What is Giardia?
Giardia lamblia (also called Giardia duodenalis or Giardia intestinalis) is a flagellated protozoan parasite that infects the small intestine and causes giardiasis, one of the most common intestinal parasitic infections worldwide with an estimated 2.8 million human infections annually 1.
The Organism
Giardia exists in two forms during its life cycle:
- Trophozoites: The active, multiplying form that lives and reproduces in the duodenum and small intestine 2
- Cysts: The dormant, infectious form that is excreted in feces and can survive in the environment 2
The parasite is considered an "early-branching" eukaryote with unique evolutionary characteristics, notably lacking mitochondria while having developed alternative pathways to perform some mitochondrial functions 3.
Transmission and Risk Factors
Giardia spreads through the fecal-oral route via 1:
- Contaminated water (most common - waterborne transmission)
- Contaminated food (foodborne transmission)
- Person-to-person contact
High-risk populations include 1:
- Children in daycare settings and daycare workers
- Travelers to endemic areas (traveler's diarrhea)
- Institutionalized individuals
- People consuming contaminated or recreational water
- Immunocompromised individuals
- Patients with cystic fibrosis
- Those engaging in oral-anal sexual contact
Clinical Presentation
The infection shows variable clinical manifestations:
- 50-75% of infected children are completely asymptomatic 1
- Symptomatic patients present with acute or chronic diarrhea
- Classic symptoms include: watery diarrhea (without blood), abdominal cramps, bloating, and flatulence 2
- Chronic infection can contribute to malnutrition, stunting, and cognitive impairment in children in developing regions 3
Public Health Significance
Giardia is a cosmopolitan pathogen with a very wide host range affecting both humans and numerous domestic and wild animal species 4. It represents an important cause of daycare center outbreaks and is the most frequently identified enteric parasite in the United States 5.