LO 1

ON EXAM — Entamoeba histolytica, Giardia, Cryptosporidium, Cyclospora/Cystoisospora

Entamoeba histolytica

Infective form

Cyst (mature, 4 nuclei)

Pathogenic form

Trophozoite (with ingested RBCs)

Transmission

Fecal-oral (contaminated food/water)

Ulcer shape

Flask-shaped ulcers in colon

Extraintestinal

Liver abscess (RUQ pain, fever)

Abscess contents

"Anchovy paste" (brown-red necrotic debris)

Stool microscopy

Trophozoites with ingested RBCs (distinguishes from E. dispar)

Serology use

Helpful in extraintestinal disease (liver abscess)

Treatment — invasive

Metronidazole (or tinidazole)

Treatment — luminal

Paromomycin (or iodoquinol) to eliminate cysts

Giardia lamblia

Morphology

Pear-shaped trophozoite with "owl-eye" appearance (2 nuclei); flagellated

Transmission

Fecal-oral; cyst ingestion from contaminated water

Classic exposure

Hikers/campers (untreated stream water); daycare; MSM

Host defect

IgA deficiency (and cystic fibrosis)

Clinical

Foul-smelling, greasy, floating stools (steatorrhea); bloating, flatulence, weight loss

Best test

Stool antigen (EIA) — more sensitive than microscopy

Treatment

Metronidazole or tinidazole (or nitazoxanide)

Cryptosporidium

At risk

HIV/AIDS (CD4 <100); also waterborne outbreaks in immunocompetent

Immunocompetent

Self-limited watery diarrhea (1–2 weeks)

Immunocompromised

Severe chronic profuse watery diarrhea, dehydration, wasting

Stain

Modified acid-fast oocysts (4–6 μm)

Treatment

Nitazoxanide (immunocompetent); restore CD4 with ART in HIV (no fully effective antimicrobial)

Modified acid-fast oocyst trio

Cryptosporidium parvum/hominis (4–6 μm)

Cyclospora cayetanensis (~10 μm; raspberries, basil)

Cystoisospora (Isospora) belli (oval, large; TMP-SMX)

🪤 TRAP: Chronic foul fatty diarrhea after hiking/well water → Giardia. Crypto in HIV → restore CD4.

LO 2

ON EXAM — Leishmania, Chagas (T. cruzi), Malaria (falciparum vs vivax/ovale/malariae), Toxoplasma, Babesia

Leishmania

Vector

Sandfly (Phlebotomus / Lutzomyia)

Form in vector

Promastigote (flagellated, extracellular)

Form in macrophages

Amastigote (no flagellum; intracellular)

Visceral disease

Kala-azar (L. donovani)

Visceral clinical

Fever, massive hepatosplenomegaly, pancytopenia, hyperpigmentation

Cutaneous clinical

Chronic ulcer with raised borders ("volcano crater") at bite site

Treatment

Liposomal amphotericin B (visceral); sodium stibogluconate; miltefosine

Chagas (T. cruzi)

Vector

Reduviid ("kissing") bug

Transmission to human

Bug feces rubbed into bite/mucosa (conjunctiva)

Geography

Latin America (Central + South)

Acute: Romaña sign

Unilateral periorbital swelling at bite/inoculation site

Chronic cardiac

Dilated cardiomyopathy w/ apical aneurysm; conduction disease (RBBB + LAFB)

Chronic GI

Megaesophagus + megacolon (destruction of myenteric plexus)

Diagnosis

Blood smear (acute); serology (chronic)

Treatment

Benznidazole or nifurtimox

Plasmodium species

Species	Periodicity	Key feature
P. falciparum	Irregular (malignant tertian)	Severe; cerebral malaria, blackwater fever; banana gametocyte; parasitizes RBCs of all ages
P. vivax	Tertian (48 h)	Hypnozoites (relapse); Duffy antigen required (rare in West Africa)
P. ovale	Tertian (48 h)	Hypnozoites; mostly West Africa
P. malariae	Quartan (72 h)	Chronic low-grade; can cause nephrotic syndrome
P. knowlesi	Quotidian (24 h)	SE Asia (monkey reservoir); can be severe

Malaria treatment

Uncomplicated

Artemisinin-based combo therapy (ACT); chloroquine only if known sensitivity

Severe falciparum

IV artesunate

Hypnozoite eradication

Primaquine (or tafenoquine) — for P. vivax / P. ovale

Check before primaquine

G6PD status (hemolysis risk)

