Residency · Residency · Infectiousdisease

Parasitic Infections

Classification and Approach

Major Categories

Parasitic infections encompass a vast and diverse array of organisms that are broadly divided into two major categories: protozoa and helminths. Protozoa are single-celled eukaryotic organisms and include several pathogens of major clinical significance in infectious disease practice, including Toxoplasma gondii, Cryptosporidium species, Giardia lamblia, Entamoeba histolytica, Leishmania species, Trypanosoma species, and Babesia microti (covered in the tick-borne illness lecture). Malaria, caused by Plasmodium species, is the most important protozoan infection globally and is addressed in a separate dedicated lecture.

Helminths, or parasitic worms, are multicellular organisms classified into three major groups. Nematodes, or roundworms, include Ascaris lumbricoides, hookworm species (Ancylostoma duodenale and Necator americanus), Strongyloides stercoralis, filarial parasites, Toxocara species, Trichinella spiralis, and Enterobius vermicularis (pinworm). Cestodes, or tapeworms, include Taenia solium (the cause of neurocysticercosis), Echinococcus species (the cause of hydatid disease), and Diphyllobothrium latum. Trematodes, or flukes, include Schistosoma species and the liver flukes Clonorchis sinensis, Opisthorchis viverrini, and Fasciola hepatica.

Diagnostic Clues

A fundamental diagnostic principle in parasitology is that peripheral eosinophilia is the hallmark of helminthic infections but is characteristically absent in protozoan infections. This distinction is clinically valuable because eosinophilia in a traveler or immigrant should immediately direct the evaluation toward helminthic causes. The tissue migration phase of helminthic infections typically produces the highest eosinophil counts, as exemplified by Loeffler syndrome, a transient pulmonary eosinophilia caused by larval migration through the lungs during infection with Ascaris, hookworm, or Strongyloides.

Protozoan infections that characteristically do not cause eosinophilia include malaria, giardiasis, amebiasis, cryptosporidiosis, leishmaniasis, and trypanosomiasis. Rare exceptions to this general rule include infections with Cystoisospora (formerly Isospora) belli and Sarcocystis, which may be associated with mild eosinophilia.

Protozoan Infections

Entamoeba histolytica

Entamoeba histolytica is transmitted through the fecal-oral route and is endemic in Mexico, Central and South America, South Asia, and Africa. The clinical spectrum includes intestinal amebiasis and extraintestinal disease. Intestinal amebiasis presents as bloody diarrhea, often termed amoebic dysentery, with colitis that produces characteristic flask-shaped ulcers visible on colonoscopy. Amoebic liver abscess is the most common extraintestinal manifestation and presents with right upper quadrant pain and fever. The abscess is typically solitary, large, and located in the right hepatic lobe, and aspiration yields the classic "anchovy paste" material representing necrotic hepatocytes and trophozoites.

Diagnosis of intestinal amebiasis relies on stool antigen testing by enzyme immunoassay or stool PCR, as microscopy alone cannot distinguish pathogenic E. histolytica from the morphologically identical but non-pathogenic E. dispar. For amoebic liver abscess, serology (IgG) has sensitivity exceeding 90 percent.

Treatment requires a two-pronged approach that addresses both tissue invasion and intestinal colonization. The tissue agent is metronidazole at 750 milligrams orally three times daily for 7 to 10 days, or tinidazole 2 grams orally daily for 3 to 5 days as an alternative. This must be followed by a luminal agent, paromomycin at 25 to 35 milligrams per kilogram per day divided into three doses for 7 days, to eradicate cysts persisting in the intestinal lumen. Omission of the luminal agent leads to relapse in 40 to 60 percent of cases. Drainage of amoebic liver abscess is not routinely required, as approximately 90 percent of cases respond to medical therapy alone. Drainage is reserved for abscesses failing medical therapy, those exceeding 10 centimeters, left-lobe abscesses (which carry a risk of pericardial rupture), or cases in which bacterial superinfection is suspected.

Giardia lamblia (duodenalis)

Giardia lamblia is the most common intestinal parasite worldwide and is transmitted through the fecal-oral route, including through contaminated water sources such as mountain streams, and in institutional settings such as daycare centers. The clinical presentation consists of watery, foul-smelling, non-bloody diarrhea accompanied by bloating, flatulence, and weight loss. Chronic giardiasis can cause malabsorption, particularly of fats and fat-soluble vitamins.

Diagnosis is most reliably established through stool antigen testing by enzyme immunoassay, which provides sensitivity exceeding 95 percent, or stool PCR. Traditional ova and parasite examination by microscopy can identify cysts and trophozoites but has lower sensitivity of 60 to 80 percent on a single specimen. Treatment of choice is tinidazole as a single 2-gram oral dose, which is preferred for its convenience and superior efficacy, or metronidazole 250 milligrams three times daily for 5 to 7 days.

