Giardiasis – Causes, Symptoms, Diagnosis, Treatment and Ongoing care



  • Intestinal infection caused by the protozoan parasite Giardia lamblia (1)
    • G. lamblia is also called G. duodenalis and G. intestinalis.
  • Infection results from ingestion of the cysts, which excyst into trophozoites. These colonize the small intestine and cause symptoms.
  • Cycle is continued when the trophozoites encyst in the small intestine and water, food, or hands are contaminated by feces of the infected person.
  • Most infections result from fecal–oral transmission or ingestion of contaminated water (such as while swimming) and are less commonly the result of contaminated food.


  • Predominant age: All ages, but most common in early childhood ages 1–9 and adults 35–44 (2)[A]
  • Predominant gender: Male > Female (slightly)

Pediatric Considerations

Common in early childhood


  • 5% of patients with stools submitted for ova and parasite exams
  • >19,000 cases/year in the United States (although it is not reportable in Indiana, Kentucky, Mississippi, North Carolina, and Texas)

Risk Factors

  • Daycare centers
  • Anal intercourse
  • Wilderness camping
  • Travel to developing countries
  • Children adopted from developing countries
  • Public swimming pools


No known genetic risk factors

General Prevention

  • Good hand-washing when caring for diapered children
  • Water purification when camping and when traveling to developing countries
  • Cooking all foods


Giardia trophozoites colonize the surface of the proximal small intestine. The mechanism by which they cause diarrhea is unknown.


Protozoan parasite (G. lamblia) infection acquired through fecal–oral transmission or ingestion of contaminated water, less commonly from contaminated food

Commonly Associated Conditions

Hypogammaglobulinemia and possibly IgA deficiency; diarrhea more severe and prolonged in these patients

Paromomycin, Small intestine, Lactose intolerance, Crohn, public swimming pools, protozoan parasite, water purification, contaminated water, diarrhea,



  • ∼25–50% of infected persons are symptomatic.
  • Chronic diarrhea (lasting >5–7 days and frequently weeks)
  • Abdominal bloating
  • Flatulence
  • Loose, greasy, foul-smelling stools
  • Weight loss
  • Nausea
  • Lactose intolerance

Physical Exam

Nonspecific; abdominal bloating and afebrile

Diagnostic Tests & Interpretation


Initial lab tests

  • Stool for ova and parasites:
    • Repeated 3 times if necessary
    • Cysts are seen in fixed or fresh stools and, occasionally, trophozoites are found in fresh diarrheal stools.
  • Fluorescent antibody (FA) and ELISA tests of fecal specimens are available. A single FA or ELISA is at least as sensitive as 3 stools for ova and parasites.
  • Polymerase chain reaction (PCR) techniques have been found to be more sensitive than microscopy, but have not been widely adopted secondary to cost (3).

Follow-Up & Special Considerations

String test (Enterotest): A gelatin capsule on a string is swallowed and left in the duodenum for several hours or overnight. The end of the string is then visualized microscopically.

Diagnostic Procedures/Surgery

Esophagogastroduodenoscopy with biopsy and sample of small intestinal fluid

Pathological Findings

Intestinal biopsy shows flattened, mild lymphocytic infiltration and trophozoites on the surface.

Differential Diagnosis

  • Includes other etiologies of small intestinal diarrhea
  • Infectious causes include cryptosporidiosis, isosporiasis, and cyclosporiasis.
  • Other causes of malabsorption include celiac sprue, tropical sprue, bacterial overgrowth syndromes, and Crohn ileitis.
  • Irritable bowel is suspected when diarrhea is not accompanied by weight loss.


