Giardia lamblia and Giardiasis — Notes OmpathStudy

Introduction Giardia lamblia (also called Giardia intestinalis or Giardia duodenalis ) is a protozoan parasite that causes giardiasis , an intestinal infection. It is the only protozoan parasite found in the lumen of the small intestine (duodenum and upper jejunum). Transmission occurs through ingestion of cysts in contaminated food and water or via direct person-to-person contact. Morphology of Giardia lamblia 1. Trophozoite (Vegetative Form) Shape : Pear-shaped and bilaterally symmetrical. Size : 9-21 µm long and 5-15 µm wide. Motility : Resembles a falling leaf movement. Key Features : Two nuclei with a central karyosome. Four pairs of flagella (arising from a blepharoplast). Axostyle (a supportive rod along the midline). Ventral sucking disc (for attachment to the intestinal wall). Two median bodies (sausage-shaped, located posterior to the sucking disc). 2. Cyst (Infective Form) Shape : Oval or ellipsoid. Size : 8-12 µm. Key Features : Thick protective wall (resistant to environmental conditions). Four nuclei (mature cyst). Remnants of flagella and median bodies . Infective Stage : Mature cysts are highly infectious and can survive in soil and water for weeks. Life Cycle of Giardia lamblia 1. Infective Stage Mature cysts are ingested via contaminated water, food, or direct fecal-oral contact. 2. Excystation Occurs in the small intestine within 30 minutes of ingestion. Each cyst releases two trophozoites , which colonize the duodenum and jejunum . 3. Multiplication and Colonization Trophozoites multiply by binary fission . They attach to the intestinal mucosa using their ventral sucking disc , interfering with absorption but not invading tissue . 4. Encystation Triggered by unfavorable conditions (dehydration, bile salts). Encystment occurs in the colon , producing infective cysts that are excreted in feces. 5. Transmission Cysts remain viable in the environment (water, soil, food) for weeks. Infective dose: 10–100 cysts . . Life Cycle of Giardia lamblia 1 Infective Stage → Mature cysts are ingested via contaminated food, water, or direct fecal-oral contact.2 Excystation → In the small intestine , each cyst releases two trophozoites within 30 minutes .3 Multiplication & Colonization → Trophozoites multiply by binary fission and attach to the duodenal and jejunal mucosa using their sucking disc .4 Encystation → Triggered by unfavorable conditions (dehydration, bile salts) in the colon , forming cysts.5 Excretion & Transmission → Cysts are excreted in feces and survive in soil, water, and food for weeks.6 Infection of New Host → Another person ingests cysts, continuing the cycle. Summary of Life Cycle Mature cyst ingested → Excystation in small intestine → Trophozoites multiply → Colonization of duodenum & jejunum → Encystation in colon → Cysts excreted in stool → Ingestion by a new host Pathogenesis and Clinical Features Pathogenesis Does not invade tissue , but adheres tightly to the intestinal epithelium , causing: Villous atrophy (shortening of intestinal villi). Apoptosis of enterocytes . Malabsorption of fats and carbohydrates (steatorrhea). Variant-Specific Surface Proteins (VSSP) allow Giardia to evade the immune system, leading to chronic infections . Clinical Features 1. Asymptomatic Carrier State Majority (50-70%) of infected individuals show no symptoms . 2. Acute Giardiasis Onset : 1-2 weeks after ingestion. Symptoms : Watery diarrhea with excess mucus (no blood). Foul-smelling stools , steatorrhea (fatty stools). Dull epigastric pain, bloating, and flatulence . Weight loss and malnutrition (due to fat, protein, and vitamin A malabsorption). Lactose intolerance (temporary). 3. Chronic Giardiasis Persistent diarrhea leading to malnutrition . Vitamin A deficiency , weight loss, failure to thrive (in children). Sprue-like syndrome (malabsorption syndrome). Chronic fatigue . 4. Extraintestinal Manifestations Biliary Giardiasis :Colonization of the gallbladder . Causes biliary colic, jaundice, and cholecystitis . Diagnosis of Giardiasis 1. Stool Examination (Microscopy) Wet mount : Detects trophozoites and cysts in fresh stool. Concentration techniques (e.g., zinc sulfate flotation) improve detection. Trichrome stain enhances visibility. 2. Entero-Test (String Test) A gelatin capsule with a string is swallowed. After 2 hours , it is retrieved and examined for trophozoites. Not widely used due to high cost . 3. Serology and Molecular Tests ELISA (Enzyme-Linked Immunosorbent Assay) – Detects Giardia antigens in stool. PCR (Polymerase Chain Reaction) – Most sensitive test for diagnosis. 4. Duodenal Aspirate or Biopsy Performed in chronic cases when stool tests are negative. Detects trophozoites adhered to the intestinal mucosa . Treatment of Giardiasis 1. First-Line Drugs Metronidazole : 250 mg, thrice daily for 5–7 days (adults). 15 mg/kg/day in 3 doses for 5 days (children). Cure rate: 90% . Tinidazole : 2 g single dose (adults). 50 mg/kg single dose (children). More effective than metronidazole wi