Parasites: Protozoa & Helminths
Vector, geography, and stool clues that name the parasite
Protozoa versus helminths
Two classes. Protozoa are single-celled organisms: the intestinal Giardia, Entamoeba and Cryptosporidium; the tissue parasites Toxoplasma, Trypanosoma and Leishmania; and the blood parasites Plasmodium and Babesia. Helminths are multicellular worms, divided into nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes).
How the organism is named. Three clues usually settle it: the vector, the exposure or geography, and where the organism is found (stool, blood smear or tissue).
Eosinophilia suggests a tissue-invasive helminth (Strongyloides, Trichinella, Schistosoma, migrating Ascaris larvae). Protozoa generally do not raise the eosinophil count.
Intestinal protozoa
Giardia lamblia. Cysts are swallowed in unfiltered stream water (campers, backpackers) or spread in daycare centers; IgA deficiency predisposes to recurrent infection. The trophozoite clings to small-bowel mucosa without invading, so it causes malabsorption: foul-smelling, greasy, non-bloody steatorrhea with bloating and no fever. The pear-shaped trophozoite has 2 nuclei (an "old man" face) and falling-leaf motility; stool antigen or microscopy confirms. Treat with metronidazole.
Entamoeba histolytica. Fecal-oral cysts carry up to 4 nuclei. Invasive trophozoites cut flask-shaped ulcers in the colon and cause bloody dysentery. Spread through the portal vein gives a right-upper-quadrant liver abscess filled with "anchovy paste" exudate. Trophozoites containing ingested RBCs are diagnostic, and serology helps with liver abscess. Treat with metronidazole for invasive disease plus luminal paromomycin to clear cysts left in the gut.
Cryptosporidium. Acid-fast oocysts resist chlorine and contaminate water supplies. Immunocompetent hosts have a self-limited watery diarrhea; once CD4 falls below 100, the diarrhea becomes severe and chronic. A modified acid-fast stain of stool shows the oocysts. Nitazoxanide is given, but antiretroviral immune reconstitution is what controls disease in AIDS.
| Feature | Giardia | Entamoeba | Cryptosporidium |
|---|---|---|---|
| Stool | Fatty, foul, non-bloody | Bloody dysentery | Watery; chronic when CD4 <100 |
| Key finding | Pear-shaped, 2 nuclei, falling-leaf motility | Trophozoites with ingested RBCs; liver abscess | Acid-fast oocysts |
| Treatment | Metronidazole | Metronidazole + paromomycin | Nitazoxanide + antiretrovirals |
Tissue protozoa
Toxoplasma gondii. Humans ingest oocysts from cat feces or tissue cysts in undercooked meat, and the parasite stays latent in brain and muscle. In AIDS with CD4 <100, reactivation produces multiple ring-enhancing brain lesions, whereas EBV-driven primary CNS lymphoma is usually solitary. Congenital infection causes the triad of chorioretinitis, hydrocephalus and diffuse intracranial calcifications (CMV gives periventricular calcifications instead). Treat with sulfadiazine plus pyrimethamine, adding leucovorin to protect the marrow. TMP-SMX prevents reactivation in seropositive patients with CD4 <100.
Trypanosoma cruzi (Chagas disease). The reduviid "kissing" bug of Latin America bites and defecates, and trypomastigotes in its feces are rubbed into the bite wound or the eye; entry through the conjunctiva gives unilateral periorbital swelling (Romaña sign). Chronic infection destroys myocardium and enteric neurons, producing dilated cardiomyopathy, megaesophagus and megacolon. Treat with nifurtimox or benznidazole.
Trypanosoma brucei (African sleeping sickness). Carried by the tsetse fly. Antigenic variation of the surface glycoprotein coat produces recurrent waves of fever with lymphadenopathy, followed by CNS invasion with somnolence and coma. Suramin treats the blood-borne stage; melarsoprol, which reaches the CNS, treats late disease.
Leishmania. The sandfly (Phlebotomus) transmits it, and amastigotes multiply inside macrophages. Cutaneous leishmaniasis forms a chronic skin ulcer. Visceral leishmaniasis (kala-azar, L. donovani) fills the reticuloendothelial system, causing fever, massive hepatosplenomegaly and pancytopenia. Tissue biopsy shows macrophages packed with amastigotes. Treat with sodium stibogluconate or amphotericin B.