Sickle cell trait

HbAS provides partial protection (impaired parasite growth in RBCs); also Duffy-negative resists P. vivax

Toxoplasma gondii

Definitive host

Cats

Transmission

Cat oocysts (litter), undercooked meat (cysts), transplacental

Immunocompetent

Usually asymptomatic or mononucleosis-like (lymphadenopathy)

AIDS (CD4 <100)

Multiple ring-enhancing brain lesions (reactivation)

Congenital triad

Chorioretinitis + hydrocephalus + intracranial calcifications (classic triad)

Treatment

Pyrimethamine + sulfadiazine + leucovorin

Babesia

Vector

Ixodes scapularis tick

US geography

Northeast (Nantucket, Martha's Vineyard) + Upper Midwest

Blood smear

Intra-erythrocytic rings + "Maltese cross" (tetrad)

At-risk host

Asplenic/splenectomized, elderly, immunocompromised

Treatment

Atovaquone + azithromycin (mild); clindamycin + quinine (severe)

🪤 TRAP: Maltese cross = Babesia. Ring forms + banana gametocyte = P. falciparum.

LO 3

ON EXAM — Ascaris, Enterobius, hookworm, Strongyloides, Trichinella, Toxocara

Nematode master table

Nematode	Exposure	Signature	Treatment
Ascaris lumbricoides	Fecal-oral (egg ingestion)	Largest roundworm; pulmonary migration (Löffler); intestinal obstruction, biliary obstruction	Albendazole / mebendazole
Enterobius vermicularis	Fecal-oral (egg ingestion; self-reinfection)	Nocturnal perianal pruritus; Scotch-tape test	Albendazole or pyrantel pamoate; treat household; repeat in 2 wk
Hookworm (Necator / Ancylostoma)	Larval skin penetration (walking barefoot)	Iron-deficiency anemia from blood sucking; ground itch; Löffler syndrome	Albendazole + iron replacement
Strongyloides stercoralis	Larval skin penetration; autoinfection cycle	Chronic asymptomatic infection → hyperinfection on immunosuppression; larva currens	Ivermectin (first-line); albendazole alternative
Trichinella spiralis	Undercooked pork/bear/wild meat	Periorbital edema, myositis, eosinophilia; muscle cysts	Albendazole + steroids
Toxocara canis / cati	Ingestion of eggs from soil with dog/cat feces	Visceral larva migrans (hepatomegaly, eosinophilia) or ocular larva migrans	Albendazole ± steroids (esp. OLM)

Enterobius specifics

Classic symptom

Nocturnal perianal itching

Test

Scotch-tape test (morning) — flattened oval eggs

Treatment

Albendazole or pyrantel pamoate; treat household contacts; repeat dose in 2 weeks

Hookworm pathophysiology

Why IDA

Adult worm attaches to intestinal mucosa and consumes blood (chronic GI blood loss)

Skin sign

"Ground itch" — pruritic rash at larval entry site (feet)

Pulmonary

Löffler syndrome — transient eosinophilic pneumonitis from larval migration

Strongyloides — unique

Autoinfection

Rhabditiform larvae become filariform in gut → re-penetrate bowel wall or perianal skin → persistent decades-long infection

Hyperinfection trigger

Corticosteroids, HTLV-1, chemotherapy, solid-organ transplant

Hyperinfection picture

Overwhelming larval dissemination → ARDS, gram-negative sepsis (gut translocation), purpura

Treatment

Ivermectin

Trichinella clinical triad

GI (d 1–7)

Nausea, diarrhea, cramps

Muscle phase (wk 2+)

Myalgia, periorbital edema, weakness

Lab

Marked eosinophilia; ↑ CPK; serology

Toxocara

VLM

Hepatomegaly, pulmonary symptoms, eosinophilia, hyperglobulinemia (children)

OLM

Unilateral retinal granuloma / strabismus / leukocoria (mimics retinoblastoma)

Host

Toddlers (geophagia / sandbox)

Treatment

Albendazole ± corticosteroids for OLM/inflammation

🪤 TRAP: Screen for Strongyloides BEFORE steroids/immunosuppression in at-risk patients.

