second-line, alternative agent.[42,43] Long-term suppressive therapy may be needed for patients with AIDS.[43]

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10 Community-Acquired Infections

From outbreaks of Legionnaires disease to the emergence of antimicrobial-resistant disorders, community-acquired infections have become an increasing focus in the healthcare community and in the media.Slideshows, November 2018
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Ascariasis

Ascariasis is the most common helminthic infection, with an estimated worldwide prevalence of 804 million cases in 2013. Usually asymptomatic, ascariasis is most prevalent in children of tropical and developing countries, where they are perpetuated by contamination of soil by human feces or use of untreated feces as fertilizer.Diseases/Conditions, July 2018
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Amebiasis

Amebiasis is caused by Entamoeba histolytica, a protozoan that is found worldwide. The highest prevalence of amebiasis is in developing countries where barriers between human feces and food and water supplies are inadequate.Diseases/Conditions, July 2018
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References