Gut Parasites

Giardia Lamblia, Symptoms, Causes, Diagnosis, Treatment and Prevention

Saskia Knight, Gut Health Nutritionist
Written by Saskia Knight, DipNT Published 22 January 2025
Sonia Khan, Pharmacist
Medically reviewed by Sonia Khan, Pharmacist Last reviewed 17 August 2026

What is Giardia lamblia?

Giardia lamblia (also known as Giardia duodenalis) is a parasite that infects the small intestine, causing giardiasis: a gut infection that brings watery diarrhoea, stomach cramps and bloating. It is highly contagious, one of the most common intestinal parasites worldwide, and it affects both humans and animals.

It spreads mostly through contaminated food and water, or through close contact with an infected person or animal. With treatment, symptoms usually settle within about a week; without it, they can drag on for weeks and sometimes months.

How does Giardia lamblia spread?

Giardia spreads when someone swallows the parasite’s cysts. The cysts act as a protective outer shell, which lets the parasite survive outside the body for weeks or months, for example in soil or water. Once swallowed, the cysts 'hatch' in the small intestine, releasing the active form of the parasite, which attaches to the gut lining and causes infection.

Common ways Giardia can spread:

Contaminated water: drinking or accidentally swallowing untreated water from rivers, lakes or wells is a common source of infection, especially on camping and hiking trips. When a Glasgow and Public Health Scotland team reviewed 75 giardiasis outbreaks in high-income countries, contaminated drinking water was one of the two most common routes.

Contaminated food: raw or unwashed fruit and vegetables, particularly any washed in untreated water, can carry Giardia. Contaminated food alone is estimated to cause around 28 million cases of diarrhoea worldwide each year, and food outbreaks usually trace back to infected food handlers.

Contaminated surfaces: touching surfaces an infected person has touched, then touching your mouth or food.

Person-to-person transmission: the parasite spreads easily in nurseries, care homes and households, and contact with children of nappy-wearing age was the other leading route in that outbreak review. Poor hand hygiene after using the toilet or changing nappies lets it spread. It can also be passed on during sex, particularly anal and oral sex.

Animal contact: infected animals, including pets, livestock and wildlife, can carry Giardia cysts in their faeces, which can contaminate water sources or surfaces. Genetic studies suggest that catching Giardia from pets is less common than once believed, although it does happen.

Is Giardia found in the UK?

Yes. Giardia is often assumed to be an infection you only pick up abroad, but the same Glasgow-led review found UK giardiasis is regularly acquired at home and is under-reported, partly because of that travel assumption.

In 2017 there were 4,793 reported cases of giardiasis in England and Wales, making it the most common intestinal parasite infection in the region, according to Patient.info.

What symptoms does Giardia lamblia cause?

Giardiasis can cause a range of uncomfortable digestive symptoms, which usually appear 1 to 2 weeks after swallowing the parasite. Symptoms vary from person to person, and some people have none at all.

Common symptoms:

Giardia lamblia, Cryptosporidium, Cyclospora and many other types of gut parasites can share many of the same symptoms, which is why testing usually checks for several parasites at once.

How long does giardiasis last?

With prompt treatment, symptoms usually settle within a few days to a week. Without treatment, giardiasis can last several weeks or even months. In some cases symptoms appear to improve and then return, known as relapsing symptoms.

Can you have Giardia without symptoms?

Yes. Some people carry Giardia without any symptoms at all: research suggests around 50 to 75% of infected children are symptom-free. Even without symptoms, carriers can still pass the parasite on through poor hand hygiene.

Can giardiasis cause long-term symptoms after the infection has cleared?

For most people, giardiasis ends when the infection does. Research from one large outbreak suggests a minority are left with symptoms that continue long after the parasite has gone.

In 2004, contaminated drinking water caused a large giardiasis outbreak in Bergen, Norway. Researchers followed 1,252 people with laboratory-confirmed infection, alongside a matched comparison group, for the next ten years.

Three years after the outbreak, roughly 45 to 48% of the previously infected group had IBS, compared with roughly 12 to 24% of the comparison group. Ten years on, the previously infected group still reported a measurably lower quality of life, and the gap was explained by higher rates of IBS and chronic fatigue.

A 2025 review pulled the wider evidence together: IBS, chronic fatigue and other long-term conditions can follow giardiasis, sometimes lasting months or years, and sometimes after infections that caused no symptoms at all. Why this happens is not yet fully understood.

These findings show an association, drawn largely from one well-studied outbreak; they do not mean Giardia will cause IBS or fatigue in any one person, and most people recover completely. But if your bowel habit has not settled weeks after treatment, tell your GP you had giardiasis.

When should you see a GP?

If you think you could have giardiasis, your GP is the right first step. NHS testing is free: a GP can send a stool sample to an NHS laboratory to confirm what is causing your symptoms. Tell them about any recent travel abroad, because it changes what the laboratory looks for.

The NHS advises asking for an urgent GP appointment, or calling 111, if you have had diarrhoea for more than 7 days, or you have bloody diarrhoea or bleeding from your bottom.

