Giardiasis

This is a 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 (1,2,3,4,5).

• up to 15% of individuals infected with G duodenalis are asymptomatic
• if the patient is symptomatic then common presentations are
  • diarrhoea (>90%),
  • flatulence (75%),
  • abdominal pain (78%), and bloating
  • less common symptoms include vomiting and fever
  • bloody diarrhea is uncommon (<12%) and the clinician should consider alternative or concurrent pathology with this presentation
  • acute symptoms may resolve spontaneously within 1 to 4 weeks

If left untreated in the acute setting, some patients may progress to chronic giardiasis

• chronic cases (lasting more than 6 weeks) more often present with intermittent, waxing, and waning diarrhoea
• is associated with malabsorption, particularly of carbohydrate and fat
• irritable bowel syndrome (IBS) and chronic fatigue have been reported as sequelae of human giardiasis (5)
• life-long consequences, including impaired physical and cognitive development have been reported in G. intestinalis-infected young children

Summary:

Reservoir:

• Gastrointestinal tracts of humans and animals

Epidemiology

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

Transmission:

• 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

Infectivity:

• 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

Notes:

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

Reference:

1. Center for Disease Control and Prevention. CDC Yellow Book 2024: health information for international travel. Section 5: travel-associated infections & diseases - ​giardiasis. May 2023 [internet publication].
2. Centers for Disease Control and Prevention. Giardia: about Giardia infections. May 2024 [internet publication].
3. PHE (2019). Recommendations for the Public Health Management of Gastrointestinal Infections.
4. Stephen T, Tobin EH, Juergens AL. Giardiasis. [Updated 2025 Dec 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-.
5. Salinas-Varas C et al. Giardia intestinalis trophozoites activate human PMN and induce NET formation but dampen neutrophil ROS production. Front Immunol. 2026 Feb 20;17:1724948.