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Giardia: Cysts
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Giardia parasites have two forms: an active form that lives in the intestine of an infected animal or human and an inactive cyst form that can survive for months in the environment. When you ingest the cysts, their hard shells break down in your stomach, releasing the parasites. The parasites then attach to the wall of your small intestine, eventually reproducing themselves millions of times and damaging the intestinal wall. This interferes with your ability to absorb food, leading to diarrhea and weight loss.
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Most dogs that are infected with Giardia do not have diarrhea or any other signes of illness. When the eggs (cysts) are found in the stool of a dog without diarrhea, they are generally considered a transient, insignificant finding. However, in puppies and debilitated adult dogs, they may cause severe, watery diarrhea that may be fatal. The reason that puppies are more susceptible is that they may have immature immune systems, and their immune system may be weakened from the stress of changing environments.
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Control of Giardia, from a public health standpoint, should start with municipal drinking water. The prevalence of Giardia in humans within industrialized countries is 2-5%. The prevalence of Giardia in humans within developing countries is 20-30%. As many as 95% of human travelers to St. Petersburg, Russia have shown signs of giardiasis. In children that attend day care centers, the prevalence of Giardia has been found to be as high as 35%. Filtration can be quite effective for removing Giardia cysts from water.
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Giardia trophozoites are binucleate and replicate in the small intestines by binary fission. Once they exit the jejunum, they encyst as they begin to lose moisture content. Within their cyst, they undergo one further division to produce a tetranucleate cyst. This cyst that is passed in the feces is very resistant in the environment and is the infective form for new hosts. When ingested, the tetranucleate parasite emerges from the cyst in the duodenum, rapidly divides into (2) binucleate trophozoites, and proceeds to undergo continued binary fission.
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Giardia can be difficult to detect even for professionals. It is too small to be seen by the unaided eye. A high quality microscope is needed for proper diagnosis; phase contrast microscopy is helpful. A definitive negative diagnosis should include stools collected on multiple days since cyst production tends to be cyclic with millions produced one day and few the following day. The cyst is the diagnostic stage of Giardia. Cysts tend to be approximately 9-15 micrometers in length and 4-5 um in width.
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Giardia is typically found in surface water bodies, like lakes, reservoirs, rivers, and streams. It is found in human and animal fecal matter, so water bodies that are contaminated with sewage or animal waste likely contain Giardia. For this reason, water sources near farms are particularly vulnerable to Giardia contamination. Shallow aquifers that are recharged by surface water may ... be susceptible to Giardia contamination. But generally, only surface water sources are affected by the cyst. If you do not know where your drinking water comes from, call your water utility and ask.
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