Giardia duodenalis

• A member of the Giardia that causes intestinal disease (giardiasis) in humans and animals
• Also known as beaver fever

Background

Microbiology

• Flagellated protozoan that infects the small bowel
• Host specificity related to genotype ("assemblage")
  • Assemblages A & B infect humans, but also numerous non-human hosts including primates, dogs, cats, cattle, sheep, deer, rodents, horses, and beavers
  • Assemblages C & D affect dogs; E affects cattle, goats, sheep, and pigs; F affects cats; G affects rodents; and H affects marine vertebrates
• Antigenic variation is determined by the variant-specific surface protein (VSP)
  • Replaced every few generations

Life Cycle

• A cyst is ingested via fecal-oral contamination
• In the small bowel, the cyst releases two trophozoites (free-living form) via excystation
• Trophozoites multiply by binary fission
• When they reach the large bowel, they encyst

Epidemiology

• Worldwide distribution
• Generally transmitted fecal-oral route (via environment), but person-to-person transmission is possible in daycares
• Numerous animal reservoirs, not fully understood how important they are
• Cysts can survive several months in cold water
• Peak prevalence occurs in children of up to 15 to 30% in poor countries
• Reinfection is common

Pathophysiology

• Infected with as few as 10 to 25 cysts
• Trophozoites colonize the upper small bowel, adhering to the mucosal enterocytes
• Host response involves B- and T-cells, including production of IgM and IgG antibodies, possibly IgA antibodies as well
• Causes villous atrophy, which can contribute to malabsorption
• Inhibits disaccharidases, which contributes to lactose intolerance

Clinical Manifestations

• Can have asymptomatic cyst passage (5 to 15%)
  • Can last up to 6 months in children
• Most common syndrome is acute diarrhea (25 to 50%)
  • Incubation period 1 to 2 weeks
  • Usually presents after 7 to 10 days
  • Malaise, flatulence, cramps, bloating, nausea, anorexia, vomiting, and malabsorption
  • Sulfuric belching is classic
  • Fevers are uncommon but possible
• Can also cause chronic diarrhea
  • More common in CVID, X-linked agammaglobulinemia, and AIDS
  • Malaise, occasional headache, diffuse abdominal and epigastric pain worse with food
  • May have malabsorption, often has weight loss
  • Symptoms can wax and wane, with periods of remission or constipation
• Post-Giardia syndrome with irritable bowel syndrome and chronic fatigue may last for years
  • Can have significant malabsorption
• Also, can have post-Giardia lactose intolerance lasting several weeks

Diagnosis

• Stool microscopy
  • May see active trophozoites in saline wet prep
  • Look for cysts after iodine staining
  • Sensitivity 60-80% with one stool sample, up to 90% with three
• Antigen assays are sometimes used during outbreaks or for screening
  • Sensitivity 85-98% and specificity 90-100%
• PCR is becoming more common, and is the current standard in Ontario
  • Becomes negative by about 7 days after successful treatment1
• String test
  • Put a string down into small bowel and then pull it back; it should have bile
  • Look with a microscope
  • Done when there is high suspicion but O&P negative

Management

• Tinidazole 2 g po once, which is the gold standard
• Nitazoxanide 500 mg p.o. bid for 3 days
• Metronidazole 250 mg po tid (or 500 mg bid) for 5 to 7 days
  • Has a higher failure rate
• Alternatives: albendazole, paromomycin, quinacrine, and furazolidone
  • Albendazole 400 mg p.o. daily for 5 days
  • Mebendazole 200 mg p.o. TID for 5 days
  • Paromomycin 10 mg/kg p.o. TID for 5 to 10 days
  • Furazolidone 100 mg p.o. QID for 7 to 10 days
  • Quinacrine 100 mg p.o. TID for 5 days
• Avoid caffeine, lactose, and smoking
• Can consider Saccharomyces boulardii probiotics (Florastor)