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Giardia is a parasite that can infect your bowels (intestines) and cause giardiasis which can lead to sudden-onset (acute) or persistent (chronic) diarrhoea. Infection often occurs after drinking contaminated water containing the parasite, or eating food prepared or washed using contaminated water. Those most at risk of giardial infection include travellers to areas where giardia is common (countries with poor sanitation). Treatment includes antibiotic medicines and drinking plenty of fluids to prevent lack of body fluid (dehydration).

A parasite is a general term for any living thing that lives in, or on, another living organism. It may feed off its host, or obtain shelter using its host but contributes nothing to its host's well-being or welfare. Human parasites include fungi, protozoa and worms.

Giardia intestinalis (G. intestinalis) - also known as Giardia lamblia or Giardia duodenalis - is a living thing that lives in, or on, another living organism (a parasite). It can infect your bowels (intestines) and cause giardiasis. So, giardiasis is an infection of your bowels (a gastroenteritis infection). G. intestinalis is a protozoan parasite. A protozoan is a microscopic, single-celled organism.

Giardia is a living thing that lives in, or on, another living organism (a parasite) and can infect the bowels (intestines) of both humans and animals. 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 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 2013, 3,624 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.

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 lack of body fluid (dehydration)

If diarrhoea caused by giardial infection is severe, dehydration can occur. You should consult a doctor quickly if you suspect that you or your child are becoming dehydrated. Mild dehydration is common and is usually easily reversed by drinking lots of fluids. Severe dehydration can be fatal unless quickly treated because the organs of your body need a certain amount of fluid to function.

  • Symptoms of dehydration in children include: passing little urine, a dry mouth, a dry tongue and lips, fewer tears when crying, sunken eyes, weakness, being irritable or lacking in energy (lethargic).
  • Symptoms of severe dehydration in children include: drowsiness, pale or mottled skin, cold hands or feet, very few wet nappies, fast (but often shallow) breathing. This is a medical emergency and immediate medical attention is needed.
  • Symptoms of dehydration in adults include: tiredness, dizziness or light-headedness, headache, muscular cramps, sunken eyes, passing little urine, a dry mouth and tongue, weakness, and becoming irritable.
  • Symptoms of severe dehydration in adults include: profound loss of enthusiasm (apathy), weakness, confusion, rapid heart rate, coma, and producing very little urine. This is a medical emergency and immediate medical attention is needed.

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 as described above. 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 of giardia is with antibiotics. Metronidazole is the antibiotic that is most commonly used. An alternative that may be used is tinidazole.

Fluids for adults

Drinking plenty of fluids is also important if you have giardial infection. The aim is to prevent lack of body fluid (dehydration), or to treat dehydration if it has developed. (Note: if you suspect that you are dehydrated, you should contact a doctor.)

  • As a rough guide, drink at least 200 mls after each bout of diarrhoea (after each watery stool).
  • This extra fluid is in addition to what you would normally drink. For example, an adult will normally drink about two litres a day but more in hot countries. The above '200 mls after each bout of diarrhoea' is in addition to this usual amount that you would drink.
  • If you are sick (vomit), wait 5-10 minutes and then start drinking again but more slowly. For example, a sip every 2-3 minutes but making sure that your total intake is as described above.
  • You will need to drink even more if you are dehydrated. A doctor will advise on how much to drink if you are dehydrated.

For most adults, fluids drunk to keep hydrated should include water, fruit juice and soups. It is best not to have drinks that contain a lot of sugar, such as cola or pop, as they can sometimes make diarrhoea worse.

Rehydration drinks are usually recommended in people who are frail, or over the age of 60, or who have underlying health problems. They are made from sachets that you can buy from pharmacies. (The sachets are also available on prescription.) You add the contents of the sachet to water. Rehydration drinks provide a good balance of water, salts and sugar. The small amount of sugar and salt helps the water to be absorbed better from the gut (intestine) into the body. They do not stop or reduce diarrhoea. As with children (see below) do not use home-made salt/sugar drinks, as the quantity of salt and sugar has to be exact.

