Giardiasis

Giardiasis is a common parasitic infection caused by the protozoan Giardia duodenalis. Transmission usually occurs via the fecal-oral route (e.g., from contaminated drinking water) when traveling or living in an endemic region. Giardia exist in two states: as active trophozoites in the human body and as infectious cysts surviving in various environments. Following the ingestion of the cyst, individuals may experience abdominal cramps and fatty diarrhea. Diagnosis of giardiasis involves analyzing stool using direct fluorescent antibody testing and microscopic confirmation of cysts or trophozoites. Treatment consists of supportive care and antibiotic therapy (e.g., tinidazole).

• Giardia duodenalis (formerly Giardia lamblia) is widespread throughout the world and affects ∼ 200 million people per year worldwide.
• Incidence: estimated 5–8/100,000 per year in the US
• In the US, giardiasis is the most common intestinal disease caused by parasites.

Epidemiological data refers to the US, unless otherwise specified.

• Pathogen: Giardia duodenalis (formerly known as Giardia lamblia and Giardia intestinalis), a protozoan ^[1]
• Transmission ^[2]
  • Waterborne: from drinking recreational water (e.g., lakes, rivers, ponds, swimming pools)
    • Swallowing cysts in contaminated water → entry of Giardia into the gastrointestinal tract
    • Most commonly affects hikers or campers
  • Fecal-oral (e.g., through food handlers, people in daycare and nurseries, oral-anal sexual contact): Giardia cysts are passed into the environment from the feces of infected people and animals. ^[2][3]
  • Infection is more likely to occur after traveling to endemic regions such as the tropics, subtropics, and North American mountain regions.
• Incubation period: 1–3 weeks ^[4]

• Life cycle ; ^[5][6]
  • Giardia have 2 stages in the life cycle.
    1. Trophozoite: active form of the pathogen that multiplies, lives within the host's body
       • Morphology: long oval shape with two nuclei and four pairs of flagella that resemble a kite
    2. Cysts: excreted, infectious form of the pathogen, able to survive in moist environments
       • Morphology: oval, four nuclei
  • Ingestion of cysts → excystation and conversion to trophozoite form → rapid multiplication, adhesion to intestinal walls → encystation in large bowel → excretion of cysts → possible reinfection
• Mechanism ^[7]
  • Although several theories exist, it is commonly suspected that infection with Giardia leads to impaired function and structure of intestinal tissue , resulting in malabsorption and diarrhea.
  • IgA deficiencies (e.g., selective IgA deficiency, X-linked agammaglobulinemia, common variable immunodeficiency) increases susceptibility to giardiasis because of the disruption of gastrointestinal protective barrier. ^[8][9][10]

Patients with giardiasis may be asymptomatic or have significant symptoms, including: ^[1][4]

• Diarrhea: foul-smelling, voluminous, occasionally watery, and fatty stools (stools tend to float and do not appear bloody)
• Excessive gas (flatulence, bloating), abdominal pain, and cramps
• Fatigue, malaise
• Nausea and vomiting
• Anorexia
• Weight loss
• Dehydration

Test all patients with prolonged diarrhea and recent exposure to routes of transmission (see “Etiology” section). ^[11]

• Stool diagnostic studies ^[1][11]
  • Direct fluorescent antibody testing (gold standard): microscopic detection of Giardia antigens in stool with immunofluorescence
  • Stool microscopy: microscopic confirmation of cysts or multinucleated trophozoites ^[1]
  • PCR: typically tests for a range of GI infectious pathogens, including Giardia
• Gastroduodenoscopy: confirms trophozoites in duodenal fluids; may be considered if stool diagnostics are negative ^[12]

Giardiasis is a notifiable disease in the US. ^[4]

• See “Etiology of acute and persistent diarrhea.”
• See “Etiology of chronic diarrhea.”
• See “Viral gastroenteritis.”
• See “Bacterial gastroenteritis.”
• See “Intestinal protozoa.”
• See “Helminths.”
• See “Differential diagnosis of acute abdominal pain.”

The differential diagnoses listed here are not exhaustive.

Initiate antibiotic therapy and provide supportive therapy for gastroenteritis for symptomatic patients. ^[4]

• First line ^[12]
  • Tinidazole : preferred due to high cure rates and fewer side effects than metronidazole ^[4][13]
  • OR nitazoxanide ^[4]
• Second line: metronidazole (off-label) ^[4]
• For pregnant individuals ^[11]
  • Mild disease: Delay antibiotic treatment until after delivery.
  • Moderate-to-severe disease: paromomycin (off-label) ^[11]

For asymptomatic patients, seek specialist advice, as treatment may not be warranted if the patient is living in an endemic area and risk of rapid reinfection is high. ^[14]

• Acute: weight loss, secondary lactose intolerance ^[4]
• Chronic: postinfectious irritable bowel syndrome, reactive arthritis ^[1]
• In children: malnutrition, developmental delay ^[1]

We list the most important complications. The selection is not exhaustive.

• See “Food and water precautions.”
• See “Prevention of infectious gastroenteritis.”