Authored by , Reviewed by Dr Hayley Willacy | Last edited | Meets Patient’s editorial guidelines

Giardia intestinalis, sometimes referred to as G. intestinalis, is a living thing that lives in, or on, another living organism (a parasite). It can infect your bowels and cause giardiasis.

Giardiasis is an infection of your bowels (a gastroenteritis infection). G. intestinalis is a protozoan parasite. A protozoan is a microscopic, single-celled organism.

An infected individual will pass out the giardia in their stools (faeces). If water, food or drinks are contaminated by the infected faeces, giardia can be passed on to others. Transmission is often via contaminated drinking water containing the parasite. Giardia are not killed by standard chlorination of drinking water. If there are deficiencies in water filtration, or if there is sewage contamination of water, giardial infection can be transmitted.

You may also become infected through direct contact with someone who has giardia. This is known as faecal-oral transmission of the infection. For example, the infected person may have the parasite on their hands after going to the toilet. When you come into contact with that person, they can pass the parasite on to your hands. The parasite can then enter through your mouth and infect your intestines. This means of transmission of infection is particularly common in young children. Spread within families in this way is also common.

Sexual activity involving contact with another person's back passage (anus) is another way of acquiring giardial infection. It can also be caught by a person swimming in contaminated water (for example, in lakes or rivers).

Giardial infection is a common cause of infective diarrhoea throughout the world. As a living thing that lives in, or on, another living organism (a parasite), it is very commonly found in countries where sanitation is poor. However, it can also be found in the developed world and outbreaks have occurred in day centres and institutions.

It is the most common gut (intestinal) parasite infection in the UK. In 2017, 4,793 cases of giardia were reported in England and Wales. Those most at risk of infection include travellers to areas where giardia is common (countries with poor sanitation), young children, men who have sex with men, and people with weakened immune systems for whatever reason.

It is estimated that, in the developing world, around 1 in 5 young children have giardial infection.

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In many people, there are no symptoms following infection with giardia and the infection is usually cleared by the immune system. If symptoms do occur, they can come on one to two weeks after first contact with the giardia. Symptoms of giardial infection can vary from person to person but can generally be divided into sudden-onset (acute) and persistent (chronic) diarrhoea symptoms.

Acute diarrhoea symptoms

Acute diarrhoea is the most common presentation of giardial infection. There is a sudden onset of diarrhoea which can be explosive and watery. It is not bloody and does not contain mucus. You may also experience crampy tummy (abdominal) pain and bloating and you may pass lots of wind. Symptoms usually settle down after some days but diarrhoea can continue for more than a week. Being sick (vomiting) and high temperature (fever) are uncommon.

Chronic diarrhoea symptoms

In some people, symptoms may be less acute in their onset. A more chronic diarrhoea can develop which may be persistent or may come and go. Stools (faeces) tend to be smelly and fatty/greasy. Diarrhoea can last for between 2-6 weeks, sometimes longer. Tiredness, feeling sick (nausea), reduced appetite and weight loss can occur as well as belching, bloating and heartburn. Vitamin deficiencies can also develop in chronic infection. Chronic giardial infection can last for months, or even years, if it goes undiagnosed and untreated.

Symptoms of dehydration

If diarrhoea caused by giardial infection is severe, lack of fluid in the body (dehydration) can occur.

Giardial infection is usually diagnosed when giardia is found in your stool (faeces) after a stool sample is sent to the laboratory. Ideally, three stool specimens from different days should be examined because of potential variations in the excretion of the giardia in your stools.

Giardial infection should be considered if you develop sudden-onset (acute) or persistent (chronic) diarrhoea and you have recently travelled abroad or are in one of the 'at-risk' groups for infection. You should visit your doctor so that they can examine you and arrange for you to send off a stool sample.

Antibiotic medicines

The usual treatment for giardia is with antibiotics. Metronidazole is the antibiotic that is most commonly used. An alternative that may be used is tinidazole.

Fluids and food

You should follow the advice given for other causes of traveller's diarrhoea.

Most people who have giardial infection make a full recovery and do not have any complications or further problems. However, there is a risk of re-infection if preventative measures are not followed (see below). Complications are similar to those for other causes of traveller's diarrhoea.

Other complications specific to giardial infection include:

  • Weight loss - can occur in persistent (chronic) giardial infection.
  • Growth restriction in children - can occur in chronic giardial infection. This is especially a problem in developing countries where the condition may not be picked up and treated.
  • Malabsorption and vitamin deficiencies - are possible in chronic giardial infection. This is because the gut infection can interfere with the absorption of essential nutrients from the food that you eat.
  • Gallbladder infection and infection of the pancreas (pancreatitis) - can also occur.

In general, good hygiene is essential to prevent the spread of many infections to others and to reduce your chance of picking up infections from others.

In addition to this, when travelling to areas with poor sanitation, you should be careful what you eat or drink.

Sexual activity involving contact with the back passage (anus) is particularly risky. If your fingers come into contact with another person's anus during sex, or you touch a condom used in anal sex, make sure you wash your hands thoroughly. Contact between the mouth and the anus (sometimes called 'rimming') is very risky and should be avoided.


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