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Giardia & Giardiasis

Giardiasis in humans is caused by the infection of the small intestine by a single-celled organism called Giardia lamblia. Giardiasis occurs worldwide with a prevalence of 20–30% in developing countries. Additionally, Giardia has a wide range of human and other mammalian hosts, thus making it very difficult to eliminate.

Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada in the supergroup "Excavata" (named for the excavated groove on one side of the cell body) that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis. Their life cycle alternates between an actively swimming trophozoite and an infective, resistant cyst. The genus was named after French zoologist Alfred Mathieu Giard.

Giardiasis is caused by the ingestion of infective cysts. There are multiple modes of transmission including person-to-person, water-borne, and venereal. Person-to-person transmission accounts for a majority of Giardia infections and is usually associated with poor hygiene and sanitation. Water-borne transmission is common in United States Giardia epidemics, which are often associated with the ingestion of unfiltered water (contaminated). Venereal transmission happens through fecal-oral contamination. Additionally, diaper changing and inadequate hand washing are risk factors for transmission from infected children. Lastly, food-borne epidemics of Giardia have developed through the contamination of food by infected food-handlers.

A range of clinical syndromes may occur, with gastrointestinal syndromes being the most prevalent.

Giardia lives inside the intestines of infected humans or animals. Individuals become infected through ingesting or coming into contact with contaminated food, soil, or water. The Giardia parasite spreads when a person accidently swallows it, which can originate from contaminated items and surfaces that have been tainted by the feces of an infected human or animal. Consuming unsanitary water or food is also another way in which the parasite can transfer from being to being.

The symptoms of Giardia, which may begin to appear 1-2 weeks after infection, include diarrhea, excess gas, stomach or abdominal cramps, and upset stomach or nausea. The result of these occurrences is weight loss and/or dehydration, which can be harmful if not treated immediately. The typical infection within an individual is around 2-6 weeks, but medication can decrease that time period.

A small number of infected individuals experience an abrupt onset of abdominal cramps, explosive, watery diarrhea, vomiting, foul flatus, and fever which may last for 3–4 days before proceeding into a more sub-acute phase. The majority of infected persons develop gradual symptoms that become recurrent or resistant.

In both the acute and insidious onsets of symptoms, stools become greasy and malodorous but do not contain blood or pus because giardiasis does not involve dysenteric symptoms. Watery diarrhea may cycle with soft stools and constipation. Upper GI symptoms including nausea, early satiety, bloating, substernal burning, egg-smelling halitosis, and acid indigestion may be exacerbated by eating and are generally present in the absence of soft stools.

Prevalence rates for giardiasis range from 20–30% in most developing countries and 2–7% in developed countries. The CDC estimates that in the United States there are more than 2.5 million cases of giardiasis annually. Giardiasis occurs worldwide with increased prevalence in areas with poor water treatment facilities and unsanitary conditions. The area of highest prevalence is the tropics and subtropics. Despite this, giardiasis does affect a large number of individuals living in highly developed nations with strong infrastructure and water systems. In the United States, giardiasis is the most commonly reported pathogenic protozoan disease.

High infection rates occur in hikers and backpackers in the United States. Giardiasis is a common infection in active outdoors population because of their exposure to areas inhabited by infected wild animals and ingestion of free flowing water which may contain cysts. Furthermore, giardiasis is common in tourists and business travelers to developing countries, especially Mexico, Southeast Asia, western South America and Russia. Also, an increased prevalence of giardiasis among homosexual men has been reported by a number of studies. Because infection may be caused by poor hygiene, giardiasis has high infection rates in daycare centers and nursing homes, though the groups most at risk for infection are overseas travelers and hikers.


Currently, there is no vaccine to protect humans from acquiring giardiasis. Preventing Giardia can be accomplished by practicing good hygiene, purifying/filter water that may be contaminated or avoiding it all together, and avoiding fecal contact.

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Did You Know?
The Centers for Disease Control and Prevention reports that in the US Giardia infects over 2.5 million people annually.

Symptoms usually begin 1 to 2 weeks (average 7 days) after an individual becomes infected. In otherwise healthy individuals, symptoms may last 2 to 6 weeks. Though symptoms may last longer, medications can help decrease the duration of symptoms.

Giardia has no vector. It affects a wide range of human and mammalian reservoir hosts. Small aquatic or semi-aquatic wild mammals, such as beavers, muskrats, and small rodents harbor water-born cysts of giardia and serve as important reservoir hosts. Furthermore, a variety of birds may also serve as reservoirs of giardia.

Contaminated water supplies, such as water in rivers and lakes and improperly treated water in developing countries, are also reservoirs of the waterborne cysts. Often, contamination of surface water is caused by rain and wind carrying cysts from fields containing or fertilized by manures of infected humans, livestock, or wild animals to nearby rivers and streams. Giardia cysts can remain viable in surface water for approximately two months. As a result, it is more dangerous for hikers to consume water from rivers and lakes during and immediately after raining seasons as contamination tend to be most severe during these periods. Finally, the organism can often be found in soil, food, and surfaces contaminated with feces containing infectious cysts.

First and foremost, avoid contaminated water. Hikers and overseas travelers to developing countries should consider all water sources contaminated and thus boil, filter, or treat all water with halogenated tablets or solutions. Second, avoid foods washed in contaminated water or that cannot be cooked or peeled, which is especially important for travelers to developing countries. Using only bottled water and avoiding raw fruits and vegetables decreases risk of infection dramatically.

In the United States, many of the reported cases of Giardia occur in the summer months. According to virologists and epidemiologists ,this is the time when hikers and backpackers are avid because of the ideal climate during these months. Also, this may be due to the use of community swimming areas by young diaper-aged children during the summer.