Blood protozoa: malaria and Babesia
- Anopheles biteThe female mosquito injects sporozoites
- Liver stageSporozoites mature into merozoites; P. vivax and P. ovale can also form dormant hypnozoites
- RBC infectionRing-form trophozoites mature into schizonts
- RBC lysisMerozoites burst out together, causing cyclic fever and anemia
Plasmodium species. P. falciparum is the most dangerous: high parasitemia, several ring forms per RBC, banana-shaped gametocytes and an irregular fever. Infected RBCs stick to capillary endothelium, so it causes cerebral malaria, renal failure and pulmonary edema. P. vivax and P. ovale cycle every 48 hours and leave hypnozoites that relapse months later. P. malariae gives a 72-hour (quartan) fever. Sickle cell trait and G6PD deficiency protect against falciparum, and Duffy-negative RBCs resist vivax.
Treatment. Chloroquine, which blocks detoxification of heme into hemozoin, treats sensitive strains. Artemisinins treat resistant or severe disease. Primaquine (or tafenoquine) kills hypnozoites; both cause hemolysis in G6PD deficiency, so test G6PD before adding either for vivax or ovale.
Babesia microti. Transmitted by the Ixodes tick, the same vector as Lyme disease, mainly in the Northeast US. It causes fever and hemolytic anemia. The smear shows intraerythrocytic "Maltese cross" tetrads without the hemozoin pigment of Plasmodium. Disease is severe in asplenic patients. Treat with atovaquone plus azithromycin.
| Organism | Fever | Distinctive feature | Key drug |
|---|---|---|---|
| P. falciparum | Irregular | Most severe; banana gametocytes; cerebral malaria | Artemisinin if severe or resistant |
| P. vivax / ovale | Every 48 h | Liver hypnozoites cause relapse | Add primaquine after G6PD check |
| P. malariae | Every 72 h (quartan) | No hypnozoites | Chloroquine |
| Babesia microti | Irregular | Maltese-cross tetrads, no pigment; Ixodes tick | Atovaquone + azithromycin |
Helminths
Nematodes (roundworms). They differ mainly in how they enter:
- Enterobius (pinworm): fecal-oral eggs; nocturnal perianal itching; the Scotch-tape test finds eggs.
- Ascaris: fecal-oral eggs; larvae migrate through the lungs, and heavy worm loads can obstruct the bowel.
- Strongyloides: soil larvae penetrate skin; autoinfection lets it persist for decades, and corticosteroids can trigger hyperinfection.
- Hookworms (Necator, Ancylostoma): larvae penetrate bare feet; adults feed on blood from the gut wall and cause microcytic iron-deficiency anemia.
- Trichinella: larvae in undercooked pork encyst in skeletal muscle, causing myositis, fever and periorbital edema.
Treat with a -bendazole (albendazole, mebendazole) or pyrantel pamoate; ivermectin is the drug for Strongyloides.
Cestodes (tapeworms). Eating cysticerci in undercooked pork gives an intestinal Taenia solium tapeworm. Ingesting the EGGS lets larvae encyst in tissue, causing cysticercosis and, in the brain, neurocysticercosis with seizures and multiple cysts. Diphyllobothrium latum from raw fish takes up B12 in the gut and causes megaloblastic anemia. Echinococcus granulosus eggs come from dog feces (sheep are intermediate hosts) and form hydatid cysts in the liver; do not rupture a cyst, because spilled antigen can cause anaphylaxis. Praziquantel treats tapeworms; albendazole treats cysticercosis and Echinococcus.
Trematodes (flukes). Schistosoma uses a freshwater snail as intermediate host, and its cercariae penetrate intact skin. Eggs trapped in tissue provoke granulomas and fibrosis. S. haematobium involves the bladder (hematuria, fibrosis, squamous cell carcinoma of the bladder). S. mansoni and S. japonicum cause periportal "pipestem" fibrosis and portal hypertension. Clonorchis, the liver fluke of undercooked fish, causes biliary inflammation and cholangiocarcinoma. Praziquantel treats flukes.
Vector and drug anchors
- Vectors: female Anopheles (malaria), Ixodes tick (Babesia), reduviid bug (Chagas), tsetse fly (sleeping sickness), sandfly (Leishmania), freshwater snail (Schistosoma intermediate host).
- Metronidazole: Giardia and Entamoeba (add paromomycin for Entamoeba).
- Praziquantel: flukes and tapeworms. -Bendazoles: roundworms, plus cysticercosis and hydatid disease.
- Ivermectin: Strongyloides. Primaquine: hypnozoites, after a G6PD check.