Last edited 02/2020 and last reviewed 05/2020

This is a chronic diarrhoeal disease that is due to the flagellate protozoal parasite, Giardia spp. Giardia duodenalis (syn. Giardia lamblia; syn. Giardia intestinalis), which attaches to, but does not invade, the small bowel.

This disease is associated with malabsorption, particularly of carbohydrate and fat. Presentation is of chronic persistent, or recurrent diarrhoea with very offensive stools, abdominal distension and weight loss. Children may show failure to thrive from lack of fat soluble vitamins.



  • Gastrointestinal tracts of humans and animals


  • Cases may be associated with recent foreign travel
  • Family clusters are common


  • Faecal-oral spread, by direct or indirect contact with the faeces of infected people or animals:
    • person-to-person spread is common, particularly within families/households
    • waterborne, including swimming in contaminated recreational water
    • direct contact with infected animals
    • foodborne transmission
    • sexual transmission, particularly amongst MSM
    • direct contact with infected animals (rare)
  • Outbreaks have been associated with infected food handlers, drinking water and swimming pools

Incubation period:

  • Usually 5-16 days (median 7-10 days); extremes of 1-28 days reported


  • whilst symptomatic and for up to 2 weeks after symptoms have stopped
  • risk of transmission to others decreases after symptoms have stopped, but cysts continue to be shed after symptoms have stopped. Since cysts are resistant to normal chlorine levels used in swimming pools, cases should not go swimming for 2 weeks after symptoms have stopped due to the potential to contaminate the pool environment and cause onward transmission


  • cysts are moderately resistant to disinfection with levels of chlorination usually used in drinking water treatment and swimming pools


  • PHE (2019). Recommendations for the Public Health Management of Gastrointestinal Infections