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# Giardiasis

Giardiasis is a gut infection by the parasite Giardia, spread by contaminated water, that causes greasy diarrhoea, bloating and weight loss.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Duodenum](https://durgical.com/en/anatomy/duodenum/)
- [Jejunum](https://durgical.com/en/anatomy/jejunum/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)
- [Stomach](https://durgical.com/en/anatomy/stomach/)
- [Gallbladder](https://durgical.com/en/anatomy/gallbladder/)

## Microorganisms that cause it

- [Giardia duodenalis](https://durgical.com/en/microbiology/giardia-duodenalis/)

## Why it happens

- [Patterns of malabsorption](https://durgical.com/en/physiology/patterns-of-malabsorption/)
- [Carbohydrate digestion and absorption](https://durgical.com/en/physiology/carbohydrate-digestion-and-absorption/)
- [Fat digestion and absorption](https://durgical.com/en/physiology/fat-digestion-and-absorption/)
- [Small intestine: villi, crypts, enterocytes, Paneth cells; duodenum vs jejunum vs ileum](https://durgical.com/en/histology/small-intestine-villi-crypts-enterocytes-paneth-cells-duodenum-vs-jejunum-vs-ile/)
- [Cilia, microvilli and stereocilia](https://durgical.com/en/histology/cilia-microvilli-and-stereocilia/)

## What it is

An infection of the small intestine by the flagellated protozoan Giardia duodenalis (also called G. lamblia or G. intestinalis), causing acute or chronic non-bloody diarrhoea and malabsorption; many infections are asymptomatic.

## Who gets it

One of the most common intestinal parasites in the world and in Latin America. In Colombia it is frequent in children, especially in rural areas and day-care settings without safe water; it also causes outbreaks from contaminated water supplies and illness in travellers and hikers.

## Causes

- Giardia duodenalis cysts, which are resistant to chlorine and survive in cold water
- Faecal-oral spread through water, food, hands or surfaces; a few cysts are enough to infect
- Zoonotic transmission from some animals is possible

## Risk factors

- Drinking untreated water (wells, streams, poorly treated supplies)
- Young children, day-care centres and their caregivers
- Poor sanitation and crowding
- Travel and outdoor activities with untreated water
- Humoral immunodeficiency (common variable immunodeficiency, X-linked agammaglobulinaemia) and IgA deficiency
- Oral-anal sexual contact

## Pathophysiology

Swallowed cysts release trophozoites in the [duodenum](https://durgical.com/en/anatomy/duodenum/), which attach to the epithelium with a ventral sucking disc but do not invade. They shorten the microvilli, reduce brush-border enzymes such as lactase and increase epithelial permeability, causing malabsorption of fat and sugars. Trophozoites encyst further down the bowel and cysts are passed in the stool.

## Classification

- Asymptomatic infection (carriage)
- Acute giardiasis
- Chronic giardiasis with malabsorption and weight loss

## Symptoms and signs

- Watery, then greasy, foul-smelling, floating stools without blood
- Bloating, flatulence, abdominal cramps and sulphur-smelling belching
- Nausea and loss of appetite
- Fatigue and weight loss in chronic cases
- Poor growth in children with long-standing infection

## Red flags

- Signs of dehydration, especially in infants and pregnant women
- Blood in the stool or high fever (suggests another cause)
- Marked weight loss or faltering growth in a child
- Recurrent or persistent giardiasis (consider reinfection, resistance or immunodeficiency)

## Diagnosis

**Clinical**
Suspected with prolonged non-bloody diarrhoea, bloating and malabsorption after exposure to untreated water, day care or travel. Treatment may start on history when testing is limited.

**Tests**
Stool antigen tests (immunoassay or immunofluorescence) or molecular panels are more sensitive than microscopy; microscopy for cysts or trophozoites needs several samples because shedding is intermittent. Small-bowel biopsy or duodenal aspirate is rarely needed.

**Imaging**
Not needed for diagnosis.

## Differential diagnosis

- Viral gastroenteritis
- Cryptosporidiosis and other intestinal protozoa
- Coeliac disease
- Lactose intolerance
- Irritable bowel syndrome
- Small intestinal bacterial overgrowth

## Goal of treatment

Eradicate the parasite in symptomatic people, correct dehydration and nutrition, and prevent spread to household and day-care contacts.

## Treatment

**Acute**
Rehydration when needed, especially in infants and pregnant women.

**Medical**
Antiparasitic drugs: nitroimidazoles (tinidazole, metronidazole) or nitazoxanide; options in pregnancy are chosen carefully. Combination therapy is used for refractory cases.

**Follow-up**
Retest stool when symptoms persist, check for reinfection or incomplete treatment, and evaluate for immunodeficiency in recurrent cases; temporary lactose intolerance may last weeks after cure.

**Lifestyle**
Handwashing, safe or boiled water, and keeping ill children out of day care and swimming pools until diarrhoea resolves.

## Complications

- Dehydration
- Malabsorption of fat, fat-soluble vitamins and vitamin B12
- Acquired lactose intolerance after the infection
- Growth faltering and malnutrition in children
- Post-infectious irritable bowel syndrome and chronic fatigue

## Prognosis

Usually cured with treatment; some infections resolve on their own. Reinfection is common in endemic areas, and symptoms such as lactose intolerance may persist for weeks.

## Prevention

- Safe, filtered or boiled drinking water (chlorine alone does not reliably kill cysts)
- Handwashing, especially after nappy changes and before eating
- Washing raw produce
- Excluding children with diarrhoea from day care and pools

## Test yourself

Think of the answer, then tap the question to see it.

## Where does Giardia live and does it invade the mucosa?

In the duodenum and jejunum, attached by a ventral sucking disc; it does not invade, so stools are not bloody.

## What type of stool is classic for giardiasis?

Greasy, foul-smelling, floating stools (steatorrhoea) with bloating and flatulence.

## Which immune deficiency predisposes to chronic giardiasis?

Humoral deficiencies such as IgA deficiency, common variable immunodeficiency and X-linked agammaglobulinaemia.

## Which drug classes treat giardiasis?

Nitroimidazoles (tinidazole, metronidazole) or nitazoxanide.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/conditions/giardiasis/)

## Related conditions

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Bacterial infectious diarrhoea and dysentery](https://durgical.com/en/conditions/bacterial-infectious-diarrhoea-and-dysentery/)
- [Intestinal amoebiasis (amoebic dysentery)](https://durgical.com/en/conditions/intestinal-amoebiasis/)
- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)
- [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/)
- [Irritable bowel syndrome](https://durgical.com/en/conditions/irritable-bowel-syndrome/)
- [Small intestinal bacterial overgrowth (SIBO)](https://durgical.com/en/conditions/small-intestinal-bacterial-overgrowth/)
- [X-linked (Bruton) agammaglobulinaemia](https://durgical.com/en/conditions/x-linked-agammaglobulinaemia/)
- [Failure to thrive (faltering growth)](https://durgical.com/en/conditions/failure-to-thrive/)
- [Soil-transmitted helminthiasis (ascariasis, hookworm, trichuriasis)](https://durgical.com/en/conditions/soil-transmitted-helminthiasis/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Giardia infection: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000288.htm) (MedlinePlus (U.S. National Library of Medicine), 2024); [Patient Care for Giardia Infection | Giardia | CDC](https://www.cdc.gov/giardia/hcp/clinical-care/index.html) (Centers for Disease Control and Prevention, 2024).

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Source: https://durgical.com/en/conditions/giardiasis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/giardiasis/