Giardia infection (giardiasis)

Giardia infection (giardiasis) is a form of gastroenteritis (gastro). Symptoms including diarrhoea and stomach cramps usually appear between 7 and 10 days after infection. Good hygiene can help stop it spreading.

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What is giardia?

Giardia is a parasite found in the gut of infected humans and animals such as cattle, sheep, cats, dogs, rats and possums. It is passed on in the faeces (poo, tūtae) of infected humans and animals. People become infected when they swallow the parasites, usually in contaminated water. Some people can get very sick from giardia.

Giardia can live in the environment for long periods, especially in lakes, rivers, streams and roof water. There is no way of telling by taste, sight or smell if soil, food, water or a surface has giardia parasites.

How is giardia infection spread?

You get infected when you swallow the parasites. This is commonly by drinking contaminated water or touching your mouth with contaminated hands. Once a person or animal has giardia, they pass on the parasites in their faeces (poo). The parasites can contaminate (make unsafe) soil, food or water, or surfaces such as toys, bathroom taps or doors, and nappy change tables.

Not everyone with giardia gets unwell — it is possible to be a carrier and spread the disease without being ill.

Who is most at risk of giardia infection?

Anyone can become infected with giardia. However, some situations do increase your risk of picking up the infection.

People at risk of giardia infection 
  • People in childcare settings.
  • People who are in close contact with someone who has the disease.
  • Travelers within areas that have poor sanitation.
  • People who have contact with poo during sexual activity.
  • Backpackers or campers who drink untreated water from springs, lakes, or rivers
  • Swimmers who swallow water from swimming pools, hot tubs, interactive fountains, or untreated. recreational water from springs, lakes, or rivers.
  • People with weakened immune systems.
  • People who have contact with infected animals or animal environments contaminated with poo.

Note: The chances of people getting a giardia infection from dogs or cats are small. The type of giardia that infects humans is usually not the same type that infects dogs and cats. 

What are the symptoms of giardia infection?

Giardia infection is a form of gastroenteritis (gastro). The symptoms of giardia infection include:

  • runny poo (diarrhoea) is the most common
  • stomach cramps or tummy pain
  • feeling sick or being sick (nausea or vomiting)
  • gas and bloating.

Giardia symptoms appear between 3 and 25 days (usually 7 to 10 days) after becoming infected. You can be ill for 3–4 days, then feel better, then the symptoms may come back. If you don’t get treatment, this can continue and you can be infectious for months.

How is giardia diagnosed?

Most cases of gastro do not require special investigations. If your symptoms are severe or ongoing, or your doctor suspects you might have giardia, you may be asked to provide a stool (poo) sample. This can be tested in the laboratory for giardia and other germs.

Giardia is a notifiable disease. This means that if you are found to have giardia, your doctor will notify the Public Health Service. Someone from your local public health team may contact you to find out how you picked up the parasite. This helps them trace the source of infection to reduce the risk of a large outbreak.

How is giardia infection treated?

Most people with giardia will get better without treatment. If your doctor decides that you do need treatment,  you may be prescribed antibiotics such as ornidazole or metronidazole, which kill the parasite in your poo and prevent the spread of the infection. 

It is also important to drink plenty of fluids often to prevent dehydration, especially if you have diarrhoea or have been sick.  Older adults and children are most at risk of dehydration. Read more about dehydration.

Medicines for diarrhoea, such as loperamide and Diastop®, are not routinely recommended. They can make things worse as it can prolong the diarrhoea and there is the risk of the serious complication, toxic megacolon. Medicines for diarrhoea are also not recommended for use in children under 12 years. Before taking them, check with your doctor whether they are safe for you. 

Many people develop a temporary milk intolerance after giardiasis so you may find it best to not consume milk and lactose-containing foods for approximately one month after treatment.

How long do I need to take off work or school?

Stay away from school, early childhood centres or work until 48 hours (2 days) after the symptoms have gone. However, as giardia infection is a notifiable disease (see diagnosis above) you will need to check with your doctor first.

Do not swim in pools (private or public) for at least 2 weeks after your symptoms have gone.

How can giardia infection be prevented?

The best way to prevent giardia infection is to practice good hygiene by the following:

  • Washing your hands frequently and properly after going to the toilet, handling animals, changing nappies, and before preparing food and drinks.
  • Clean, sanitize, or disinfect toys and surfaces.
  • At the pool, lake, and other swim areas, do not swallow the water. When tramping or walking in parks, farms and national reserves, check if the streams or rivers are known to have any giardia.
  • Avoid drinking unboiled tap water and eating uncooked foods when travelling in countries where the water supply may be unsafe.

Learn more

Cryptosporidium and giardia Ministry of Health, Health Promotion Agency


  1. Giardia Auckland Regional Public Health Service
  2. Parasites - Giardia Centers for Disease Control and Prevention 
Credits: Editorial team. Last reviewed: 21 Sep 2021