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Giardia lamblia (G. duodenalis, G. intestinalis)
- Most common intestinal parasitic infection in USA; worldwide
- Cyst-trophozoite life cycle
- Transmission: contaminated water (untreated, well, recreational, streams), fecal-oral, daycare, MSM
- Resistant to chlorine â requires UV or filtration
- Clinical:
- Acute (1-2 wk): watery / foul-smelling / greasy diarrhea, bloating, flatulence, abdominal cramps, weight loss
- Chronic (months): persistent foul-smelling diarrhea, steatorrhea + malabsorption, lactose intolerance, weight loss
- Often asymptomatic
- Diagnosis: stool antigen test (high sens) or PCR; stool O+P Ã 3 (lower sens)
- Treatment:
- Tinidazole 2 g PO Ã 1 (single dose, preferred)
- Metronidazole 250-500 mg PO tid à 5-7d
- Nitazoxanide 500 mg PO bid à 3d (pediatric, pregnancy)
- Paromomycin for pregnancy 1st trimester
Cryptosporidium parvum / hominis
- Acid-fast intracellular protozoan
- Chlorine-resistant â drinking water + recreational water + zoonotic
- Milwaukee 1993 outbreak: 400,000+ cases (water treatment failure)
- Cattle reservoir (zoonotic with C. parvum)
- Clinical:
- Immunocompetent: 1-2 wk acute watery diarrhea, self-limited
- HIV CD4 < 100: chronic severe watery diarrhea + wasting + cholangiopathy + pancreatitis
- Pediatric malnutrition + diarrhea â global LMIC burden
- Diagnosis: stool antigen (sensitive, sensitivity > 95%), modified acid-fast stain, PCR
- Treatment:
- Nitazoxanide 500 mg PO bid à 3d in immunocompetent
- HIV: ART critical + nitazoxanide adjunct (immune reconstitution required for cure)
- Paromomycin limited efficacy
Cyclospora cayetanensis
- Acid-fast intracellular protozoan
- Foodborne outbreaks (especially raspberries, basil, cilantro, leafy greens)
- USA seasonal summer outbreaks
- Clinical:
- Persistent watery diarrhea + fatigue + flatulence + cramping
- Often weeks-months untreated
- Self-limited but prolonged
- Diagnosis: modified acid-fast stain (autofluoresce under UV â distinctive); PCR
- Treatment: TMP-SMX 1 DS PO bid à 7-10d (longer in HIV)
- éå ± (outbreaks)
Microsporidia (Encephalitozoon, Enterocytozoon)
- Spore-forming intracellular fungi (reclassified from protozoa)
- HIV-associated: chronic diarrhea, ocular, biliary, disseminated
- Diagnosis: modified trichrome stain, PCR
- Treatment:
- Enterocytozoon bieneusi: fumagillin (limited availability)
- Encephalitozoon spp.: albendazole
Blastocystis hominis
- Controversial pathogenicity
- Common (10-20% of stool samples)
- Treatment with metronidazole / tinidazole / nitazoxanide for symptomatic with no other cause
- Asymptomatic carriage â donât treat
Dientamoeba fragilis
- Pinworm co-infection common (eggs may transmit)
- Mild diarrhea
- Metronidazole or iodoquinol
1ïžâ£ Giardia
Microbiology
- Giardia lamblia (also called G. duodenalis, G. intestinalis)
- 8 genotypes (A-H); A + B human-infectious mainly
- Two forms:
- Cyst: environmental form (infectious; resistant to chlorine)
- Trophozoite: motile, âkite-shapedâ, 8 flagella, intestinal
Transmission
- Fecal-oral
- Contaminated water (most common; streams, wells, recreational pools, daycares)
- Daycare outbreaks
- MSM (anal-oral)
- Pets (rare zoonotic)
- Beavers + other wild animals (especially in N America wilderness)
Clinical
Asymptomatic Carriage
- Common (especially endemic areas)
Acute Giardiasis
- 1-2 weeks incubation
- Watery diarrhea initially â foul-smelling, greasy, malodorous stool
- Bloating, flatulence
- Abdominal cramps
- Anorexia, weight loss
- Self-limited in immunocompetent (weeks)
Chronic Giardiasis
- Months
- Steatorrhea + fat malabsorption
- Lactose intolerance (post-infectious)
- Weight loss, growth failure (pediatric)
- B12 + fat-soluble vitamin deficiency
- Iron deficiency anemia
Diagnosis
PCR
- Multiplex GI panels (BioFire) â include Giardia
- High sensitivity
Treatment
Tinidazole
- 2 g PO Ã 1 dose (single dose advantage)
- Preferred for adherence + efficacy
Metronidazole
- 250-500 mg PO tid à 5-7 d
- Common, cost-effective
- Side effects: GI, metallic taste, disulfiram reaction, peripheral neuropathy long-course
Nitazoxanide
- 500 mg PO bid à 3 d
- Pediatric OK
- Pregnancy OK
- Preferred in pediatric outbreaks
Paromomycin
- 25-35 mg/kg/d PO divided à 7-10 d
- Pregnancy 1st trimester preferred (metronidazole avoided)
Prevention
- Water filtration / boil / UV (chlorine alone insufficient)
- Wilderness travelers: portable filter or chemical
- Hygiene + handwashing
- Daycare outbreak management
- Sexual safety (MSM)
2ïžâ£ Cryptosporidium
Microbiology
- Cryptosporidium parvum (zoonotic, cattle reservoir)
- C. hominis (anthroponotic)
- Other species (rare)
- Acid-fast staining (modified Ziehl-Neelsen)
- Intracellular but extracytoplasmic (within parasitophorous vacuole in apical surface of epithelial cells)
- Chlorine-resistant
Transmission
- Fecal-oral
- Waterborne (most common): drinking water, recreational water (pools, lakes)
