Helminth & Parasitic Infections
A board-focused tour of the helminths (nematodes, cestodes, trematodes) and the key parasitic protozoa, mapping each classic exposure and buzzword to its diagnosis, confirmatory test, and first-line therapy. The lesson anchors on the high-yield treatment clusters (albendazole/ivermectin vs. praziquantel vs. metronidazole) and on next-best-step decisions like screening for Strongyloides before immunosuppression.
How Boards Test Parasites
Helminths (multicellular worms) split into three Step 1 groups: nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Tissue-invasive or migrating worms drive peripheral eosinophilia and high IgE (a Th2 response); purely luminal adults (e.g., Enterobius, adult Taenia) usually do not. Learn the treatment clusters — they are frequently the answer: most nematodes → albendazole/mebendazole or ivermectin, whereas cestodes and trematodes → praziquantel (with albendazole substituted for tissue cysts: neurocysticercosis and hydatid disease). The single-celled protozoa are grouped as "parasites" too and often hinge on metronidazole (Giardia, Entamoeba, Trichomonas). A classic vignette gives you an exposure clue — travel, undercooked meat/fish, freshwater, an insect vector, or barefoot soil contact — plus a signature finding, then asks for the diagnosis, the confirmatory test, or the next best step.
- Perianal nocturnal itch + Scotch-tape test → Enterobius vermicularis (pinworm)
- Microcytic iron-deficiency anemia + barefoot soil exposure → hookworm (Necator/Ancylostoma)
- Löffler pneumonitis, eosinophilia, biliary/bowel obstruction → Ascaris lumbricoides
- Larva currens; hyperinfection after corticosteroids → Strongyloides stercoralis
- Periorbital edema + myalgia after undercooked pork/bear → Trichinella spiralis
- Megaloblastic (B12) anemia after raw freshwater fish → Diphyllobothrium latum
- New-onset seizures + cystic brain lesions in an immigrant → neurocysticercosis (swallowed Taenia solium eggs)
- Terminal-spine egg + painless hematuria → Schistosoma haematobium (→ bladder squamous cell carcinoma)
- River blindness + subcutaneous nodules, blackfly bite → Onchocerca volvulus (Tx ivermectin)
- Hydatid liver cyst, "eggshell" calcification, anaphylaxis if ruptured → Echinococcus granulosus
Nematodes (Roundworms)
| Organism | Exposure / classic clue | Treatment |
|---|---|---|
| Ascaris lumbricoides | Fecal–oral eggs; Löffler pneumonitis, GI/biliary obstruction | Albendazole |
| Enterobius vermicularis | Perianal nocturnal itch; Scotch-tape test | Albendazole or pyrantel pamoate |
| Hookworm (Necator/Ancylostoma) | Barefoot soil; iron-deficiency anemia | Albendazole |
| Strongyloides stercoralis | Autoinfection; hyperinfection with steroids/HTLV-1 | Ivermectin |
| Trichinella spiralis | Undercooked pork/bear; periorbital edema, myalgia | Albendazole (± steroids) |
| Onchocerca volvulus | Blackfly (Simulium); river blindness, nodules | Ivermectin |
| Loa loa | Deer/mango fly (Chrysops); Calabar swellings, eye-crossing worm | Diethylcarbamazine |
| Wuchereria bancrofti | Mosquito; lymphatic filariasis → elephantiasis | Diethylcarbamazine |
Vignette. A 58-year-old man who emigrated from Southeast Asia decades ago is started on high-dose corticosteroids for a new autoimmune disease. Days later he develops abdominal pain, periumbilical purpura ("thumbprint" petechiae from disseminating larvae), cough, and then Gram-negative bacteremia and meningitis. Eosinophilia may be absent during hyperinfection.
- Diagnosis: Strongyloides stercoralis hyperinfection / disseminated strongyloidiasis — autoinfective larvae multiply, penetrate the gut wall, and carry enteric bacteria into blood and CSF.
- Best diagnostic test: serial stool exams for rhabditiform larvae (larvae, not eggs — eggs hatch in the gut); serology (ELISA) is more sensitive; sputum larvae if pulmonary.
- Treatment: ivermectin.
- Next best step / prevention pearl: screen (serology) and treat Strongyloides BEFORE immunosuppression in patients from endemic areas. In chronic infection the classic skin clue is larva currens — a rapidly migrating serpiginous urticarial track over the buttocks/trunk; new periumbilical purpura instead signals dissemination.
