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Giardiasis

BASICS

  • Caused by protozoan Giardia lamblia (also G. duodenalis, G. intestinalis).
  • Life cycle: ingestion of cysts → excystation in duodenum releasing trophozoites → colonize small intestine → encyst in large intestine → cysts excreted.
  • Transmission: fecal-oral, contaminated water (unfiltered surface water), less commonly food.

EPIDEMIOLOGY

  • Most common in children <5 years and adults 25-44 years; more common in males.
  • Slight seasonal increase in summer and early fall.
  • High-risk regions: South/Southeast Asia, North Africa, Caribbean, South America.
  • In the US: ~14,887 cases reported in 2019 (~6/100,000), but likely underreported.
  • Prevalence: 2% adults, 6-8% children in developed countries; up to 33% in developing countries.

ETIOLOGY AND PATHOPHYSIOLOGY

  • Trophozoites attach to proximal small intestinal enterocytes via ventral suction disc.
  • Parasite excretion damages epithelium, disrupts brush border enzymes → malabsorption, diarrhea.
  • Eight genotypes (assemblages A-H); only A and B infect humans.

RISK FACTORS

  • Daycare centers, anal intercourse, wilderness camping.
  • Travel to developing countries.
  • Untreated water from lakes, streams, wells.
  • Swimming in contaminated recreational waters.
  • Contact with infected pets.
  • Consumption of raw produce.

GENERAL PREVENTION

  • Hand hygiene.
  • Water purification (boiling is most effective).
  • Proper cooking of food.
  • Protect water supplies from fecal contamination.
  • Barrier protection during anal intercourse.

COMMONLY ASSOCIATED CONDITIONS

  • Immunodeficiency states (hypogammaglobulinemia, IgA deficiency, CVID) linked to prolonged disease and treatment failure.

DIAGNOSIS

HISTORY

  • 50-75% asymptomatic.
  • Symptomatic: diarrhea, foul-smelling/fatty stools, flatulence, abdominal cramps, nausea, weight loss, vomiting, urticaria.
  • Chronic: malabsorption, fatigue, depression, failure to thrive in children.

PHYSICAL EXAM

  • Typically normal vital signs.
  • Abdominal tenderness, bloating, increased bowel sounds.
  • Assess dehydration, malnutrition, weight loss.

DIFFERENTIAL DIAGNOSIS

  • Other protozoal infections: cryptosporidiosis, microsporidiosis, amebiasis.
  • Viral gastroenteritis, traveler's diarrhea.
  • Malabsorption syndromes: lactose intolerance, celiac disease, Crohn disease.

DIAGNOSTIC TESTS

  • Stool microscopy: 3 serial stool samples increase sensitivity >90%.
  • DFA test and ELISA antigen detection: high sensitivity and specificity (gold standard).
  • PCR assays: high sensitivity/specificity, replacing microscopy in some regions.
  • Serology (IgG/IgM) not useful clinically.
  • String test (entero-test): rarely used now.
  • EGD with biopsy may show trophozoites on intestinal surface.

TREATMENT

GENERAL MEASURES

  • No treatment for asymptomatic patients except prophylaxis in close contacts.
  • Fluid replacement for dehydration.

MEDICATION

  • First line:
  • Tinidazole 2 g PO single dose (or 50 mg/kg for children ≥3 years)
  • Nitazoxanide 500 mg PO BID for 3 days
  • Second line alternatives:
  • Metronidazole 250 mg TID or 500 mg BID for 5-7 days
  • Albendazole, mebendazole, paromomycin, furazolidone, quinacrine
  • Avoid alcohol during nitroimidazole treatment (disulfiram-like reaction).
  • Treatment failure may require combination or prolonged therapy; increasing resistance noted.

SPECIAL POPULATIONS

  • Pregnancy: delay treatment to 2nd trimester if possible; paromomycin preferred.
  • Pediatrics:
  • <12 months: metronidazole
  • 12-36 months: nitazoxanide
  • ≥36 months: tinidazole preferred

ISSUES FOR REFERRAL

  • Treatment failure or complicated cases: refer to specialist; exclude immunodeficiency or malabsorption disorders.

ADDITIONAL THERAPIES

  • Supportive fluids to prevent dehydration.

COMPLEMENTARY & ALTERNATIVE MEDICINE

  • Zinc and vitamin A may have protective effects in children.

ONGOING CARE

  • Monitor symptoms, weight, stool exams if symptoms persist.
  • Recommend low-lactose or lactose-free diet post-infection due to secondary lactose intolerance.

PATIENT EDUCATION

  • Handwashing more important than water purification in some outdoor settings.
  • Avoid swimming if symptomatic.
  • CDC pool guidelines: https://www.cdc.gov/healthywater/pdf/swimming/resources/giardia-factsheet.pdf

PROGNOSIS

  • Excellent in most cases; many are asymptomatic.
  • Chronic infections or severe disease in immunocompromised may occur.
  • Rare mortality due to dehydration, mainly in infants or malnourished children.

COMPLICATIONS

  • Malabsorption, growth retardation, postinfectious IBS, lactose intolerance.
  • Rare: cholecystitis, cholangitis, granulomatous hepatitis.

ALERT

  • Giardiasis is a reportable disease to CDC.

REFERENCES

  1. Minetti C, Chalmers RM, Beeching NJ, et al. Giardiasis. BMJ. 2016;355:i5369.
  2. Adam RD. Giardia duodenalis: biology and pathogenesis. Clin Microbiol Rev. 2021;34(4):e0002419.
  3. Centers for Disease Control and Prevention. Giardiasis Summary Report—National Notifiable Disease Surveillance System, United States, 2019. Atlanta, GA: CDC; 2021.
  4. Leung AKC, Leung AAM, Wong AHC, et al. Giardiasis: an overview. Recent Pat Inflamm Allergy Drug Discov. 2019;13(2):134-143.

CODES

  • ICD10: A07.1 Giardiasis [lambliasis]

CLINICAL PEARLS

  • Daycare and public pools are common transmission sources; camping/travel history not always present.
  • Abdominal bloating and foul-smelling loose stools are classic symptoms.
  • Tinidazole and nitazoxanide are first-line treatments; metronidazole effective but less tolerated.
  • Consider combination or prolonged therapy for treatment failures.
  • DFA testing is gold standard diagnostic test.