- Foodborne outbreaks
- Zoonotic (calves on farms, petting zoos)
- Person-to-person (daycares)
- MSM (sexual)
Major Outbreaks
- Milwaukee 1993: 400,000+ cases; water treatment failure; cryptosporidiosis caused largest waterborne outbreak in US history
- 2024: ongoing recreational water outbreaks worldwide
- LMIC: major pediatric diarrhea + mortality contributor
Clinical
Immunocompetent
- 7-day incubation
- Watery, profuse, non-bloody diarrhea
- Cramping, nausea, low-grade fever
- 1-2 weeks self-limited
- Severe in young children + elderly
HIV / Immunocompromise (CD4 < 100)
- Severe chronic watery diarrhea
- Massive fluid loss (5-10 L/day in severe)
- Wasting + malnutrition
- Cholangiopathy (papillary stenosis, sclerosing cholangitis)
- Pancreatitis
- Reactive arthritis
- Long-term morbidity + mortality
Pediatric (LMIC)
- Major pediatric diarrhea agent
- Contributes to malnutrition + stunting + cognitive delay
- Mortality especially malnourished
Diagnosis
- Stool antigen test (sensitivity > 95%)
- Modified acid-fast stain of stool (oocysts visible)
- PCR (multiplex GI panels)
- Direct fluorescent Ab (DFA)
Treatment
Immunocompetent
- Nitazoxanide 500 mg PO bid à 3 d
- Adults + children
- Effective in immunocompetent
HIV / Immunocompromise
- ART critical (immune reconstitution = main therapy)
- Nitazoxanide adjunct
- Paromomycin (limited efficacy)
- Combination: nitazoxanide + paromomycin sometimes
- Supportive (hydration, ORS, electrolyte)
- Persistent infection until immune recovery
Prevention
- Water filtration (chlorine alone insufficient â chlorine-resistant)
- Boiling for traveler / camping
- Avoid recreational water (pools especially) if diarrhea
- Hand hygiene
- Pets + petting zoo precautions
3ïžâ£ Cyclospora cayetanensis
Microbiology
- Apicomplexan, related to Toxoplasma + Cryptosporidium
- Acid-fast staining
- Autofluorescence under UV â distinctive
Transmission
- Foodborne: contaminated raspberries, basil, cilantro, leafy greens
- Waterborne (less common)
- Person-to-person rare (oocysts require sporulation time outside)
Epidemiology
- USA outbreaks summer (May-August peak)
- Imported produce from endemic areas (Guatemala, Mexico, Peru historically)
- Continued outbreaks 2024+
Clinical
- 7-10 day incubation
- Watery diarrhea (persistent, weeks-months untreated)
- Fatigue (often prominent)
- Cramping, flatulence, anorexia
- Weight loss
- Self-limited but prolonged
- Severe in HIV / immunocompromise (longer, more severe)
Diagnosis
- Modified acid-fast stain of stool
- UV autofluorescence (oocysts fluoresce blue)
- PCR
- Stool O+P with concentration
Treatment
- TMP-SMX 1 DS PO bid à 7-10 d (preferred)
- Pediatric weight-based
- HIV: longer course (often + secondary prophylaxis)
- Alternative: ciprofloxacin (rare clinical use)
Prevention
- Wash produce thoroughly
- Avoid raw produce in endemic areas during travel
- éå ± outbreaks
- Imports surveillance (FDA, USDA)
4ïžâ£ Microsporidia
Reclassification
- Formerly classified as protozoa
- Now classified as fungi (small, spore-forming)
- Multiple species:
- Enterocytozoon bieneusi (most common HIV diarrhea cause among Microsporidia)
- Encephalitozoon intestinalis (intestinal + dissemination)
- Encephalitozoon cuniculi, E. hellem (disseminated, ocular, sinus, urinary)
Transmission
- Spore inhalation / ingestion / direct contact
- Animal reservoirs (rabbits, dogs, etc.)
- Waterborne
- Person-to-person rare
Clinical
HIV-Associated
- Chronic diarrhea + wasting (CD4 < 200)
- Biliary disease
- Cholangiopathy
Disseminated
- HIV + organ transplant + immunocompromise
- Multi-organ involvement
- Ocular (keratitis especially after corneal trauma in healthy)
- Sinusitis
- Urinary tract
Diagnosis
- Modified trichrome stain (Weber stain) â spores visible
- PCR
- Calcofluor white stain (epifluorescence)
- Tissue biopsy + IHC
Treatment
Enterocytozoon bieneusi
- Fumagillin (oral, limited availability) â drug of choice for E. bieneusi
- ART critical
- 30% spontaneous improvement on ART alone
Encephalitozoon species
- Albendazole 400 mg PO bid à 3-4 wk â effective
- ART
- Longer course in severe / disseminated
Prevention
- Immune restoration (ART)
- No vaccine
6ïžâ£ Blastocystis hominis
Background
- Controversial pathogenicity
- Common (10-20% stool samples globally)
- Multiple subtypes (1-9, some pathogenic some not)
Clinical
- Many asymptomatic carriers
- Symptomatic: chronic diarrhea, IBS-like, bloating, abdominal pain
- Often co-infection with other GI pathogens
Treatment
- Donât treat asymptomatic carriers
- Symptomatic + no other pathogen identified + persistent: trial of treatment
- Metronidazole, tinidazole, nitazoxanide, paromomycin options
7ïžâ£ Dientamoeba fragilis
Background
- No cyst stage (unique â requires direct contact transmission)
- Often co-infection with pinworm (pinworm eggs may transmit)
- Mild diarrhea or asymptomatic
Treatment
- Metronidazole or iodoquinol for symptomatic