STEPTRACKERMICROBIOLOGY + GASTROENTEROLOGY | INTESTINAL PROTOZOA | HIGH-YIELD REVIEW

Intestinal Protozoa

Single-celled gut parasites spread by cysts in water and food — sort them by stool type, the host they target and the form seen on stool microscopy.

FOCUS ON
Giardia lamblia: campers, daycare, well water, IgA deficiency → greasy, foul-smelling, non-bloody diarrhea, bloating, malabsorption; pear-shaped trophozoites with two nuclei ("face"), cysts; stool antigen → metronidazole/tinidazole. Entamoeba histolytica: invades colon → bloody dysentery, flask-shaped ulcers, liver abscess ("anchovy paste"); trophozoites with ingested RBCs → metronidazole then paromomycin/iodoquinol (luminal). Cryptosporidium: acid-fast oocysts, chlorine-resistant, severe watery diarrhea in AIDS → nitazoxanide. Cyclospora: imported produce (raspberries), → TMP-SMX.
PARASITE
SOURCE · MECHANISM
CLINICAL PICTURE
DIAGNOSIS · TREATMENT
Giardia lamblia (duodenalis)
Cysts in streams, wells, daycare, MSM; trophozoites attach by ventral disc to duodenum (no invasion)
Bloating, flatulence, greasy foul stools, weight loss, lactose intolerance; no blood, no fever
Stool antigen/PCR; trophozoites with 2 nuclei and flagella · tinidazole or metronidazole
Giardia risk hosts
Low secretory IgA
Recurrent giardiasis in selective IgA deficiency, CVID, X-linked agammaglobulinemia
Check immunoglobulins
Amebic dysentery
E histolytica cysts in water/food (travel, institutions); trophozoites lyse colonic cells
Bloody diarrhea, cramps, tenesmus; flask-shaped ulcers in cecum/colon
Stool antigen/PCR; trophozoites with ingested RBCs
Amebic liver abscess
Spread via portal vein
Fever, RUQ pain, single right-lobe abscess; may occur without diarrhea; "anchovy paste" pus
Serology · metronidazole → luminal agent (paromomycin, iodoquinol)
Cryptosporidium parvum / hominis
Oocysts resist chlorine (swimming pools, city water outbreaks); attach to brush border
Healthy: self-limited watery diarrhea · AIDS: chronic severe watery diarrhea, biliary disease
Modified acid-fast oocysts · nitazoxanide; ART in HIV · filters, not chlorine
Cyclospora cayetanensis
Imported fresh produce (berries, basil, lettuce)
Prolonged relapsing watery diarrhea, fatigue
Acid-fast, UV autofluorescent oocysts · TMP-SMX
Cystoisospora belli
Fecal-oral, tropics
Watery diarrhea in AIDS
Acid-fast oval oocysts · TMP-SMX
Microsporidia
Spore-forming (now fungi)
Chronic diarrhea in AIDS
Albendazole
Balantidium coli
Pigs; only ciliated human pathogen
Dysentery
Tetracycline
Luminal vs tissue drugs
Metronidazole is absorbed; cysts persist in lumen
Without luminal agent → relapse, carrier
Asymptomatic cyst passers: luminal agent alone

Mechanism — invasive vs non-invasive

GiardiaAttaches, coats mucosa
→
MalabsorptionFatty stools, no blood
E histolyticaLyses epithelium
→
Flask ulcers → blood
→
Portal vein → liver abscess

Protozoal diarrhea compared

PARASITE
STOOL
STOOL FORM
TREAT
Giardia
Greasy, no blood
2-nucleus trophozoite
Tinidazole, metronidazole
E histolytica
Bloody
Trophozoite with RBCs
Metronidazole + luminal
Cryptosporidium
Watery
Acid-fast oocyst (small)
Nitazoxanide
Cyclospora
Watery, relapsing
Acid-fast oocyst (larger)
TMP-SMX

How it's tested

COMMON CONFUSIONAmebic liver abscess (single, travel, no bacteria, metronidazole) vs pyogenic abscess (biliary disease, polymicrobial, drainage + antibiotics).
THE TRAPChlorine doesn't kill Cryptosporidium oocysts — pool outbreaks persist despite chlorination.
YOUR APPROACHDiarrhea + exposure → bloody (Entamoeba) vs fatty (Giardia) vs watery in AIDS (Cryptosporidium) → stool test → drug (± luminal agent).

If you see… think…

Hiker drinking stream water, foul greasy stools, bloating → Giardia
Recurrent giardiasis + sinopulmonary infections → IgA deficiency or CVID
Pear-shaped trophozoite with two nuclei → Giardia
Bloody diarrhea after travel, flask-shaped colonic ulcers → Entamoeba histolytica
Trophozoites containing red cells → E histolytica
Single right-lobe liver abscess with "anchovy paste" pus → amebic abscess
AIDS patient with chronic watery diarrhea, acid-fast oocysts → Cryptosporidium
Pool outbreak of watery diarrhea despite chlorination → Cryptosporidium
Diarrhea after imported raspberries → Cyclospora (TMP-SMX)
Drug added after metronidazole to clear amebic cysts → paromomycin or iodoquinol