Cryptosporidium

Cryptosporidium is a waterborne protozoan pathogen of particular importance because its oocysts are resistant to standard chlorination. In immunocompetent individuals, cryptosporidiosis causes a self-limited watery diarrhea lasting one to two weeks. In HIV/AIDS patients with CD4 counts below 100, Cryptosporidium produces profuse, chronic watery diarrhea that can lead to severe dehydration and wasting, and may extend to involve the biliary tract, causing a sclerosing cholangitis-like syndrome.

Diagnosis is established through modified acid-fast staining of stool, which reveals characteristic red-staining oocysts against a green background, stool antigen testing, or stool PCR. Treatment in immunocompetent patients consists of nitazoxanide 500 milligrams orally twice daily for three days. In HIV/AIDS patients, antiretroviral therapy with immune reconstitution is the primary and most effective treatment, as nitazoxanide has limited efficacy in the setting of severe immunosuppression.

Leishmaniasis

Leishmaniasis is transmitted by the bite of sandflies and encompasses three major clinical forms caused by different Leishmania species. Visceral leishmaniasis (kala-azar), caused by the Leishmania donovani complex, presents with fever, massive splenomegaly, pancytopenia, hypergammaglobulinemia, and progressive wasting, and is fatal if untreated. Cutaneous leishmaniasis, caused by L. tropica, L. major, or L. mexicana, produces painless ulcers at the site of the sandfly bite with characteristic central ulceration and raised borders. Mucocutaneous leishmaniasis, caused by L. braziliensis and found in Latin America, results in destructive involvement of nasal and oropharyngeal tissues.

Diagnosis requires demonstration of amastigotes, also known as Leishman-Donovan bodies, within macrophages on tissue biopsy or aspirate, or by PCR from tissue. For visceral leishmaniasis, the rK39 rapid diagnostic test provides sensitivity exceeding 95 percent and is widely used in endemic settings. Treatment of visceral leishmaniasis consists of liposomal amphotericin B at 3 milligrams per kilogram per day for five days followed by doses on days 14 and 21, for a total dose of 21 milligrams per kilogram, which is the FDA-approved regimen. Treatment of cutaneous leishmaniasis varies by species, lesion size, and location, with options including cryotherapy, intralesional antimonials, and oral miltefosine. Mucocutaneous disease requires systemic therapy with liposomal amphotericin B.

Chagas Disease (American Trypanosomiasis)

Chagas disease, caused by Trypanosoma cruzi and transmitted by the triatomine bug (the "kissing bug"), is endemic throughout Latin America, and over 300,000 individuals in the United States, predominantly immigrants from endemic regions, are estimated to be infected. The acute phase presents with the Romana sign, a unilateral painless periorbital edema at the site of conjunctival inoculation, accompanied by fever and hepatosplenomegaly, though the majority of acute infections are asymptomatic.

The chronic phase develops decades after initial infection and is dominated by cardiac involvement. Chagas cardiomyopathy, a dilated cardiomyopathy, is the most important clinical manifestation and a leading cause of heart failure in endemic regions. Gastrointestinal involvement produces megaesophagus and megacolon through destruction of the enteric nervous system.

Diagnosis of acute disease relies on microscopic identification of trypomastigotes in blood or PCR. Chronic disease requires serologic confirmation using two different assay formats, as no single assay has sufficient performance characteristics to be used alone. Treatment consists of benznidazole (preferred) or nifurtimox, with the greatest benefit observed in acute, congenital, and early chronic disease. The BENEFIT trial in 2015 demonstrated that benznidazole reduced parasitemia in patients with established Chagas cardiomyopathy but did not reduce cardiac disease progression, underscoring the importance of early treatment before irreversible cardiac damage has occurred. Screening for T. cruzi infection should be performed in all blood donors, organ donors and recipients from endemic areas, and pregnant women from endemic regions.

<image>A clinical presentation comparison of four major protozoan infections seen in ID practice. Four quadrants: (1) "Entamoeba histolytica" showing a cross-section of the liver with a large right-lobe abscess and a colonoscopy image showing flask-shaped colonic ulcers. (2) "Leishmania" showing three clinical forms: visceral (cachetic patient with massive splenomegaly), cutaneous (volcano-like skin ulcer), and a microscopy image of amastigotes within macrophages. (3) "Trypanosoma cruzi (Chagas)" showing Romana sign (periorbital edema), dilated cardiomyopathy on echocardiogram, and megaesophagus/megacolon on barium study. (4) "Cryptosporidium" showing modified acid-fast stain with red oocysts against green background, and a clinical context box showing HIV patient with chronic diarrhea. Include organism life stage illustrations and geographic distribution for each. Medical illustration style.</image>

Helminthic Infections

Strongyloides stercoralis

Strongyloides stercoralis occupies a unique position among helminths due to its ability to complete an internal autoinfective cycle within the human host, a property that allows the parasite to persist for decades after the initial infection without reexposure. This autoinfective capacity also underpins the devastating hyperinfection syndrome that occurs when immunosuppression, particularly corticosteroid therapy, disrupts the host immune containment of the parasite. Strongyloides is endemic in tropical and subtropical regions, and historical foci of endemicity have been documented in the Appalachian region of the United States.