See a GP promptly if you have:

  • Diarrhoea lasting more than a week, and certainly if it passes two weeks
  • Blood in your stool
  • Unexplained weight loss
  • Signs of dehydration, such as a dry mouth, dizziness, or passing little urine
  • An inability to keep fluids down because of vomiting
  • Severe stomach pain or bloating
  • Symptoms that started during or after travel abroad

That timing matters: short-lived diarrhoea is usually caused by viruses or bacteria, but once diarrhoea persists, parasites such as Giardia move towards the top of the list.

This guide is for information only and does not replace medical advice. A home test is never a substitute for seeing a GP about the symptoms above.

Who is most at risk of Giardia infections?

Giardiasis can affect anyone, but some groups are at higher risk.

Travellers: people visiting regions with poor sanitation or untreated water supplies.

Campers and hikers: those drinking untreated water from rivers, lakes or streams.

Children in nurseries, and the staff and parents who care for them: Giardia spreads easily wherever nappies are changed.

People with weakened immune systems: infection is more likely to take hold and can be harder to clear.

How is giardiasis diagnosed?

Giardiasis is confirmed with a stool test, which looks for the parasite’s cysts or its DNA in a sample of faeces. Parasites are shed intermittently, so a single sample can miss the infection; testing several samples over a few days gives a more reliable answer.

Modern DNA-based methods such as PCR (polymerase chain reaction) testing are generally more sensitive at detecting Giardia than examining a sample under a microscope.

Our gut parasite stool test identifies 28 types of parasites, pathogenic bacteria and viruses. It can be paired with our comprehensive gut microbiome test kit for a fuller picture of your gut health, including the balance of your gut bacteria. No home test diagnoses a condition: results need interpreting alongside your symptoms, and a GP should always come first if you are unwell.

How to test for Giardia lamblia at home

Home stool test kits let you collect a sample at home and post it to a laboratory, with results usually available within a few days. Because so many parasites cause near-identical symptoms, testing for a wide range of parasites, pathogenic bacteria and viruses at once is more useful than testing for Giardia alone.

How is giardiasis treated?

Giardiasis is usually treated with a short course of prescription antibiotics, most commonly metronidazole or tinidazole. With treatment, symptoms usually stop in about a week. These are prescription-only medicines, and whether to treat is a decision for your GP.

A review pooling 30 randomised trials, 3,930 people in total, found high cure rates across the metronidazole family of drugs.

Treatment does not always work first time. In some settings, up to 45% of people are not cleared by their first course, and simply repeating the same drug rarely helps; alternative drugs and combinations do better. If your symptoms persist after treatment, go back to your GP, who may try a different approach or refer you to a specialist.

What helps with recovery?

Giardia can unsettle the balance of bacteria in your gut, so it helps to support your recovery with gut-friendly foods and plenty of fluids. The NHS also advises not drinking alcohol while taking the antibiotics used for giardiasis, because the two can react.

Foods to eat:

High fibre foods: fruits, vegetables, whole grains, nuts, legumes and seeds. Introduce fibre slowly and keep drinking plenty of fluids.

Fermented foods with live cultures: yoghurt, kefir and fermented vegetables may help support the balance of bacteria in your gut after antibiotic treatment.

Hydrating fluids: oral rehydration solutions or drinks containing electrolytes help prevent dehydration. A pharmacist can advise if you have signs of dehydration.

Foods to avoid:

Alcohol, caffeine, greasy foods, spicy foods and sugary snacks, as these can irritate the gut and worsen diarrhoea.

Read our guide on good gut foods for a full breakdown of foods that can help to improve your gut health.

Can Giardia infections be prevented?

You cannot remove every chance of coming into contact with Giardia, but simple precautions reduce your risk of infection.

Food safety:

Wash fresh fruit and vegetables thoroughly, and cook food properly: heat kills the parasite.

Water safety:

Drink bottled, boiled or filtered water when travelling in high-risk areas, and avoid swallowing water when swimming in lakes, rivers or public pools. If you have had giardiasis yourself, the NHS advises staying out of swimming pools until two weeks after your symptoms stop.

Hygiene:

Wash your hands thoroughly with soap and water after using the toilet, after changing nappies, and before eating or preparing food. Teach children good hygiene habits to prevent infections spreading in nurseries and schools.

The role of gut health in recovery

Research suggests the make-up of your gut bacteria may influence how a parasite infection plays out, and that the infection can in turn unsettle that balance. Much of this work comes from laboratory and animal studies, so it is best read as promising rather than proven. Eating a balanced diet that includes prebiotic foods and fermented foods with live cultures may help support your microbiome as it recovers.

Conclusion

Giardia lamblia is a common intestinal parasite spread through contaminated food, water and close contact, at home as well as abroad. Infections are unpleasant but treatable, and most people recover fully. Good hygiene and care with food and water, especially when travelling, will lower your risk.

If anything on the warning-signs list above applies to you, speak to your GP before reaching for any home test. NHS testing is free, and a clinician can interpret the result properly.

Sources

We have also drawn on the NHS guidance page on giardiasis (last reviewed by the NHS in August 2023), which is clinical guidance rather than a study, and on Patient.info for the UK case figures above.

Reading next

Cryptosporidium, Symptoms, Causes, Diagnosis, Treatment and Prevention

Jan 22, 2025

How the gut microbiome helps our body to synthesise vitamins and minerals

Feb 10, 2025

Medically reviewed by Nutritionist Saskia Knight BA DipNT