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Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Giardia article more useful, or one of our other health articles.

Giardia lamblia, also known as Giardia intestinalis and Giardia duodenalis, is an important cause of persistent diarrhoea or malabsorption. It is a flagellated, anaerobic protozoon.

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Giardiasis epidemiology and transmission1 2

Giardial infections occur worldwide, including in developed countries. Giardiasis is more common with:

  • Poor sanitation.

  • Travel to endemic areas.

  • Reduced immunity - malnutrition, immunocompromise or cystic fibrosis.

  • Institutions - eg, children in nurseries, and their carers.

  • Those having ano-oral sex - eg, sexually active gay men.

There are 3,000-4,000 cases in England each year, although many of these are acquired abroad.

Worldwide prevalence: giardiasis occurs at any age but is common in young children, (estimated around 20% prevalence in young children in developing countries).3


  • Transmission of Giardia spp. is via the faeco-oral route. The incubation period is 1-2 weeks.4 Giardial cysts are resistant to standard chlorination.

  • Transmission of Giardia spp is usually via contaminated drinking water.

  • Other possible sources are ingested food, contaminated swimming pools and direct contact with infected people, animals or contaminated objects.

  • In the UK, many giardiasis cases are associated with recent foreign travel.

  • Many animals are host to the organism, including pets, livestock and wild animals but it is not clear if they are a source of infection for man. Beavers may be an important reservoir host (in Canada, giardiasis is nicknamed 'beaver fever').5

Giardiasis presentation6

Suspect giardiasis in scenarios such as:

  • Acute diarrhoea lasting for more than three days .

  • Traveller's diarrhoea with symptoms lasting more than ten days, symptoms that begin after return, and associated weight loss.

  • Diarrhoea in immunocompromised or palliative care patients.2

Giardiasis symptoms:2

  • Acute or chronic diarrhoea.

  • Malabsorption, weight loss and, in children, failure to thrive.

  • Abdominal pain, anorexia, flatulence, bloating and nausea.

  • Vomiting and fever are uncommon.

Gardiasis signs:

  • Generally, there are few or no physical signs.

  • In acute or severe cases, signs of dehydration or malnutrition.

  • Physical examination generally is unremarkable.

  • Abdominal examination may reveal nonspecific tenderness.

Unusual presentations of giardiasis:

  • Rarely, it may present with complications (see 'Complications and prognosis', below).

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Stool microscopy is the usual test:

  • Ensure the laboratory request has a full history and a request for ova, cysts and parasites (OC&P). Note that the routine microbiological examination of a stool sample looks only for Campylobacter spp., E. coli O157, Salmonella spp., Shigella spp. and Cryptosporidium spp. Testing for other pathogens may be carried out depending on the clinical history.2

  • If parasitic infection is suspected, send three fresh specimens (5 ml each) 2-3 days apart, as OC&P are shed intermittently.2

Other tests for giardiasis are:6

  • Stool antigen tests which are available and may be the best test .

  • DNA probes for Giardia spp. - improve the detection rate .

  • Other tests: duodenal samples for microscopy, which can be obtained using the 'string test' (swallowing a gelatin capsule on a string) or duodenal biopsy, although the routine use of diagnostic biopsy is not supported.7

  • A simple test using methylthioninium chloride (methylene blue) staining to detect Giardia spp. trophozoites.8

Giardiasis differential diagnosis

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Giardiasis treatment and management

General points:

  • In areas where there is contamination of the water supply the treatment of asymptomatic patients is of dubious value, as they will become re-infected.9

  • Treatment is required where there are giardiasis symptoms, or where there is risk of infection of others who are at special risk.2

  • Giardiasis may be caused by food poisoning and therefore can be a notifiable disease.

Drug treatment for giardiasis:10

  • Rehydration, if required.

  • Clinical Knowledge Summaries (CKS) recommend tinidazole as the drug of choice for treating giardiasis but metronidazole is recommended first-line by the British National Formulary. Note cautions with pregnancy and breastfeeding with both drugs.2 10

  • The other drug recommended by the British National Formulary (BNF) is mepacrine (unlicensed).10

  • Mebendazole also has some effect against Giardia spp.11

  • Metronidazole resistance is becoming an increasing problem. Treatment failures are reported in up to 20% of cases and this has led to the search for alternatives. Auranofin, an antirheumatic agent, has shown promising results.12

  • Development of new drugs based on modification of 5-nitroimidazole, the core structure of metronidazole and other molecular targets of Giardia spp. is ongoing.13

Giardiasis complications and prognosis14


  • The prognosis is usually good.