Chronic strongyloidiasis is often asymptomatic or causes mild gastrointestinal symptoms accompanied by eosinophilia. Larva currens, a distinctive serpiginous urticarial rash on the trunk and buttocks, is a pathognomonic but infrequently observed manifestation. Hyperinfection syndrome occurs when immunosuppression, particularly with corticosteroids, HTLV-1 co-infection, or transplant-related immunosuppression, removes the immune constraints on the autoinfective cycle, resulting in massive larval burden with dissemination to the lungs, central nervous system, and other organs. A characteristic and ominous complication is the development of gram-negative bacteremia and meningitis, as the migrating larvae carry enteric bacteria through the intestinal wall into the bloodstream.

Diagnosis is challenging, as stool ova and parasite examination has a sensitivity of only approximately 30 percent on a single specimen, necessitating multiple samples. Strongyloides serology provides sensitivity of 85 to 95 percent and is the preferred screening test. Stool PCR is an emerging diagnostic modality. Treatment of uncomplicated chronic infection consists of ivermectin 200 micrograms per kilogram per day for two days, with a repeat course at day 14 recommended for immunocompromised patients. Hyperinfection syndrome requires daily ivermectin until larvae are undetectable, often for two to four weeks. The critical preventive measure is to screen all patients from endemic areas before immunosuppression, particularly before transplant or initiation of corticosteroid therapy.

Schistosomiasis

Schistosomiasis is caused by trematodes of the genus Schistosoma, with three major species of clinical importance: S. mansoni (Africa and South America), S. haematobium (Africa and the Middle East), and S. japonicum (East Asia). Infection is acquired through skin penetration by cercariae during freshwater exposure. The cercariae migrate to the portal venous system, where S. mansoni and S. japonicum reside, or to the vesical venous plexus, where S. haematobium localizes.

Acute schistosomiasis, known as Katayama fever, develops two to eight weeks after exposure and presents with fever, urticaria, eosinophilia, and hepatosplenomegaly, representing an immune complex-mediated reaction to the developing schistosomulae. Chronic schistosomiasis is driven by the host immune response to eggs deposited in tissues. S. mansoni and S. japonicum produce hepatosplenic disease with periportal fibrosis (Symmers pipe-stem fibrosis) and portal hypertension. S. haematobium produces urogenital disease with hematuria, hydroureter, and an increased risk of squamous cell carcinoma of the bladder.

Diagnosis relies on stool ova and parasite examination for S. mansoni and S. japonicum, urine ova and parasite examination for S. haematobium, serology, and quantitative egg counts. Treatment consists of praziquantel at 40 to 60 milligrams per kilogram administered in divided doses as a single-day treatment. For acute schistosomiasis, the treatment should be repeated at four to six weeks, because praziquantel is effective only against adult worms and cannot kill immature schistosomulae that may not have matured at the time of initial treatment.

Neurocysticercosis

Neurocysticercosis, caused by the larval stage of the pork tapeworm Taenia solium, is the most common helminthic infection of the central nervous system worldwide. A critical distinction in understanding the epidemiology is that humans become the accidental intermediate host through ingestion of T. solium eggs from fecal contamination, not through eating pork. Consumption of undercooked pork containing cysticerci causes intestinal taeniasis (the adult tapeworm infection), not cysticercosis.

The larvae, or cysticerci, encyst within the brain parenchyma and other tissues. Seizures are the most common clinical presentation. Other manifestations include hydrocephalus, focal neurologic deficits, and diffuse encephalitis with multiple viable cysts. MRI reveals ring-enhancing or cystic lesions, and the visualization of the scolex as a "dot within the cyst" is pathognomonic. Cysts progress through well-defined stages: vesicular (viable), colloidal (degenerating), granular-nodular (involuting), and calcified (dead).

