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Tuesday, August 12, 2025

Giardiasis


Definition

Giardiasis is an intestinal infection caused by the protozoan parasite Giardia duodenalis (also called G. lamblia or G. intestinalis). It is one of the most common causes of waterborne parasitic diarrhoea worldwide and can affect people of all ages. The infection may be asymptomatic or present with acute or chronic gastrointestinal symptoms.

Etiology and Transmission

  • Causative organism: Giardia duodenalis, a flagellated protozoan parasite

  • Transmission routes:

    • Fecal–oral: ingestion of infective cysts from contaminated water, food, or hands

    • Person-to-person spread in households, childcare centres, institutions

    • Sexual transmission (particularly oral–anal contact)

  • Reservoirs: Humans, domestic animals, wildlife (beavers, dogs, cats)

  • Infective dose: Very low – ingestion of 10–25 cysts can cause infection

  • Incubation period: 1–3 weeks after exposure

Life Cycle and Pathophysiology

  • Infective cysts are ingested and survive gastric acid

  • In the small intestine, they excyst to form trophozoites, which attach to the mucosa via a ventral adhesive disc

  • Trophozoites multiply by binary fission, interfering with absorption of fats and carbohydrates

  • Some trophozoites encyst in the colon and are excreted in faeces, ready to infect a new host

  • The infection can cause malabsorption due to villous atrophy and epithelial damage

Risk Factors

  • Drinking untreated surface water (campers, hikers)

  • Poor sanitation and hygiene

  • Living in or travelling to endemic regions

  • Attending or working in childcare settings

  • Immunodeficiency (especially IgA deficiency)

  • Close contact with infected individuals or animals

Clinical Features

1. Asymptomatic infection – common, especially in endemic areas

2. Acute giardiasis:

  • Onset: Gradual or sudden

  • Diarrhoea – foul-smelling, greasy, floating stools (steatorrhoea)

  • Abdominal cramps, bloating, flatulence

  • Nausea, vomiting (less common)

  • Low-grade fever (occasionally)

3. Chronic giardiasis:

  • Recurrent or persistent diarrhoea

  • Malabsorption → weight loss, failure to thrive in children

  • Fatigue, malaise

  • Lactose intolerance due to mucosal injury

Complications

  • Chronic diarrhoea and weight loss

  • Growth retardation in children

  • Secondary lactose intolerance

  • Rarely, reactive arthritis or post-infectious irritable bowel syndrome

Diagnosis

Laboratory methods:

  • Stool examination: Microscopy for cysts/trophozoites – may require 3 specimens on separate days due to intermittent shedding

  • Stool antigen tests: Enzyme immunoassay (EIA) or direct fluorescent antibody (DFA) – more sensitive than microscopy

  • PCR assays: High sensitivity, can detect multiple pathogens simultaneously

  • Duodenal aspirates or biopsies – rarely needed, used in difficult cases

Differential Diagnosis

  • Other protozoal infections (Entamoeba histolytica, Cryptosporidium)

  • Bacterial gastroenteritis (Salmonella, Shigella, Campylobacter)

  • Viral gastroenteritis (rotavirus, norovirus)

  • Malabsorption syndromes (celiac disease, tropical sprue)

Management

Treatment is indicated for all symptomatic cases and for certain asymptomatic individuals (e.g., food handlers, immunocompromised patients, household contacts of children in childcare).

First-line Treatment for Adults

  • Metronidazole 250 mg orally three times daily for 5–7 days

    • Alternative dose: 500 mg twice daily for 7 days

Alternative Regimens for Adults

  • Tinidazole 2 g orally as a single dose

  • Ornidazole 1.5 g orally once daily for 1–2 days

  • Nitazoxanide 500 mg orally twice daily for 3 days

Paediatric Dosing

  • Metronidazole 15 mg/kg orally three times daily (max 250 mg/dose) for 5–7 days

  • Tinidazole 50 mg/kg orally as a single dose (max 2 g)

  • Nitazoxanide:

    • 1–3 years: 100 mg orally twice daily for 3 days

    • 4–11 years: 200 mg orally twice daily for 3 days

Notes on Treatment

  • Tinidazole offers better compliance due to single-dose regimen

  • Metronidazole is widely used and cost-effective but may cause metallic taste, nausea, and disulfiram-like reaction with alcohol

  • In treatment failures: repeat therapy with an alternative drug or combination (e.g., albendazole plus metronidazole)

  • For chronic or refractory cases, assess for reinfection, compliance, and immunodeficiency

Supportive Care

  • Maintain hydration and electrolyte balance

  • Nutritional support, including lactose-free diet if secondary lactose intolerance occurs

  • Education on hygiene and prevention to avoid reinfection

Prevention

  • Boil or filter drinking water when in endemic areas or wilderness

  • Practise good hand hygiene, especially after using toilet or handling nappies

  • Avoid swallowing water while swimming in untreated water sources

  • Wash raw fruits and vegetables before consumption

  • Exclude infected individuals from food handling or childcare duties until cleared

Prognosis

  • Most patients respond well to appropriate antimicrobial therapy

  • Chronic or relapsing infection can occur, especially if reinfection risk is high or immune function is impaired

  • Effective preventive measures can significantly reduce transmission in communities




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