STEPTRACKERMICROBIOLOGY + PHARMACOLOGY | ANTIPARASITICS & PROPHYLAXIS | HIGH-YIELD REVIEW

Antiparasitic Drugs & Prophylaxis

A short list of antiparasitic drugs covers most worms and protozoa — and a prophylaxis table pulls together every "give this before it happens" rule in microbiology.

FOCUS ON
Antihelminthics: albendazole/mebendazole (block microtubule polymerization; most nematodes, neurocysticercosis, hydatid), praziquantel (↑ Ca²⁺ permeability → paralysis; flukes and tapeworms), ivermectin (glutamate-gated Cl⁻ channels; Strongyloides, Onchocerca, scabies, lice), pyrantel (depolarizing NM block; pinworm), diethylcarbamazine (Loa loa, Wuchereria). Antiprotozoals: metronidazole (Giardia, Entamoeba, Trichomonas), nitazoxanide (Cryptosporidium), pyrimethamine-sulfadiazine (Toxoplasma), antimalarials (artemisinins, chloroquine, primaquine, atovaquone-proguanil), benznidazole/nifurtimox (Chagas), suramin/melarsoprol (sleeping sickness), sodium stibogluconate/miltefosine/amphotericin (Leishmania). Prophylaxis: by exposure and by host (see table).
SITUATION
ORGANISM
PROPHYLAXIS
NOTE
Close contact of meningococcal disease
N meningitidis
Rifampin, ciprofloxacin or ceftriaxone
Ceftriaxone in pregnancy
Household contact of Hib (unvaccinated child present)
H influenzae type b
Rifampin
—
Pertussis exposure
B pertussis
Azithromycin
Household and high-risk contacts
GBS-colonized mother in labor
S agalactiae
Intrapartum penicillin or ampicillin
Cefazolin if mild allergy
Newborn eyes
N gonorrhoeae
Erythromycin ointment
Doesn't prevent chlamydia reliably
Rheumatic fever history
Group A strep
Benzathine penicillin G monthly
Years to decades
High-risk cardiac lesion + dental work
Viridans strep
Amoxicillin before procedure
Prosthetic valve, prior IE, some congenital disease, cardiac transplant valvulopathy
Recurrent cystitis
E coli
Nitrofurantoin or TMP-SMX (post-coital or daily)
—
HIV, CD4 < 200
Pneumocystis
TMP-SMX (dapsone, atovaquone if allergic)
Stop when CD4 > 200 for 3 months
HIV, CD4 < 100, Toxo IgG ⁺
Toxoplasma
TMP-SMX
—
HIV, CD4 < 50, not starting ART
MAC
Azithromycin weekly
Not needed if ART started
Latent TB
M tuberculosis
Rifampin 4 months or INH-based
See 11.05
Travel to malaria area
Plasmodium
Atovaquone-proguanil, doxycycline, mefloquine; chloroquine where sensitive
Primaquine for terminal prophylaxis (vivax areas)
Exposures needing passive + active
Rabies, HBV, tetanus, VZV
RIG + vaccine · HBIG + vaccine · TIG + toxoid · VZIG
See 10.08

Mechanism — how antihelminthics kill

Albendazole/mebendazoleBind β-tubulin → no glucose uptake
PraziquantelCa²⁺ influx → spastic paralysis
IvermectinCl⁻ channels → flaccid paralysis
PyrantelNicotinic agonist → spastic paralysis

Albendazole is teratogenic; ivermectin can cause encephalopathy in heavy Loa loa infection.

Worm → drug

GROUP
DRUG
EXCEPTIONS
Intestinal nematodes
Albendazole / mebendazole
Pyrantel for pinworm
Tissue nematodes
Ivermectin (Strongyloides, Onchocerca)
DEC (Loa, Wuchereria)
Cestodes (gut)
Praziquantel
—
Cysticercosis, hydatid
Albendazole
± surgery
Trematodes
Praziquantel
Triclabendazole (Fasciola)

How it's tested

COMMON CONFUSIONPraziquantel for adult tapeworms and flukes vs albendazole for tissue cysts (neurocysticercosis, hydatid) — same organism, different stage, different drug.
THE TRAPMeningococcal prophylaxis goes to close contacts (household, kissing, dorm), not to every healthcare worker — only those exposed to secretions without a mask.
YOUR APPROACHOrganism or exposure → is this treatment or prevention? → drug and duration → host cautions (pregnancy, G6PD, Loa coinfection).

If you see… think…

Drug for most intestinal roundworms → albendazole or mebendazole
Drug for schistosomiasis and tapeworms → praziquantel
Drug for Strongyloides, river blindness and scabies → ivermectin
Drug for Cryptosporidium → nitazoxanide
Roommate of a meningococcal patient → rifampin, ciprofloxacin or ceftriaxone
Pregnant meningococcal contact → ceftriaxone
Prosthetic valve, dental extraction tomorrow → amoxicillin before
CD4 150 → TMP-SMX for Pneumocystis
Rheumatic heart disease in a teenager → monthly benzathine penicillin
Traveler to India returning with vivax risk → primaquine (check G6PD)