18.6
Giardiasis
Giardiasis is an enteric infection caused by the protozoan parasite Giardia duodenalis.
Giardia cysts containing immature trophozoites can enter the human host via contaminated water or food.
Once inside, the acidic conditions in the stomach trigger excystation of the trophozoites by weakening the cyst wall.
The cysts move further to the duodenum, where pancreatic enzymes trigger the release of active, motile trophozoites.
The trophozoites multiply rapidly and attach to the mucosa of the duodenum.
The trophozoites secrete proteases and parasite-derived factors that damage brush-border enzymes and disrupt the microvilli structure, impairing the host’s nutrient absorption.
As trophozoites move toward the colon, they encounter a neutral pH and secondary bile salts, which prompt encystation.
Cysts formed in the large intestine are passed in feces and can survive in the environment, enabling transmission to new hosts.
Giardiasis is a globally prevalent intestinal infection caused by the protozoan parasite Giardia duodenalis (also known as G. lamblia or G. intestinalis). This flagellated protozoan is the most frequently identified intestinal parasite in the United States and worldwide. Transmission primarily occurs via the fecal-oral route, with infection arising from ingestion of water or food contaminated with cysts. Individuals in low-resource settings, international travelers, outdoor enthusiasts, daycare workers, and men who have sex with men (MSM) are at heightened risk. While many infections remain asymptomatic, symptomatic cases commonly present with flatulence, watery diarrhea, abdominal cramps, and fatigue. In children from developing countries, chronic giardiasis may contribute to growth impairment.
G. duodenalis has a direct lifecycle that does not involve intermediate hosts. Infection begins when cysts are ingested through contaminated sources. These cysts travel to the stomach and reach the duodenum. These changes in pH and exposure to bile and pancreatic enzymes trigger excystation, the process by which parasites emerge from their protective cysts to become active trophozoites. Trophozoites, the active form of the parasite, emerge and colonize the small intestine. Rapid replication occurs through binary fission. As the parasites transit toward the colon, encystation is induced by changes in bile concentration and nutrient availability. The newly formed cysts are excreted in feces, capable of infecting new hosts.
The precise mechanisms of symptom development remain under investigation. Trophozoites adhere to the epithelial cells of the small intestine via a ventral adhesive disk, disrupting cellular junctions and brush border enzyme activity. They secrete lectins and thiol proteinases that exert cytopathic effects, leading to increased intestinal permeability and malabsorption, particularly of carbohydrates. These disruptions in epithelial function and gastrointestinal motility contribute to the characteristic clinical manifestations.
Giardiasis is an enteric infection caused by the protozoan parasite Giardia duodenalis.
Giardia cysts containing immature trophozoites can enter the human host via contaminated water or food.
Once inside, the acidic conditions in the stomach trigger excystation of the trophozoites by weakening the cyst wall.
The cysts move further to the duodenum, where pancreatic enzymes trigger the release of active, motile trophozoites.
The trophozoites multiply rapidly and attach to the mucosa of the duodenum.
The trophozoites secrete proteases and parasite-derived factors that damage brush-border enzymes and disrupt the microvilli structure, impairing the host’s nutrient absorption.
As trophozoites move toward the colon, they encounter a neutral pH and secondary bile salts, which prompt encystation.
Cysts formed in the large intestine are passed in feces and can survive in the environment, enabling transmission to new hosts.
From Chapter 18:
Now Playing
18.6 : Giardiasis
Fungal and Parasitic Infections
33 Views
18.1 : Leishmaniasis
Fungal and Parasitic Infections
60 Views
18.2 : Cryptococcal Meningitis
Fungal and Parasitic Infections
31 Views
18.3 : Malaria
Fungal and Parasitic Infections
34 Views
18.4 : Toxoplasmosis
Fungal and Parasitic Infections
44 Views
18.5 : American Trypanosomiasis
Fungal and Parasitic Infections
43 Views
18.7 : Amebiasis
Fungal and Parasitic Infections
36 Views
18.8 : Candidiasis
Fungal and Parasitic Infections
42 Views
18.9 : Trichomoniasis
Fungal and Parasitic Infections
36 Views