  • Resistance to treatment or re-infection can occur.

Possible complications

Giardiasis prevention

  • Handwashing and hygiene around infected people and in institutions.

  • UK tap water is very unlikely to be contaminated with Giardia spp. but the same cannot be said for water from rivers and lakes. When camping, water from these sources should be boiled before use.

  • Swimming pools and other recreational facilities can become contaminated. It should not be assumed that chlorinated water is safe.

  • Travellers to endemic areas should avoid eating uncooked foods.

  • Breast-feeding is protective.

  • People who frequently engage in anal sex are at higher risk of acquiring giardiasis. This can be minimised by washing the hands after touching the anus of another person or after touching a condom that has been used for anal sex, and by avoiding oro-anal contact.

Further reading and references

  1. Yoder JS, Gargano JW, Wallace RM, et al; Giardiasis surveillance - United States, 2009-2010. MMWR Surveill Summ. 2012 Sep 7;61(5):13-23.
  2. Gastroenteritis; NICE CKS, August 2020 (UK access only)
  3. Ankarklev J, Hestvik E, Lebbad M, et al; Common coinfections of Giardia intestinalis and Helicobacter pylori in non-symptomatic Ugandan children. PLoS Negl Trop Dis. 2012;6(8):e1780. doi: 10.1371/journal.pntd.0001780. Epub 2012 Aug 28.
  4. Katz DE, Heisey-Grove D, Beach M, et al; Prolonged outbreak of giardiasis with two modes of transmission. Epidemiol Infect. 2006 Oct;134(5):935-41. Epub 2006 Mar 29.
  5. Fayer R, Santin M, Trout JM, et al; Prevalence of Microsporidia, Cryptosporidium spp., and Giardia spp. in beavers (Castor canadensis) in Massachusetts. J Zoo Wildl Med. 2006 Dec;37(4):492-7.
  6. Dunn N, Juergens AL; Giardiasis
  7. Chew TS, Hopper AD, Sanders DS; Is there a role for routine duodenal biopsy in diagnosing giardiasis in a European population? Scand J Gastroenterol. 2008;43(10):1219-23. doi: 10.1080/00365520802101853.
  8. Rajurkar MN, Lall N, Basak S, et al; A simple method for demonstrating the giardia lamblia trophozoite. J Clin Diagn Res. 2012 Nov;6(9):1492-4. doi: 10.7860/JCDR/2012/4358.2541.
  9. Giardiasis; Department of Health, Australia, 2007
  10. British National Formulary (BNF); NICE Evidence Services (UK access only)
  11. Canete R, Escobedo AA, Gonzalez ME, et al; A randomized, controlled, open-label trial of a single day of mebendazole versus a single dose of tinidazole in the treatment of giardiasis in children. Curr Med Res Opin. 2006 Nov;22(11):2131-6.
  12. Tejman-Yarden N, Miyamoto Y, Leitsch D, et al; A reprofiled drug, auranofin, is effective against metronidazole-resistant Giardia lamblia. Antimicrob Agents Chemother. 2013 May;57(5):2029-35. doi: 10.1128/AAC.01675-12. Epub 2013 Feb 12.
  13. Lalle M, Hanevik K; Treatment-refractory giardiasis: challenges and solutions. Infect Drug Resist. 2018 Oct 24;11:1921-1933. doi: 10.2147/IDR.S141468. eCollection 2018.
  14. Granados CE, Reveiz L, Uribe LG, et al; Drugs for treating giardiasis. Cochrane Database Syst Rev. 2012 Dec 12;12:CD007787. doi: 10.1002/14651858.CD007787.pub2.
  15. Savioli L, Smith H, Thompson A; Giardia and Cryptosporidium join the 'Neglected Diseases Initiative'. Trends Parasitol. 2006 May;22(5):203-8. Epub 2006 Mar 20.
  16. Grazioli B, Matera G, Laratta C, et al; Giardia lamblia infection in patients with irritable bowel syndrome and dyspepsia: a prospective study. World J Gastroenterol. 2006 Mar 28;12(12):1941-4.
  17. Verdu EF, Mauro M, Bourgeois J, et al; Clinical onset of celiac disease after an episode of Campylobacter jejuni enteritis. Can J Gastroenterol. 2007 Jul;21(7):453-5.
  18. Elliott EJ; Acute gastroenteritis in children. BMJ. 2007 Jan 6;334(7583):35-40.
  19. Parasite Summary Tables; Ethnomed

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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