Giardia duodenalis, also known as Giardia intestinalis and Giardia lamblia, is a flagellated parasitic protozoan microorganism of the genus Giardia that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The parasite attaches to the intestinal epithelium by a ventral disc (syn. adhesive disc or sucker), and reproduces via binary fission. G. duodenalis is a non-invasive parasite, that does not spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine. The parasite exists in two forms; trophozoites and cysts. The microorganism can undergo encystation, transforming into a dormant cyst that enables it to survive outside of its host. Giardia trophozoites are anaerobic, and absorb their nutrients from the intestinal lumen. If the organism is stained, its characteristic pattern resembles the familiar "smiley face" symbol. Chief pathways of human infection include ingestion of untreated drinking water (which is the most common method of transmission for this parasite), food, soil contaminated with human feces, and sewage, a phenomenon particularly common in many developing countries. Contamination of natural waters also occurs in watersheds where intensive grazing occurs. Giardia infections occur worldwide. It is the most commonly identified intestinal parasite among children in day-care centers, hikers and immunocompromised patients. About 20,000 cases per year in the United States are reported. Almost half of those infected with giardiasis remain asymptomatic. For those who do experience symptoms, they usually appear 1 to 2 weeks after infection. Common symptoms include abdominal pain, nausea, and bloating, along with large, watery, foul-smelling, and greasy stools. Due to frequent loose stools, individuals with giardiasis often experience dehydration. It has also been shown that G. intestinalis damages the intestinal epithelium, which directly affects nutrient absorption. In severe cases, giardiasis can lead to chronic diarrhea, chronic fatigue syndrome and cognitive impairment in children.
Life cycle
G. duodenalis takes on two morphologically distinct forms during its lifecycle. Trophozoites are the replicative stage of the parasite, characterized by a pear-shaped, motile, flagellated cell that survives only in the small intestine of the host. The trophozoites do not penetrate host cells, but rather attaches to the intestinal epithelium cells to establish an infection. A cyst is the environmentally stable stage of the parasite, that facilitates transmission between hosts. The infection process in the host begins with the ingestion of cysts, which pass through the stomach into the first part of small intestine, or the duodenum. Exposure to digestive enzymes and an acidic pH triggers excystation, where the cysts release trophozoites. Trophozoites swim through the intestinal mucus until they eventually adhere to the intestinal epithelium using their ventral disc. Adhered trophozoites can then feed, divide by binary fission and cause the disease. Trophozoites cause structural and functional damage to the host epithelial cells, impairing the intestine's ability to absorb nutrients effectively. Some trophozoites differentiate back into cysts under specific conditions, such as high organism density. As the trophozoites travel through the intestinal lumen to the large intestine, the alkaline environment and the presence of pancreatic proteases trigger the encystation. Trophozoites develop into cysts through encystation, which involves specialized vesicles (ESVs) that facilitate the formation of the cyst wall. Cysts and trophozoites pass through the host's large intestine and are shed in the feces. Trophozoites cannot survive outside of the host, whereas cysts can remain viable in the external environment for several months. The cysts remain dormant until ingested by a host animal. When a new potential host ingests water or food contaminated with this feces, the cysts gain entry to the gastrointestinal tract of the new host, repeating the cycle.
Structure The trophozoite has an elaborate structure with two nuclei and four pairs of flagella which allow it to swim within the intestinal lumen of the host. It also has an adhesive disk on its ventral surface that is associated with the parasite's attachment to the intestinal epithelium. The adhesive disk is composed of microtubules, and is found only in Giardia. The parasite lacks Golgi apparatus and mitochondria but has mitosomes, which probably evolved from mitochondria. The mitosome is a double-membraned organelle, that lacks the enzymatic components required for classic mitochondrial functions, such as ATP synthesis and lipid metabolism. However, they do contain certain mitochondrial genes associated with iron-sulfur complex biosynthesis, which suggests that this organism likely lost its mitochondria during evolution. The cyst contains four nuclei, axonemes, median bodies as well as the rigid cyst wall. Additionally, the cysts contain fragments of the adhesive disc within the cytoplasm. The two-layered cyst wall protects the parasite against different environmental conditions such as chemical treatments. The metabolic activity of the cyst is significantly lower compared to that of the trophozoites, which allows the parasite to survive for longer periods in harsh conditions, such as in cold environments.
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