Fluids for children to prevent dehydration  

You should encourage your child to take plenty of fluids. The aim is to prevent dehydration. The fluid lost in their diarrhoea needs to be replaced. Your child should continue with their normal diet and usual drinks. In addition, they should also be encouraged to drink extra fluids. However, avoid fruit juices or fizzy drinks as these can make diarrhoea worse.

Babies under 6 months old are at increased risk of dehydration. You should seek medical advice if they develop bowel infection (gastroenteritis). Breast- or bottle-feeds should be encouraged as normal. You may find that your baby's demand for feeds increases. You may also be advised to give extra fluids (either water or rehydration drinks) in between feeds.

Rehydration drinks may be suggested by a healthcare professional for children at increased risk of dehydration. They are made from sachets available from pharmacies and on prescription. You should be given instructions about how much to give. Rehydration drinks provide a perfect balance of water, salts and sugar. The small amount of sugar and salt helps the water to be absorbed better from the gut into the body. Home-made salt/sugar mixtures are used in developing countries if rehydration drinks are not available. However, they have to be made carefully, as too much salt can be dangerous to a child. Rehydration drinks are cheap and readily available in the UK and are the best treatment for your child.

If your child vomits, wait 5-10 minutes and then start giving drinks again but more slowly (for example, a spoonful every 2-3 minutes). Use of a syringe can help in younger children who may not be able to take sips. (Note: if you suspect that your child is dehydrated, or is becoming dehydrated, you should seek medical advice.)

Fluids to treat dehydration in children

If your child is mildly dehydrated, this may be treated by giving them rehydration drinks. Read the instructions carefully for advice about how to make up the drinks and about how much to give. This can depend on the age and the weight of your child. If you are breast-feeding, you should continue with this during this time. Otherwise, don't give your child any other drinks unless the doctor or nurse has said that this is OK. It is important that your child should be rehydrated before they have any solid food.

Sometimes a child may need to be admitted to hospital for treatment if they are dehydrated. Treatment in hospital usually involves giving rehydration solution via a special tube called a nasogastric tube. This tube passes through your child's nose, down their throat and directly into their stomach. An alternative treatment is with fluids given directly into a vein (intravenous fluids).

Eat as normally as possible

In children, correcting any dehydration is the first priority when they have giardial infection. However, if your child is not dehydrated (most cases) or, once any dehydration has been corrected, then encourage your child to have their normal diet. Do not 'starve' your child. This used to be advised but is now known to be wrong. So:

  • Breast-fed babies should continue to be breast-fed if they will take it. This will usually be in addition to extra rehydration drinks (described above).
  • Bottle-fed babies should be fed with their normal full-strength feeds if they will take them. Again, this will usually be in addition to extra rehydration drinks (described above).
  • Older children - offer them some food every now and then. However, if he or she does not want to eat, that is fine. Drinks are the most important and food can wait until their appetite returns.

For adults, it is also advised not to 'starve' yourself but to eat small, light meals if you can. Be guided by your appetite. You may not feel like food and most adults can do without food for a few days. Eat as soon as you are able but don't stop drinking. If you do feel like eating, avoid fatty, spicy or heavy food. Plain foods such as wholemeal bread and rice are good foods to try eating.

