Giardia infection - including symptoms, treatment and prevention

Giardia infection is an infection of the bowel caused by the parasite Giardia duodenalis, also known as Giardia lamblia or Giardia intestinalis. It is a single-celled organism found worldwide.

Although Giardia infection occurs in many animals including dogs, cats, sheep and cattle, there is still some uncertainty about the extent of disease transmission between people and animals.

How Giardia is spread

Spread takes place when hands, objects or food become contaminated with faeces of infected people or animals, or by drinking contaminated water. The parasites must be taken in by mouth to cause infection. In institutions and preschool centres, person-to-person transmission may be a significant means of spreading the illness. Transmission can occur with some sexual practices where there is contact with faecal matter. Re-infection can occur.

Signs and symptoms 

While some people have no symptoms, the most common symptoms include:

  • stomach cramps
  • excessive gas or bloating
  • diarrhoea, which may be watery, usually lasting 1 to several weeks
  • frequent loose or pale, greasy faeces which may float in the toilet bowl
  • fatigue
  • weight loss
  • lactose intolerance may occur in 20 to 40% of cases and last several weeks.

Giardia infections do not usually cause fever or bloody diarrhoea.


The infection is diagnosed by examining the faeces by a microscope or using a PCR (polymerase chain reaction) test in a pathology laboratory.

Incubation period

(time between becoming infected and developing symptoms)

3 to 25 days or longer (usually 7 to 10 days).

Infectious period

(time during which an infected person can infect others)

For as long as the organism is present in the faeces (often months), whether or not the person is ill. A person with diarrhoea is more likely to spread infection, but an infected person without symptoms can still spread the parasite to others.


Treatment of an ill person with appropriate antibiotic medication relieves symptoms and usually makes the person non-infectious within a few days.


  • Exclude people with Giardia infection from childcare, preschool, school and work until there has been no diarrhoea for at least 24 hours. If working as a food handler in a food business, the exclusion period should be until there has been no diarrhoea or vomiting for 48 hours.
  • Infants, children and adults with Giardia infection should not swim until there has been no diarrhoea for 24 hours.
  • Follow good hand washing procedures.
  • Water suspected of contamination should be boiled before drinking.
  • Babies and small children without diarrhoea who are not toilet trained should wear tight fitting waterproof pants or swimming nappies in swimming pools and be changed regularly in the change room. When faecal accidents occur, swimming pools should be properly disinfected.
  • Treatment of infected people reduces spread.

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