Giardia lamblia

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

Background

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

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

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

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

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 wx 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

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
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

Tinidazole 2 g po once, which is the gold standard
Metronidazole 250 mg po tid for 5 to 7 days, but has a higher failure rate
Alternatives: nitazoxanide, albendazole, paromomycin, quinacrine, and furazolidone
Avoid caffeine, lactose, and smoking
Can consider Saccharomyces boulardii probiotics (Florastor)