If you or your child have giardia, the following are recommended to prevent the spread of infection to others:

  • Wash your hands thoroughly after going to the toilet. Ideally, use liquid soap in warm running water but any soap is better than none. Dry properly after washing. If your child wears nappies, be especially careful to wash your hands after changing nappies and before preparing, serving, or eating food.
  • If a potty has to be used, wear gloves when you handle it, dispose of the contents into a toilet, then wash the potty with hot water and detergent and leave it to dry.
  • Don't share towels and flannels.
  • Don't prepare or serve food for others.
  • If clothing or bedding is soiled, first remove any stools (faeces) into the toilet. Then wash in a separate wash at as high a temperature as possible.
  • Regularly clean with disinfectant the toilets that you use. With hot water and detergent, wipe the flush handle, toilet seat, bathroom taps, surfaces and door handles at least once a day. Keep a cloth just for cleaning the toilet (or use a disposable one each time).
  • You should stay off work, school, college, etc, while you have giardial infection. Your doctor will advise you when it is safe to return. Avoid contact with other people as far as possible during this time.
  • Food handlers: if you work with food and develop diarrhoea or being sick (vomiting), you must inform your employer and immediately leave the food-handling area. If giardia is confirmed, you should inform your employer and stay away from work until your doctor advises it is safe to return.

If you have giardial infection and you work with vulnerable groups of people such as the elderly, the unwell or the young, you should inform your employer.

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). If complications do occur, they can include:

  • Salt (electrolyte) imbalance in your body and lack of body fluid (dehydration). This occurs if the salts and water that are lost in your stools (faeces) are not replaced by your drinking adequate fluids. If you can manage to drink plenty of fluids then dehydration is unlikely to occur, or is only likely to be mild and will soon recover as you drink. Severe dehydration can lead to a drop in your blood pressure. This can cause reduced blood flow to your vital organs. If dehydration is not treated, kidney failure may also develop. Some people who become severely dehydrated need a 'drip' of fluid directly into a vein. This requires admission to hospital.
  • Lactose intolerance can sometimes occur for a period of time after giardial infection. This is known as 'secondary' or 'acquired' lactose intolerance. Your gut (intestinal) lining can be damaged by the episode of bowel infection (gastroenteritis). This leads to lack of a chemical (enzyme) called lactase that is needed to help your body digest a sugar called lactose in milk. Lactose intolerance leads to bloating, tummy (abdominal) pain, wind and watery stools after drinking milk. The condition gets better when the infection is over and the gut (intestinal) lining heals. It is more common in children.
  • 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.
  • Irritable bowel syndrome is sometimes triggered by giardial infection.
  • Gallbladder infection, infection of the pancreas (pancreatitis), arthritis and some types of eye problems (for example, retinal arteritis and iridocyclitis) can also occur.
  • Reduced effectiveness of some medicines. During any episode of diarrhoea, certain medicines that you may be taking for other conditions or reasons may not be as effective. This is because the diarrhoea means that reduced amounts of the medicines are taken up (absorbed) into your body. Examples of such medicines are those for epilepsy, diabetes and contraception. Speak to your doctor or practice nurse if you are unsure of what to do if you are taking other medicines and have diarrhoea.

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.

Hand washing is the most important thing that you and your child can do. In particular, always wash your hands and dry them thoroughly; teach children to wash and dry theirs:

  • After going to the toilet (and after changing nappies or helping an older child to go to the toilet).
  • Before preparing or touching food or drinks.
  • Before eating.

If you smoke, you should also wash your hands before smoking. The simple measure of washing and drying hands regularly and properly is known to make a big difference to the chance of developing many infections.

In addition to this, when travelling to areas with poor sanitation, you should avoid eating or drinking the following to help reduce your chance of developing giardial infection. This is because giardia is often passed on by drinking contaminated water. Avoid:

  • Tap water.
  • Ice cream.
  • Ice cubes.
  • Shellfish.
  • Eggs.
  • Salads.
  • Raw or undercooked meat.
  • Fruit that has already been peeled.
  • Mayonnaise or sauces.

Sealed bottled water, tea, coffee and alcohol are thought to be safe to drink. Also, avoid swimming in water that may be contaminated with giardia - for example, lakes or rivers.

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.

Further help & information

Original Author:
Dr Michelle Wright
Current Version:
Peer Reviewer:
Dr Laurence Knott
Document ID:
12504 (v4)
Last Checked:
Next Review:
The Information Standard - certified member
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