Giardiasis

Overview

Giardiasis is an infection of the small intestine (the first part of the bowel after the stomach). It is caused by a microscopic parasite (a tiny living thing that lives inside another) called Giardia, also known as Giardia lamblia or Giardia intestinalis. The parasite passes out in the stool (poo) of infected people and animals, and you catch it by swallowing it. Only a few germs are needed to make you ill.

The parasite forms tough cysts (resting forms with a protective shell) that can survive in water, soil and on surfaces for weeks or months. The cysts are fairly resistant to chlorine, so water needs to be boiled or filtered as well. Giardiasis is found all over the world, but it is much more common where clean drinking water and proper sewage systems are lacking. Children get it most often.

Symptoms usually start 1 to 2 weeks after swallowing the parasite and often last 2 to 6 weeks. Some people have no symptoms at all but can still pass the infection on. In others the illness lingers for months, and some people keep having bowel symptoms even after the parasite has gone. Most people get better fully with fluids and the right medicine.

Symptoms

• Diarrhoea, often smelly, greasy stools that may float
• Stomach pain or cramps
• Wind (flatulence) and a bloated tummy
• Burps that smell like rotten eggs
• Feeling sick (nausea) and loss of appetite
• Tiredness that gets worse over time
• Weight loss
• Signs of dehydration (losing too much body water), such as dark urine and passing less urine than usual
• Vomiting or a mild fever in some people, though these are less common
• No symptoms at all in some people, who can still spread the infection

Causes and risk factors

Causes

• Swallowing Giardia cysts that have passed out in the stool of an infected person or animal
• Drinking untreated or dirty water, for example from shallow wells, springs, rivers or lakes
• Eating raw or undercooked food, or food washed in dirty water or handled by an infected person
• Close contact with an infected person, such as changing nappies or caring for someone who is ill, without washing hands
• Touching surfaces or objects with the germ on them and then touching your mouth
• Swallowing water while swimming in pools, lakes or rivers

What makes it more likely

• Being a young child, especially a child who wears nappies or goes to a nursery or day-care centre
• Living in an area without safe drinking water or a proper sewage system
• Living with or caring for someone who has giardiasis
• Travelling to places where the infection is common
• Living in a rural area, and lower parental education (seen in a Pakistani study of children)
• A weak immune system
• Sexual contact that involves contact with stool, such as unprotected anal sex
• Contact with infected animals or their droppings

Complications

The most common problems are dehydration and weight loss. Dehydration can be dangerous, even life-threatening, for babies and is also a special worry in pregnancy. Giardia can damage the lining of the gut, so many people find they cannot digest milk sugar (lactose intolerance) for some weeks after the infection has cleared. Long-lasting infection can stop the body taking in food properly (malabsorption), leading to low vitamin levels, anaemia (low red blood cells) and, in children, poor weight gain and growth. Some people have bowel symptoms like irritable bowel syndrome for a long time afterwards. Rarely, it can cause joint pain (reactive arthritis), eye problems or skin rashes such as hives.

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Diagnosis

A doctor will ask about your symptoms, where you get your drinking water, recent travel and whether anyone at home is ill. Giardiasis is confirmed by testing a stool sample. Because the parasite is not passed in every stool, the doctor may ask for three samples collected over several days. The lab may look for the parasite under a microscope, or use more sensitive tests that detect parts of the parasite (antigen tests) or its genetic material (PCR tests). Microscopy is less sensitive but can also find other parasites.

Blood tests are usually normal and are not needed to make the diagnosis. A camera test with a small tissue sample (biopsy) is not routinely needed. If symptoms continue after treatment, the stool test is repeated. A negative result after three samples suggests the parasite has gone, and ongoing symptoms may be lingering effects, such as lactose intolerance. If the parasite will not clear even after combined treatment, the doctor may check for a weak immune system or other bowel conditions.

Treatment

1. Drink plenty of fluids. Use oral rehydration salts (ORS) mixed with clean water to replace lost water and salts, especially for babies, children and pregnant women. Keep breastfeeding or bottle-feeding babies as usual.
2. See a doctor for a stool test if diarrhoea lasts more than a few days. If giardiasis is confirmed and you have symptoms, the doctor usually prescribes a short course of an antiparasitic medicine. Your doctor decides the medicine and the dose.
3. Take the full course exactly as prescribed. Do not drink alcohol while taking metronidazole or tinidazole, as it can cause a bad reaction.
4. Household members or close contacts may also need testing and treatment, especially if they have symptoms, to stop the infection going round the family.
5. People with no symptoms may not need treatment, but the doctor may still treat them if they work with food, children or vulnerable people.
6. If symptoms continue a week after starting treatment, go back to the doctor. They may repeat the stool test, check for re-infection from water or family members, try a different medicine or a combination, or refer you to a specialist.
7. If you are pregnant, the doctor will weigh up whether treatment can wait until after the birth and which medicine is safest. All medicine decisions in pregnancy are made by your doctor.

