Giardiasis (gee-ar-dye-a-sis) is an infection of the digestive system caused by tiny parasites called Giardia intestinalis (also known as Giardia lamblia, or Giardia duodenalis).

Diarrhoea is the most common symptom of giardiasis.

Other symptoms can include:

abdominal cramps

foul-smelling flatulence and belching

nausea (feeling sick)



fatigue (extreme tiredness)


loss of appetite

weight loss caused by malnutrition

You may also experience vomiting and a mild fever of 37-38C (98.6-100.4F) but these symptoms are less common.

Although these symptoms are often unpleasant, giardiasis doesn't usually pose a serious threat to health and can be easily treated.

When to see a doctor

See your GP if you have symptoms of diarrhoea, cramps, bloating and nausea that last for more than a week, particularly if you've recently travelled abroad.

If your baby or child has diarrhoea lasting for more than two or three days, or they've had six or more episodes of diarrhoea in the last 24 hours, take them to see your GP.

Your GP may have to send stool samples to be tested in a laboratory to diagnose giardiasis. Up to three samples may need to be taken over a number of days to confirm giardiasis.

Treating giardiasis

Giardiasis is usually treated successfully with antibiotic medicine that kills the giardia parasite. In most cases, medicines calledmetronidazole or tinidazole are used.

Nausea is the most common side effect of metronidazole. On rare occasions, some people may also feel dizzy or sleepy. If this happens to you, avoid driving or using power tools or machinery.

Don't drink alcohol while taking metronidazole or tinidazole, or for 48 hours after finishing your dose. Mixing alcohol with these types of medication can make the side effects worse.

If you're diagnosed with giardiasis, other members of your household may be advised to have treatment. This may be recommended as a precautionary measure in case they've also been infected. Your GP will be able to tell you if treatment is necessary.

How is giardiasis spread?

Most people become infected with giardiasis by drinking water contaminated with the Giardia parasite, or through direct contact with an infected person.

The giardiasis infection can also be passed on if an infected person doesn't wash their hands properly after using the toilet, then handles food eaten by others. Food can also be contaminated if it is washed with infected water.

Practising good hygiene – such as regularly washing your hands with soap and water – and taking care when drinking water in countries with poor levels of sanitation can help to reduce your risk of developing giardiasis.

Who is affected?

Giardiasis occurs almost everywhere in the world, but is particularly widespread where access to clean water is limited and sanitation is poor.

It can affect people of all ages but is most common in young children and their parents. This is because things like nappy changing increase the risk of infection.

There are more than 3,500 cases of giardiasis reported in England and Wales each year, although the true number is likely to be higher as many cases go undiagnosed. Around one quarter of these cases are thought to be contracted abroad, but many people don't develop the symptoms until they return home.

Most cases of giardiasis are one-offs, but small outbreaks can occur in households, among family members, or at nurseries. Larger outbreaks are usually traced to contaminated water sources, such as drinking wells or water parks.

Risk areas for giardiasis

Places where giardiasis is widespread include:

sub-Saharan Africa – all the countries south of the Sahara desert, such as South Africa, Gambia and Kenya

south and southeast Asia, particularly India and Nepal

Central America

South America





the countries of the former Yugoslavia (Croatia, Serbia, Montenegro, Slovenia, Macedonia, and Bosnia and Herzegovina)

Causes of giardiasis 

Giardiasis is caused by microscopic parasites known as Giardia intestinalis. The parasites live in the intestines of humans and animals. In most cases, the infection is caught from other humans.

The parasites don't usually cause any symptoms, and people have no idea they're infected. In parts of the world where giardiasis is widespread, an estimated one in five people could be infected.

How giardiasis is spread

While inside the intestines, the parasites form a hard protective shell known as a giardia cyst.

When someone with the giardiasis infection goes to the toilet, some of the cysts inside the intestines can be passed out of the body inside faeces. Giardia cysts can survive outside the body for several weeks or months.

Once outside the body, giardiasis is usually spread by drinking water that's been contaminated with infected faeces. This most commonly occurs in countries with poor sanitation and limited access to clean water.

Giardiasis can also be spread through direct contact between people. Less commonly, giardiasis is spread when an infected person doesn't wash their hands properly after going to the toilet and transfers the parasites on to surfaces, utensils or food.

Who's at risk?

Parents or childcare workers who change the nappy of a baby with giardiasis have an increased risk of developing the condition by accidentally transferring faeces into their mouth. The risk is higher in places where there are many babies and frequent nappy changing, such as day care centres and nurseries.

There have been cases of hikers and campers developing giardiasis after drinking contaminated water from streams and lakes. You should always avoid drinking untreated water (water that hasn't been boiled or chemically treated), even if it looks clean.

A small number of outbreaks of giardiasis have been linked to recreational water areas, such as water parks and swimming pools, that have become contaminated with giardiasis parasites.

People travelling to parts of the world where standards of water hygiene are poor have an increased risk of developing giardiasis. However, because of the time it takes for symptoms to appear after becoming infected, most people won't have any symptoms until they return home.

People who have regular anal sex are also at an increased risk of contracting giardiasis, as the giardia parasite can be passed from the anus (back passage) to the mouth during sexual intercourse. 

Preventing giardiasis 

Giardiasis can often be prevented by practising good hygiene and taking commonsense precautions.

Wash your hands

The most effective way to prevent giardiasis is to wash your hands regularly, particularly:

after going to the toilet

after changing a nappy

before handling and eating food

Wash your hands with soap and water. You should also encourage your children to wash their hands regularly.

Water safety

Avoid drinking untreated water from rivers and lakes.

Swimming pools, paddling pools and water parks can sometimes become contaminated, particularly if they're used by younger children. Giardia parasites can survive in chlorinated water, so you shouldn't assume chlorinated water is safe.

If you're going camping, it's recommended you boil water before drinking it.

Travelling abroad

If you're travelling to countries where giardiasis is widespread and sanitation is poor, only drink bottled water. Make sure the bottle is properly sealed before using it. You should also use bottled water when brushing your teeth.

Also avoid eating raw fruit and vegetables as they may have been handled by someone with giardiasis. Read about food and water abroad for more information and advice.

Places where giardiasis is widespread include:

sub-Saharan Africa – all the countries south of the Sahara desert, such as South Africa, Gambia and Kenya

south and southeast Asia, particularly India and Nepal

Central America

South America





the countries of the former Yugoslavia (Croatia, Serbia, Montenegro, Slovenia, Macedonia, and Bosnia and Herzegovina)

Preventing the spread of infection

If you're diagnosed with giardiasis (or if you have diarrhoea), it's important to take precautions to prevent other members of your household becoming infected. You should:

wash your hands regularly

not cook or handle food that will be eaten by other members of your household

avoid sharing utensils or towels

It's recommended you stay away from work or college and avoid swimming pools until you've been completely free from symptoms for 48 hours. Your child should also stay away from school or nursery until they've been completely free from symptoms for 48 hours.


If you have frequent anal sex, make sure you wash your hands after handling a used condom. Kissing or licking also increases the risk of infection and should be avoided.