LO 4

ON EXAM — Taenia solium/saginata, Diphyllobothrium (B12), Echinococcus (hydatid), Schistosoma species

Cestode framework

Definitive host

Harbors the adult/sexual form of the parasite

Intermediate host

Harbors the larval/asexual form (cysts)

Cestode master table

Cestode	Exposure	Signature	Treatment
Taenia solium (pork)	Undercooked pork (cysticerci) → intestinal taeniasis; OR ingestion of eggs → cysticercosis (human = intermediate host)	Neurocysticercosis (seizures, brain cysts)	Praziquantel (taeniasis); albendazole + steroids + antiepileptics (NCC)
Taenia saginata (beef)	Undercooked beef (cysticerci)	Intestinal taeniasis; usually asymptomatic; no cysticercosis	Praziquantel
Diphyllobothrium latum (fish)	Raw/undercooked freshwater fish (sushi, sashimi)	Megaloblastic anemia (competes for B12)	Praziquantel
Echinococcus granulosus	Dog feces (eggs); sheep-raising areas	Hydatid cyst in liver (eggshell calcification, daughter cysts); anaphylaxis if ruptured	Albendazole + PAIR (Puncture-Aspirate-Inject-Reaspirate) or surgical resection

Neurocysticercosis

How humans become intermediate host

Ingestion of T. solium EGGS (fecal-oral from a tapeworm carrier), NOT larvae in pork

MRI

Multiple cystic CNS lesions with scolex visible; calcified cysts (chronic)

Symptom

Seizures (most common presentation)

Treatment

Albendazole + corticosteroids + anticonvulsants

Echinococcus (hydatid)

Reservoir

Dogs (definitive host); sheep (intermediate host); humans accidental

Organ

Liver (right lobe most common); also lung

Needle risk

Cyst rupture → anaphylaxis + peritoneal seeding (daughter cysts)

Treatment

Albendazole + PAIR or surgical resection with perioperative albendazole

Diphyllobothrium

Source

Raw/undercooked freshwater fish

Deficiency

Vitamin B12 → megaloblastic anemia

Mechanism

Worm competes with host for dietary B12 in the small intestine

Schistosoma species

Species	Geography	Site	Chronic
S. mansoni	Africa, S. America, Caribbean	Mesenteric veins (inferior) → liver via portal	Periportal ("pipestem") fibrosis, portal HTN, esophageal varices
S. japonicum	East Asia	Mesenteric veins (superior)	Severe hepatic fibrosis, portal HTN
S. haematobium	Africa, Middle East (esp. Egypt)	Bladder venous plexus	Hematuria; squamous cell carcinoma of bladder

Schistosoma general

Intermediate host

Freshwater snails

Infection mode

Cercariae penetrate skin in freshwater

Katayama fever

Acute hypersensitivity: fever, urticaria, eosinophilia, hepatosplenomegaly 2–8 wk after exposure

Treatment

Praziquantel (all species)

🪤 TRAP: S. haematobium → SQUAMOUS cell bladder cancer (not TCC). Terminal spine eggs = haematobium; lateral spine = mansoni; no spine/small = japonicum.

LOs 1–4

Q1: Hikers and foul fatty diarrhea

32 F returns from camping where she drank untreated stream water. 3 weeks of foul-smelling greasy floating stools, bloating, weight loss.

A) Cryptosporidium; nitazoxanide
B) Giardia lamblia; metronidazole (or tinidazole)
C) Entamoeba histolytica; metronidazole + paromomycin
D) Cyclospora; TMP-SMX
E) Rotavirus; supportive

Correct Answer: B

Giardia — foul fatty diarrhea, hikers/daycare, untreated water. Treat with metronidazole or tinidazole.

Q2: HIV + watery diarrhea

38 M with AIDS (CD4 = 42) has 3 weeks of profuse watery diarrhea. Modified acid-fast stain shows 4–6 μm oocysts.

A) Giardia
B) Entamoeba
C) Cryptosporidium parvum
D) Cyclospora
E) Cystoisospora

Correct Answer: C

Cryptosporidium — small acid-fast oocysts; severe chronic in AIDS. Nitazoxanide limited; ART to restore CD4.

