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Giardia
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Giardia is a pathogenic parasite that is commonly found in lakes and rivers, particularly those contaminated by sewage and animal wastes. The parasite exists as a cyst, an egg-shell-like covering that protects the microorganism from such hazards as chlorine, a disinfectant. The cyst ... protects the microorganism from destruction outside the host, leaving it viable in water. When this water is ingested, viable cysts can infect their new hosts.
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According to the Centers for Disease Control and Prevention, Giardia is one of the most common causes of waterborne diseases in humans in the United States. Many people get Giardia from other people and and contaminated water. Symptoms in people include diarrhea, abdominal cramps and nasuea. They appear within 2 weeks of exposure to the parasite. Exposure comes from many sources. They include swallowing water from swimming pools, lakes, rivers or streams that have been contaminated with animal or human feces.
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Giardia has a two-stage life cycle consisting of trophozoite and cyst. The life cycle begins with ingested cysts, which release trophozoites (10-20 µm x 5-15 µm) in the duodenum. These trophozoites attach to the surface of the intestinal epithelium using a ventral sucking disk and then reproduce by binary fission. The trigger for encystment is unclear, but the process results in the inactive, environmentally resistant form of Giardia -- a cyst (11-14 µm x 7-10 µm) that is excreted in feces.
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Giardia has a direct life cycle. Once the cyst stage is ingested by a suitable host, excystation occurs within the duodenum. It is believed that excystation occurs as a result of exposure to the low gastric pH in addition to contact with pancreatic enzymes such as chymotrypsin and trypsin. During excystation, two binuclear trophozoites arise from each quadrinuclear cyst. The trophozoite form uses its large adhesive disk located on its ventral surface to attach to the epithelium of the duodenum and jejunum. The trophozoites reproduce asexually by binary fission.
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The ideal prevention of water-borne Giardiasis is to boil your water…it is proven that Giardia cysts are killed instantly by boiling water; yet can live in cold water for at least 2 months! Water filtration is a very complex process, and filter operation or maintenance problems in such systems may not be detected until there is an outbreak of Giardia recognized in the community. It is even more complicated due to the fact that when you have a filter small enough to keep out the Giardia cysts (6 to 9 micrometers x 8 to 12 micrometers), the pores of the filters tend to clog easily, and it may pose a significant health hazard to clean and/or replace the filter element. According to recent studies, only the "First Need" and "Katadyn" filters removed 100% of the Giardia cysts. Chlorine concentrations in the 0.1 mg. per liter to 0.5 mg. per liter range are largely ineffective against Giardia. Much more study needs to be done to provide safe and effective water treatment programs in all parts of the world.
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The life cycle of Giardia consists of two stages: the fecal-orally transmitted cyst and the disease-causing trophozoite. Cysts are passed in a host's feces, remaining viable in a moist environment for months. Ingestion of at least 10 to 25 cysts can cause infection in humans. When a new host consumes a cyst, the host's acidic stomach environment stimulates excystation. Each cyst produces two trophozoites. These trophozoites migrate to the duodenum and proximal jejunum, where they attach to the mucosal wall.
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Giardia