Cestodes & Trematodes
| Organism | Exposure / classic clue | Treatment |
|---|---|---|
| Taenia solium | Undercooked pork = taeniasis; swallowing eggs = neurocysticercosis (seizures) | Praziquantel (taeniasis); albendazole + steroids (NCC) |
| Taenia saginata | Undercooked beef; usually asymptomatic | Praziquantel |
| Diphyllobothrium latum | Raw freshwater fish; B12 deficiency | Praziquantel |
| Echinococcus granulosus | Dogs/sheep; hydatid liver cyst, anaphylaxis if ruptured | Albendazole ± surgery/PAIR (avoid spillage) |
| Schistosoma haematobium | Freshwater snail (swimming); hematuria, bladder SCC | Praziquantel |
| S. mansoni / japonicum | Freshwater; periportal "pipestem" fibrosis, portal HTN | Praziquantel |
| Clonorchis sinensis | Undercooked freshwater fish; cholangiocarcinoma | Praziquantel |
| Paragonimus westermani | Undercooked crab/crayfish; hemoptysis (mimics TB) | Praziquantel |
- Praziquantel = Platyhelminths (the flatworms): trematodes (flukes) + cestodes (tapeworms). Exceptions swapped to albendazole: neurocysticercosis and *Echinococcus* tissue cysts.
- "-bendazoles Bind β-tubulin" → most intestinal nematodes (Ascaris, hookworm, pinworm, whipworm, Trichinella).
- Ivermectin for the African eye/skin worm — river blindness (Onchocerca). Use diethylcarbamazine (DEC) for *Loa loa, but with high Loa* microfilaremia either DEC or ivermectin can trigger fatal encephalopathy — check the microfilarial load first.
- *Diphyllobothrium Depletes B12* → megaloblastic anemia (the fish tapeworm from raw freshwater fish).
- *Taenia Solium → Skull — the tapeworm whose swallowed eggs* lodge in the brain (neurocysticercosis).
Vignette. A 34-year-old man returns from Mexico with 3 weeks of fever, right-upper-quadrant pain, and tender hepatomegaly; weeks earlier he had bloody, mucoid diarrhea. Ultrasound shows a single right-lobe liver abscess; aspirate (if performed) is "anchovy-paste" brown, odorless, and sterile at the wall.
- Diagnosis: Entamoeba histolytica amebic liver abscess — trophozoites with ingested RBCs; flask-shaped colonic ulcers.
- Best diagnostic test: serology (anti-amebic antibodies) plus stool antigen/PCR; imaging for the abscess.
- Treatment / next best step: metronidazole (or tinidazole) FIRST, then a luminal agent (paromomycin) to clear intestinal carriage. Drainage is usually NOT needed — reserve it for very large or left-lobe abscesses, impending rupture, or failure to respond.
- Giardia lamblia: campers/hikers, well or stream water, daycare; fatty, foul, non-bloody diarrhea + bloating; stool antigen / pear-shaped trophozoites; Tx metronidazole/tinidazole.
- Cryptosporidium: watery diarrhea in AIDS (CD4 <100); acid-fast oocysts; Tx supportive ± nitazoxanide, restore immunity with ART.
- Trichomonas vaginalis: strawberry cervix, frothy discharge, motile trophozoites; treat patient AND partner — metronidazole.
- Trypanosoma cruzi (Chagas): reduviid "kissing" bug, Latin America; Romaña sign, dilated cardiomyopathy, megacolon/megaesophagus; Tx benznidazole/nifurtimox.
- Toxoplasma gondii: cat feces / undercooked meat; ring-enhancing brain lesions in AIDS, congenital triad (chorioretinitis, hydrocephalus, intracranial calcifications); Tx pyrimethamine + sulfadiazine + leucovorin.
- Plasmodium (malaria): female *Anopheles* mosquito; cyclic fevers + hemolytic anemia; Giemsa thick/thin smears (ring forms; banana-shaped gametocytes = *P. falciparum*, the deadliest → cerebral malaria). *P. vivax/ovale* form dormant hypnozoites → add primaquine (check G6PD first); severe falciparum → IV artesunate.
- Babesia microti: Ixodes tick (NE US), asplenia; hemolysis + "Maltese cross" intraerythrocytic tetrads; Tx atovaquone + azithromycin.
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