How parasites are tested on Step 1

Vignettes name an exposure (travel, raw fish, undercooked pork, freshwater swimming, walking barefoot, a cat) and a finding (anemia, eosinophilia, seizures, hematuria) and ask for the parasite or the drug.

workspace_premium Why eosinophils?

Worms are too large to phagocytose. Th2 responses drive IgE production and eosinophil recruitment; eosinophils bind IgE-coated worms and release major basic protein.

Protozoa

Group protozoa by organ: gut, blood, central nervous system, and genitourinary tract.

ProtozoaIntestinalGiardia,Entamoeba,CryptosporidiumBloodPlasmodium,BabesiaCNSToxoplasma,Naegleria,TrypanosomabruceiVisceral,skinLeishmania, T.cruziGenitourinaryTrichomonasvaginalis
Protozoa by organ. Cryptosporidium causes severe watery diarrhea in AIDS; acid-fast oocysts are the clue.
OrganismTransmissionDiseaseTreatment
Giardia lambliaCysts in stream waterFatty, foul diarrhea, bloating; IgA deficiency riskMetronidazole or tinidazole
Entamoeba histolyticaCysts in food or waterBloody diarrhea, flask-shaped ulcers, liver abscessMetronidazole, then iodoquinol or paromomycin
Toxoplasma gondiiCat feces, undercooked meat, transplacentalRing-enhancing brain lesions (AIDS); congenital chorioretinitisSulfadiazine + pyrimethamine
Naegleria fowleriWarm freshwater through the cribriform plateRapidly fatal meningoencephalitisAmphotericin B (rarely effective)
Trypanosoma cruziReduviid ("kissing") bugChagas: dilated cardiomyopathy, megacolon, megaesophagusBenznidazole or nifurtimox
Leishmania donovaniSandflyVisceral leishmaniasis (kala-azar): fever, hepatosplenomegaly, pancytopeniaLiposomal amphotericin B
BabesiaIxodes tickHemolytic anemia; "Maltese cross" in RBCs; worse after splenectomyAtovaquone + azithromycin

Ixodes ticks can transmit Babesia, Borrelia (Lyme), and Anaplasma simultaneously.

Malaria

Plasmodium species cycle between the Anopheles mosquito and human liver and red cells.

MosquitobiteSporozoitesenter bloodLiverstageHypnozoitesin vivax,ovaleMerozoitesInvade redcellsRingformsTrophozoites,schizontsRBCruptureCyclic fever,anemia
Plasmodium life cycle. Sickle cell trait, G6PD deficiency, and Duffy-negative red cells (resistant to P. vivax) all protect against malaria.
SpeciesFever patternKey featureTreatment
P. falciparumIrregularBanana-shaped gametocytes; multiple rings per cell; cerebral malaria, renal failureArtemisinin combination; IV artesunate if severe
P. vivax, P. ovale48-hour (tertian)Schüffner dots; dormant hypnozoites cause relapseChloroquine (if sensitive) + primaquine (check G6PD)
P. malariae72-hour (quartan)Band forms; nephrotic syndromeChloroquine

Primaquine causes hemolysis in G6PD deficiency, so test before giving it.

Helminths

Worms are classified by shape. Exposure route predicts the species.

Nematodes (roundworms)Enterobius — perianal itch; tape testAscaris — intestinal obstruction,Löffler pneumonitisStrongyloides — skin penetration,hyperinfection with steroidsAncylostoma, Necator — iron deficiencyanemiaTrichinella — undercooked pork,myositis, periorbital edemaTreat: albendazole, mebendazole;ivermectin for StrongyloidesCestodes and trematodesTaenia solium — cysticercosis (pork);seizuresDiphyllobothrium — raw fish; B12deficiencyEchinococcus — dog feces; hydatid livercystSchistosoma — freshwater snails; S.haematobium → bladder SCCClonorchis — raw fish;cholangiocarcinomaTreat: praziquantel; albendazole forcysticercosis and hydatidvs
Worm classes. Never spill a hydatid cyst during surgery — the contents can cause anaphylaxis.
functions Worked example — steroids and worms

A 60-year-old man who emigrated from rural Brazil is started on high-dose prednisone for COPD. Two weeks later he has gram-negative sepsis, diffuse pulmonary infiltrates, and larvae in his sputum. What happened?

  1. Larvae in sputum point to a nematode with a pulmonary migration phase.
  2. Strongyloides stercoralis can autoinfect, persisting for decades after exposure.
  3. Glucocorticoids suppress the immune control of autoinfection, causing hyperinfection.
  4. Migrating larvae carry gut bacteria into the bloodstream, explaining gram-negative sepsis.

Answer. Strongyloides hyperinfection syndrome, triggered by corticosteroids. Treat with ivermectin; screen at-risk patients before immunosuppression.

Antiparasitic drugs

A handful of drugs cover most parasitic infections tested on Step 1.

DrugMechanismMain uses
Metronidazole, tinidazoleFree radicals damage DNAGiardia, Entamoeba, Trichomonas
Albendazole, mebendazoleInhibit microtubule (β-tubulin) assemblyMost nematodes; neurocysticercosis; echinococcosis
IvermectinOpens glutamate-gated Cl⁻ channels — paralysisStrongyloides, Onchocerca, scabies
Praziquantel↑ calcium permeability — worm contractionMost cestodes and trematodes (Schistosoma, Clonorchis, Taenia)
Pyrantel pamoateDepolarizing neuromuscular blocker in wormsEnterobius, Ascaris, hookworms
ChloroquineBlocks heme polymerization in the parasiteSensitive Plasmodium species
ArtemisininsFree radical damage to the parasiteP. falciparum
PrimaquineKills liver hypnozoitesP. vivax and P. ovale relapse prevention

Mebendazole and albendazole are poorly absorbed, which suits intestinal worms; albendazole is taken with fatty food for tissue infections.