Q3: Chagas cardiomyopathy

52 M Bolivian with CHF and dysphagia. ECG: RBBB + LAFB. Echo: apical aneurysm. Megaesophagus.

A) T. brucei; tsetse
B) T. cruzi; reduviid bug
C) L. donovani; sandfly
D) Babesia; Ixodes
E) P. falciparum; Anopheles

Correct Answer: B

Chagas disease (T. cruzi), reduviid ("kissing") bug. Chronic cardiomyopathy with apical aneurysm + RBBB/LAFB + megaesophagus/colon. Benznidazole or nifurtimox.

Q4: Malaria relapse

Treated 3 months ago for P. vivax with chloroquine; returns with recurrent fever + parasitemia. Missed drug?

A) Atovaquone-proguanil
B) Mefloquine
C) Primaquine (after G6PD testing)
D) Doxycycline
E) Artesunate

Correct Answer: C

Vivax/ovale hypnozoites → relapse. Primaquine (or tafenoquine) kills hypnozoites — check G6PD first.

Q5: Strongyloides hyperinfection

60 M Vietnamese starts high-dose prednisone → bloody sputum, abdominal pain, E. coli bacteremia, filariform larvae in sputum.

A) Stool O&P and empiric ivermectin prior to steroids
B) Nothing
C) Blood smear for malaria
D) Colonoscopy
E) CXR only

Correct Answer: A

Strongyloides persists decades via autoinfection; steroids → hyperinfection with GNR sepsis and ARDS. Screen and treat with ivermectin before immunosuppression.

Q6: Neurocysticercosis

28 M from rural Mexico with new seizures. MRI: multiple cystic CNS lesions with scolex.

A) T. saginata; undercooked beef
B) T. solium; ingestion of eggs (fecal-oral)
C) Echinococcus
D) Toxocara
E) Toxoplasma

Correct Answer: B

Neurocysticercosis = ingestion of T. solium EGGS (not larvae). Albendazole + steroids + anticonvulsants.

Q7: B12 deficiency from fish

40 M sushi enthusiast with macrocytic anemia, ↑MMA, hypersegmented neutrophils. Stool O&P: operculated eggs.

A) Diphyllobothrium latum; competes for B12
B) Ascaris
C) Giardia
D) Whipple
E) Schistosoma

Correct Answer: A

Diphyllobothrium latum competes for B12 → megaloblastic anemia. Raw fish. Praziquantel.

Q8: Hydatid cyst

45 M sheep rancher with large cystic liver lesion, eggshell calcification, daughter cysts. Echinococcus serology +.

A) Percutaneous aspiration alone
B) Albendazole + PAIR or surgical resection (avoid spillage)
C) Metronidazole
D) Observation
E) Ivermectin

Correct Answer: B

Echinococcus — albendazole + PAIR or surgery. Spillage risks anaphylaxis and seeding.

Q9: Pinworm

6 y/o with nocturnal perianal itching; tape test shows oval eggs flattened on one side.

A) Topical steroids
B) Metronidazole
C) Albendazole or mebendazole for patient AND contacts; repeat in 2 wk
D) Ivermectin alone
E) Praziquantel

Correct Answer: C

Enterobius: albendazole or pyrantel; treat household; repeat in 2 weeks.

Q10: Schistosoma + bladder cancer

35 M Egyptian with painless hematuria; UA shows terminal-spine eggs; cystoscopy: SCC of bladder.

A) S. mansoni
B) S. japonicum
C) S. haematobium
D) S. mekongi
E) S. intercalatum

Correct Answer: C

S. haematobium → bladder → SCC (terminal-spine eggs). All treated with praziquantel.

Q11: Toxoplasma in AIDS

45 y/o with AIDS (CD4 = 50), headache, hemiparesis; MRI: ring-enhancing lesions; Toxo IgG +.

A) Radiation
B) Pyrimethamine + sulfadiazine + leucovorin
C) Amphotericin
D) Artesunate
E) Ganciclovir

Correct Answer: B

AIDS + ring-enhancing lesions + Toxo IgG + → pyrimethamine + sulfadiazine + leucovorin. TMP-SMX prophylaxis prevents reactivation.

Q12: Babesia

65 y/o splenectomized man from Nantucket with fever, hemolysis; smear: intraerythrocytic rings and Maltese cross.

A) Reduviid bug
B) Ixodes scapularis tick
C) Anopheles mosquito
D) Sandfly
E) Tsetse fly

Correct Answer: B

Babesia microti via Ixodes scapularis. Severe in asplenics. Atovaquone + azithromycin; clinda + quinine if severe.

Parasitology — Answer Key