Medicines in Pakistan

Medicines a doctor may use, by group. Read the role: some are only for special cases. Each link opens our page for that medicine, with every brand and its official DRAP price. Your doctor decides which medicine and dose are right for you.

Also used (not yet in our price list): Nitazoxanide, Paromomycin, Quinacrine (mepacrine)

Self-care and prevention

Self-care

• Drink plenty of fluids so that your urine is pale yellow or clear, and use oral rehydration salts if you have a lot of diarrhoea
• Keep breastfeeding or bottle-feeding a baby with diarrhoea as usual
• Wash your hands well with soap and water after using the toilet, changing nappies and before preparing or eating food
• Stay off school or work until 2 days after your symptoms have stopped
• Do not go swimming in a pool until 2 weeks after your symptoms have stopped
• Avoid preparing food for other people while you are ill, and do not share towels, cloths or cutlery
• Wash dirty clothes and bedding separately on a hot wash, and clean toilet seats, flush handles, taps and door handles often
• If milk or dairy foods make your diarrhoea or bloating worse after the infection, tell your doctor, as this can be short-term lactose intolerance

Prevention

• Drink only safe water: boil it for at least 1 minute or use a good filter if you are not sure it is clean
• Wash hands with soap and water at key times, especially after the toilet, after changing nappies and before handling food
• Wash or peel fruit and vegetables, using safe water
• Try not to swallow water when swimming in pools, lakes or rivers
• Avoid contact with animal droppings, and clean and disinfect areas where a person or pet has had diarrhoea
• Wait several weeks after diarrhoea stops before having sex, and use safer sex practices

Common questions

Is giardiasis contagious?

Yes. The parasite passes out in stool and can spread from person to person, especially in families and nurseries, or through dirty water and food. You are most infectious from when symptoms start until 2 days after they stop. Good handwashing is the best way to stop it spreading.

How long does giardiasis last?

Without treatment, symptoms often last 2 to 6 weeks, and sometimes much longer. With treatment, most people feel better within about a week. Some people have bloating or loose stools for a few weeks after the parasite has gone, often because the gut is still healing.

Why do I still have symptoms after treatment?

Some people keep having symptoms after the infection has cleared, for example because of short-term lactose intolerance. You may also have been re-infected from water or a family member, or the parasite may not have cleared. See your doctor, who may repeat the stool test and change the treatment if needed.

Can I drink alcohol while being treated for giardiasis?

No. The main medicines used, metronidazole and tinidazole, can react badly with alcohol and make you feel very sick. Avoid alcohol during treatment and ask your pharmacist how long to wait after the course ends.

Can giardiasis be treated in pregnancy?

Drinking enough fluids is very important, as dehydration is a special risk in pregnancy. Some medicines for giardiasis may harm the unborn baby, so treatment is sometimes delayed until after the birth. Your doctor decides whether and how to treat you.

Sources

1. CDC. Patient Care for Giardia Infection (n.d.).
2. CDC. About Giardia Infection (n.d.).
3. NHS. Giardiasis (n.d.).
4. MedlinePlus (US National Library of Medicine). Giardia infection: MedlinePlus Medical Encyclopedia (n.d.).
5. StatPearls. Giardiasis (n.d.).
6. Cochrane Database of Systematic Reviews (Granados, Reveiz, Uribe, Criollo), via GOV.UK Research for Development Outputs. Drugs for treating giardiasis. (2012).
7. Sao Paulo Medical Journal (Naz, Nawaz, Rasool, Zahoor). Cross-sectional epidemiological investigations of Giardia lamblia in children in Pakistan (2018).

Prepared from the sources listed below by the MedInfoBase team. Last reviewed 16 October 2026. This page is general health information, not medical advice. It gives no doses and does not replace a doctor: talk to your doctor or pharmacist before starting, stopping or changing any medicine. How we write and check our pages.

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