Treatment is tailored to the stage and number of cysts. Calcified or dead cysts require only antiseizure medications, as no antiparasitic therapy is needed. Viable parenchymal cysts are treated with albendazole at 15 milligrams per kilogram per day, with a maximum of 800 milligrams twice daily, for 10 to 14 days, with or without praziquantel at 50 milligrams per kilogram per day. Corticosteroids, specifically dexamethasone at 0.1 milligrams per kilogram per day, must always be started before the antiparasitic agent, as killing the cysticerci triggers an intense inflammatory response that can cause fatal cerebral edema. Single enhancing lesions are treated with albendazole for 7 days plus corticosteroids. Intraventricular and subarachnoid cysts require neurosurgical consultation, and ventricular shunting is indicated for hydrocephalus. Antiparasitic therapy in these locations requires close monitoring. Household contacts of patients with neurocysticercosis should be screened with stool ova and parasite examination for intestinal T. solium carriage, and positive contacts should be treated with praziquantel or niclosamide to prevent further egg shedding.

Echinococcosis (Hydatid Disease)

Echinococcosis is caused by Echinococcus granulosus, which produces cystic echinococcosis, and E. multilocularis, which produces alveolar echinococcosis. The lifecycle involves a dog-sheep cycle, and humans become accidental intermediate hosts through ingestion of eggs shed in canine feces. Hepatic cysts are the most common manifestation, followed by pulmonary cysts. CT and MRI reveal well-defined cystic hepatic lesions with internal daughter cysts, and the "water lily sign," representing floating detached germinal membrane, is a characteristic imaging finding.

Treatment involves a combination of surgery and medical therapy. The PAIR technique (percutaneous aspiration, injection of hypertonic saline or ethanol, and re-aspiration) is a minimally invasive alternative to open surgery. Albendazole at 400 milligrams twice daily should be started one to four weeks before any procedural intervention and continued for one to three months after surgery or PAIR. A critical safety consideration is that cyst rupture can cause anaphylaxis and peritoneal dissemination, necessitating careful technique and pre-treatment with albendazole to reduce the viability of protoscoleces.

<image>A diagnostic algorithm for eosinophilia in the returning traveler or immigrant. Start with "Peripheral eosinophilia (>500 eosinophils/mm3) in patient with travel/immigration history." First step: "Detailed exposure history: travel destinations, freshwater exposure, soil contact, raw/undercooked food, animal exposure." Second step: "Screening tests: Strongyloides serology, Schistosoma serology (if freshwater exposure), stool O&P x3, Toxocara serology, filaria antigen/antibody (if tropical residence), CBC with differential." Branch based on results: "Strongyloides positive" → ivermectin; "Schistosoma positive" → praziquantel; "Stool O&P positive" → identify organism and treat specifically; "All negative but persistent eosinophilia" → consider tissue-invasive helminth (Toxocara, Trichinella, Fasciola), empiric albendazole, or tissue biopsy. Include a note: "Protozoa generally do NOT cause eosinophilia (exceptions: Cystoisospora, Sarcocystis)." Use a clinical decision tree format with organism illustrations.</image>

Key Clinical Pearls

• Eosinophilia in a traveler or immigrant should always trigger evaluation for helminthic infection -- protozoa generally do NOT cause eosinophilia
• Strongyloides hyperinfection is preventable -- screen all patients from endemic areas before starting corticosteroids or other immunosuppressive therapy
• Entamoeba histolytica treatment requires BOTH a tissue agent (metronidazole) AND a luminal agent (paromomycin) -- omitting the luminal agent leads to relapse in 40-60%
• Praziquantel is effective against adult Schistosoma worms but NOT immature schistosomulae -- repeat dosing is needed for acute infection
• Neurocysticercosis treatment ALWAYS requires corticosteroids before antiparasitic therapy -- killing the cysts without anti-inflammatory coverage can trigger fatal cerebral edema
• Cryptosporidium in HIV/AIDS is treated primarily by immune reconstitution with ART -- antiparasitic agents are of limited benefit in severe immunosuppression
• Chagas disease screening should be performed for all blood donors, organ donors/recipients, and pregnant women from endemic Latin American countries
• For amoebic liver abscess, drainage is NOT routinely needed -- 90% respond to metronidazole alone; drainage reserved for large, left-lobe, or refractory abscesses

References

1. CDC. Parasites - Resources for Health Professionals. Centers for Disease Control and Prevention. Accessed 2024.
2. Nutman TB. Evaluation and differential diagnosis of marked, persistent eosinophilia. Immunol Allergy Clin North Am. 2007;27(3):529-549.
3. White AC Jr, Coyle CM, Rajshekhar V, et al. Diagnosis and treatment of neurocysticercosis: 2017 clinical practice guidelines by the IDSA and ASTMH. Clin Infect Dis. 2018;66(8):e49-e75.
4. Buonfrate D, Bisanzio D, Giorli G, et al. The global prevalence of Strongyloides stercoralis infection. Pathogens. 2020;9(6):468.
5. Morillo CA, Marin-Neto JA, Avezum A, et al. Randomized trial of benznidazole for chronic Chagas' cardiomyopathy (BENEFIT). N Engl J Med. 2015;373(14):1295-1306.

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