16.4 Parasites: Protozoa & Helminths (Trematodes, Nematodes, Cestodes)

Parasites: Protozoa & Helminths

Quick Answer: Parasites divide into protozoa (single-celled eukaryotes) and helminths (multicellular worms). Helminths further divide into trematodes (flukes), nematodes (roundworms), and cestodes (tapeworms). PA-CAT items test the genus-vector-route-tropism-signature finding quartet.

Protozoa

Plasmodium species (P. falciparum, vivax, ovale, malariae, knowlesi) cause malaria, transmitted by the bite of female Anopheles mosquitoes. Sporozoites invade hepatocytes (exoerythrocytic schizogony), then merozoites infect red blood cells, causing cyclic fever paroxysms. P. falciparum causes the most severe disease (cerebral malaria, blackwater fever from hemolysis) and has the highest mortality. P. vivax and P. ovale can form dormant hypnozoites in the liver, requiring primaquine (or tafenoquine) to prevent relapse. Diagnosis is by thick and thin blood smears (thin smear for species identification). Chloroquine resistance is widespread in P. falciparum, for which artemisinin-based combination therapy (ACT) is first-line.

Giardia lamblia is a flagellated protozoan acquired by ingesting cysts in contaminated water (campers, day-care outbreaks). Trophozoites attach to the duodenal mucosa causing foul-smelling, greasy, non-bloody diarrhea and malabsorption. Diagnosis is by stool antigen or microscopic cyst identification; treatment is metronidazole or tinidazole. Entamoeba histolytica causes amebic dysentery (bloody, mucus-containing stools) and flask-shaped liver abscesses (anchovy-pus aspirate); diagnosis is by stool O&P and serology; treatment is metronidazole plus a luminal agent (paromomycin) to clear cysts.

Toxoplasma gondii is acquired from cat feces and undercooked meat; it causes mild illness in immunocompetent hosts but severe disease in AIDS (ring-enhancing brain abscesses, chorioretinitis) and congenital infection with the classic triad of chorioretinitis, hydrocephalus, and intracranial calcifications. Diagnosis uses serology (IgM/IgG) and PCR; treatment is pyrimethamine plus sulfadiazine plus folinic acid.

Leishmania species are transmitted by sandflies and cause cutaneous, mucocutaneous, or visceral (kala-azar) leishmaniasis; intracellular amastigotes are seen in macrophages. Trypanosoma includes T. brucei (African sleeping sickness, tsetse fly) and T. cruzi (Chagas disease, reduviid bug), the latter causing megaesophagus, megacolon, and cardiomyopathy.

Helminths

Trematodes (flukes) are flat, leaf-shaped worms. Schistosoma species (S. mansoni, haematobium, japonicum) have a freshwater snail intermediate host; cercariae penetrate human skin during swimming or bathing, migrate to mesenteric (S. mansoni/japonicum) or vesicular (S. haematobium) venules, and release eggs that cause granulomatous inflammation — S. haematobium causes hematuria and is associated with bladder squamous cell carcinoma. Diagnosis is by eggs in stool or urine; treatment is praziquantel.

Nematodes (roundworms) include:

• Ascaris lumbricoides — the largest intestinal roundworm; acquired by ingesting eggs, can cause intestinal obstruction or biliary obstruction.
• Hookworm (Necator, Ancylostoma) — larvae penetrate skin (barefoot exposure); causes iron-deficiency anemia from chronic intestinal blood loss.
• Enterobius vermicularis (pinworm) — causes perianal pruritus in children; diagnosed by the Scotch-tape test showing eggs; treated with mebendazole or albendazole.
• Trichinella spiralis — from undercooked pork; causes myositis and periorbital edema.

Cestodes (tapeworms) have a scolex with hooks/suckers and proglottids. Taenia solium (pork tapeworm) is acquired from undercooked pork; eggs ingested by humans cause cysticercosis with cysts in brain, eye, and muscle (neurocysticercosis is a leading cause of adult-onset seizures in endemic regions). Echinococcus granulosus (dog tapeworm) causes hydatid cysts in liver and lung; care must be taken to avoid spillage during surgery (anaphylaxis risk). Treatment is praziquantel or albendazole depending on the cestode.

Quick Matching Table

Antiparasitic Therapy Highlights

PA-CAT therapy pairings worth memorizing:

• Metronidazole or tinidazole for Giardia, Entamoeba, Trichomonas, and bacterial vaginosis-associated organisms.
• Chloroquine for non-falciparum malaria in sensitive regions; artemisinin-based combination therapy (ACT) for falciparum; primaquine for P. vivax/ovale hypnozoites (screen G6PD first to avoid hemolysis).
• Praziquantel for schistosomiasis and most cestodes and trematodes; albendazole/mebendazole for intestinal nematodes and as an adjunct for neurocysticercosis; ivermectin for onchocerciasis, strongyloidiasis, and scabies.
• Pyrimethamine + sulfadiazine + folinic acid for Toxoplasma; trimethoprim-sulfamethoxazole for prophylaxis when CD4 < 200.

Why Protozoa vs. Helminth Matters

The protozoan/helminth split has direct treatment implications: protozoa are single-celled and typically cleared by antimicrobials (metronidazole, antimalarials), whereas helminths are multicellular and usually expelled by anthelmintics that paralyze or damage the worm (albendazole, praziquantel, ivermectin). The IgE/eosinophil axis is prominent in helminth infections (especially tissue-stage larvae) but not in most protozoal infections — a useful lab clue. Eosinophilia suggests tissue-invading helminths (e.g., Ascaris migration, Strongyloedes, schistosomiasis, filariasis) rather than protozoa.