Giardia lamblia Life Cycle and Transmission

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10 Questions

What is the primary method of diagnosing giardiasis in formed stools?

Finding trophozoites or cysts or both in diarrheal stools

What is the purpose of the String Test in diagnosing giardiasis?

To detect trophozoites in the duodenum

What is the significance of acquired immunity in giardiasis?

It develops and is shown by evidence

What is the most severe symptom of cryptosporidiosis?


What is the purpose of chlorination, filtration, and iodine treatment in preventing cryptosporidiosis?

To remove or kill the cysts

What is the characteristic shape of the oocyst of Cryptosporidium?


What is the characteristic shape of the sporozoite of Cryptosporidium?

Rough surface and pointed apical region, rounded posterior end

What is the characteristic shape of the trophozoite of Cryptosporidium?

Smooth surface and hood-like shape

What is the significance of Type I and Type II meronts in the life cycle of Cryptosporidium?

They produce merozoites of different sizes

What is the final stage of the life cycle of Cryptosporidium?

Formation of microgametes and macrogametes

Study Notes

Giardia Lamblia

  • Also known as Giardia intestinalis, it is a flagellated parasitic microorganism that colonizes and reproduces in the small intestine, causing a diarrheal condition known as giardiasis.

Habitat and Transmission

  • Definitive host: Humans (lives in the duodenum and upper part of the jejunum)
  • Intermediate host: None
  • Cysts are found in the colon of humans and contaminated materials
  • Transmission occurs through ingestion of contaminated food and water (fecal-oral route)


  • Infective agent: Mature cyst
  • Infective dose: About 100 cysts
  • Trophozoites attach to the bowel wall, causing irritation, low-grade inflammation, and damage to epithelial cells, leading to acute or chronic diarrhea


  • Found worldwide, with approximately 50% of stool specimens in the US containing Giardia cysts
  • Half of those infected are asymptomatic carriers who continue to excrete cysts for years
  • IgA deficiency predisposes to symptomatic infection
  • Outbreaks occur related to contaminated water supplies
  • Hikers who drink untreated stream water are frequently infected
  • Many species of humans and mammals act as reservoirs
  • High incidence among children in daycare and patients in mental hospitals

Clinical Findings

  • Diarrhea (non-bloody and foul-smelling)
  • Steatorrhea due to malabsorption of fats
  • Malaise
  • Weakness
  • Anorexia
  • Weight loss
  • Abdominal cramps and flatulence
  • Distension due to gas
  • Nausea and vomiting
  • Chronic cholecystopathy (when the parasite colonizes the biliary tract)

Diagnostic Laboratory Tests

  • Microscopy (stool, duodenal aspirate, and serum)
  • Enzyme-Linked Immunosorbent Assay Test (ELIZA) to detect Giardia antigen in the stool
  • String Test (if other tests are negative and symptoms persist)


  • Acquired immunity develops in giardiasis
  • Immunocompromised individuals are more susceptible to massive infection with severe clinical manifestations


  • Tinidazole (tindamax)
  • Metronidazole (flagyl)
  • Quinacrine hydrochloride
  • Furazolidone

Prevention and Control

  • Avoiding fecal contamination of food and water
  • Good personal hygiene
  • Proper sanitation
  • Purification of municipal water supplies
  • Avoid using "night soil" (human feces) as a fertilizer
  • Eating properly cooked food
  • Chlorination does not kill the cyst, but filtration removes them
  • Drinking boiled, filtered, or iodine-treated water in endemic areas
  • No prophylactic drug or vaccine is available


  • A genus of apicomplexan parasites that causes cryptosporidiosis, characterized by diarrhea
  • Cryptosporidium hominis is the primary species that causes disease in humans
  • Cryptosporidium parvum is the former name, no longer used


  • Oocysts: oval-shaped, 5×7um, with a smooth surface and a cleft that allows for sporozoites to be released during excystation
  • Sporozoites: rough surface, pointed apical region, rounded posterior end, 5×0.5um
  • Trophozoites: smooth surface, hood-like shape, 1-2.5um in length
  • Type I and Type II meronts vary in size, with an epicellular, smooth surface and merozoites produced by them are relatively similar in size
  • Type I merozoites: rod-like shape, pointed apical region, rough surface
  • Type II merozoites: round, rough surface
  • Microgametes: spherical, rough surface
  • Macrogametes: ovular, rough surface

Life Cycle

  • Oocysts release sporozoites, which form trophozoites
  • Several stages ensue, involving the formation of schizonts and merozoites
  • Eventually, microgametes and macrogametes form

Test your knowledge of Giardia lamblia, a parasitic protozoan that infects the human small intestine. Learn about its habitat, transmission, and pathogenesis. Understand how trophozoites multiply and form colonies in the duodenum and upper part of the jejunum.

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