Outpatient for mild cases, inpatient if symptoms are severe enough to cause dehydration


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First Line

  • Metronidazole (Flagyl): 250 mg t.i.d. for 5–7 days (4)[B]
  • Tinidazole 2 g single dose (50 mg/kg up to 2 g for children) (4)[B]
  • Albendazole 400 mg once daily for 5 days:
    • Albendazole has comparable effectiveness to metronidazole with fewer side effects and low cost (5)[A].
  • Precautions:
    • Theoretical risk of carcinogenesis with metronidazole
  • Significant possible interactions: Occasional disulfiram reaction with metronidazole or tinidazole

Pregnancy Considerations

  • Concern for potential teratogenicity of medications; consult infectious disease specialist or gastroenterologist for symptomatic disease
  • Contraindications: Relatively contraindicated in pregnancy, especially 1st trimester

Second Line

  • Furazolidone: 8 mg/kg/d t.i.d. for 10 days (slightly less effective, but commonly used in pediatrics because it is well tolerated)
  • Paromomycin (Humatin): A nonabsorbable aminoglycoside that is probably less effective but commonly recommended in pregnancy because of theoretical risk of teratogenicity of other agents
  • Quinacrine: 100 mg t.i.d. for 5–7 days; was the treatment of choice for giardiasis, but is withdrawn from the market in US
  • Nitazoxanide suspension was approved by the FDA in 2003 for treatment of giardiasis in children ages 1–11. Children ages 1–4 receive 100 mg b.i.d. and ages 5–11 receive 200 mg b.i.d. for 3 days (1)[B].

Additional Treatment

Lactose intolerance may follow Giardia infection and be a cause of persistent diarrhea post treatment.

General Measures

  • Medical therapy for all infected individuals
  • Fluid replacement if dehydrated

Ongoing Care

Follow-Up Recommendations

Patient Monitoring

Symptoms, weight, stool exams


Good nutrition, low lactose, low fat, monitor for dehydration

Patient Education

Hand washing may be more important than water purification to prevent transmission in outdoor recreationalists (6)[A].


  • Untreated giardiasis lasts for weeks.
  • Patients usually (90%) respond to treatment within a few days:
    • Most nonresponders or relapses respond to a 2nd course with the same or a different agent.


Malabsorption and weight loss


1. Yoder JS, Beach MJ, Centers for Disease Control and Prevention (CDC). Giardiasis surveillance–United States, 2003–2005. MMWR Surveill Summ.2007;56:11–8.

2. Yoder JS, Harral C, Beach MJ, Centers for Disease Control and Prevention (CDC) et al. Giardiasis surveillance – United States, 2006–2008. MMWR Surveill Summ. 2010;59:15–25.

3. Haque R, Roy S, Siddique A, et al. Multiplex real-time PCR assay for detection of Entamoeba histolytica, Giardia intestinalis, and Cryptosporidium spp. Am J Trop Med Hyg. 2007;76:713–7.

4. Fallah M, Rabiee S, Moshtaghi AA. Comparison between efficacy of a single dose of tinidazole with a 7-day standard dose course of metronidazole in giardiasis. Pakistan Journal of Medical Sciences. 2007;23(1):43–6.

5. Solaymani-Mohammadi S, Genkinger JM, Loffredo CA, Singer SM et al. A meta-analysis of the effectiveness of albendazole compared with metronidazole as treatments for infections with Giardia duodenalis. PLoS Negl Trop Dis. 2010;4:e682.

6. Welch TP et al. Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data. Int J Infect Dis.2000;4:100–3.

Additional Reading

Pawlowski SW, Warren CA, Guerrant R: Diagnosis and treatment of acute or persistent diarrhea. Gastroenterology. 2009;136(6):1874–86.

Shields JM, Gleim ER, Beach MJ. Prevalence of Cryptosporidium spp. and Giardia intestinalis in Swimming Pools, Atlanta, Georgia. Emerg Infect Dis.2008;14:948–50.

See Also (Topic, Algorithm, Electronic Media Element)

Algorithm: Diarrhea, Chronic



007.1 Giardiasis


58265007 Giardiasis (disorder)

Clinical Pearls

  • Daycare facilities and public swimming pools are common sources of Giardia(don’t assume camping or travel is required).
  • Treatment with metronidazole is often poorly tolerated, but has higher cure rates.
  • Most treatment failures respond to a second course of antibiotics (whether or not you switch drugs).

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