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|Classification and external resources|
Giardia cell, SEM
|Classification and external resources|
Giardia cell, SEM
Giardiasis (popularly known as beaver fever) is a zoonotic parasitic disease caused by the flagellate protozoan Giardia lamblia (also sometimes called Giardia intestinalis and Giardia duodenalis). The giardia organism inhabits the digestive tract of a wide variety of domestic and wild animal species, as well as humans. It is the most common pathogenic parasitic infection in humans worldwide; in 2013, there were about 280 million people worldwide with symptomatic giardiasis.
Symptoms include loss of appetite, diarrhea, loose or watery stools, stomach cramps, upset stomach, projectile vomiting (uncommon), bloating, excessive gas, and burping (often sulfurous). The incubation period for giardiasis is 9–15 days. Symptoms are caused by Giardia organisms infecting the cells of the duodenum and jejunum of the small intestine and blocking nutrient absorption. Most people are asymptomatic; only about a third of infected people exhibit symptoms. If the infection is not treated, these symptoms may last for six weeks or more.
Symptomatic infections are well recognized as causing lactose intolerance, which, while usually temporary, may become permanent. Although hydrogen breath tests indicate poorer rates of carbohydrate absorption in those asymptomatically infected, such tests are not diagnostic of infection. It has been suggested that these observations are explained by symptomatic giardia infection allowing for the overgrowth of other bacteria.
Giardiasis is caused by the protozoan Giardia lamblia. The infection occurs in many animals including beavers, cows, rodents, and sheep. Animals are believed to play a role in keeping infections present in an environment.
Wilderness travel within the United States is believed to be a risk factor with poorly treated or untreated water playing a role.
Giardiasis is transmitted via the fecal-oral route with the ingestion of cysts. Primary routes are personal contact and contaminated water and food. The cysts can stay infectious for up to three months in cold water.
According to the CDC, those at greatest risk are travelers to countries where giardiasis is common, people in child care settings, those who are in close contact with someone who has the disease, people who swallow contaminated drinking water, backpackers or campers who drink untreated water from lakes or rivers, people who have contact with animals who have the disease, and men who have sex with men. Not all Giardia infections are symptomatic, and many people can unknowingly serve as carriers of the parasite.
Giardia are flagellated protozoans that cause decreased expression of brush border enzymes, morphological changes to the microvillus, and programmed cell death of small intestinal epithelial cells. There is no invasion of giardia trophozoites, and small intestinal morphology may appear normal in light microscopy.
The attachment of trophozoites causes villus flattening and inhibition of enzymes that break down dissacharide sugars in the intestines. Ultimately, the community of microorganisms that lives in the intestine may overgrow and may be the cause of further symptoms, though this idea has not been fully investigated. The alteration of the villi leads to an inability of nutrient and water absorption from the intestine, resulting in diarrhea, one of the predominant symptoms. In the case of asymptomatic giardiasis, there can be malabsorption with or without histological changes to the small intestine. The degree to which malabsorption occurs in symptomatic and asymptomatic cases is highly varied.
The species Giardia intestinalis uses enzymes that break down proteins to attack the villi of the brush border and appears to increase crypt cell proliferation and crypt length of crypt cells existing on the sides of the villi. On an immunological level, activated host T lymphocytes attack endothelial cells that have been injured in order to remove the cell. This occurs after the disruption of proteins that connect brush border endothelial cells to one another. The result is heavily increased intestinal permeability.
There appears to be a further increase in programmed cell death by Giardia intestinalis, which further damages the intestinal barrier and increases permeability. There is significant upregulation of the programmed cell death cascade by the parasite, and, furthermore, substantial downregulation of the anti-apoptotic protein Bcl-2 and upregulation of the proapoptotic protein Bax. These connections suggest a role of caspase-dependent apoptosis in the pathogenesis of giardiasis.
Giardia protects its own growth by reducing the formation of the gas nitric oxide by consuming all local arginine, which is the amino acid necessary to make nitric oxide. Arginine starvation is known to be a cause of programmed cell death, and local removal is a strong apoptotic agent.
Boiling suspect water for one minute is the surest method to make water safe to drink and kill disease-causing microorganisms such as Giardia lamblia if in doubt about whether water is infected. Chemical disinfectants or filters may be used.
The CDC recommends the washing of hands before handling food.
According to a review of the literature from 2000, there is little evidence linking the drinking of water in the N. American wilderness and Giardia. The researcher notes that treatment of drinking water for Giardia may not be as important as recommended hand-washing in wilderness regions in North America. CDC surveillance data (for 2005 and 2006) reports one outbreak (6 cases) of waterborne giardiasis contracted from drinking wilderness river water in Colorado. However, less than 1% of reported giardiasis cases are associated with outbreaks.
Treatment is not always necessary as the infection usually resolves on its own. However, if the illness is acute or symptoms persist and medications are needed to treat it, a nitroimidazole medication is used such as metronidazole, tinidazole, secnidazole or ornidazole.
While tinidazole has similar side effects and efficacy to metronidasole, it is administered with a single dose.
Current research evidence suggests albendazole is probably as effective as metronidazole, but has fewer gastrointestinal and neurological side effects and is more convenient to take or administer. Both medications need a five to 10 day long course; albendazole is taken once a day, while metronidazole needs to be taken three times a day. A comparison of multiple drugs (tinidazole, nitazoxanide, metronidazole, albendazole) also favoured albendazole in preference to metronidazole, but did not draw any conclusions on the effectiveness of the other drugs due to a lack of reliable evidence.
In the case of nitroimidazole-resistant strains of Giardia, other drugs are avaliable which have showed efficacy in treatment: quinacrine, nitazoxanide, bacitracin zinc, furazolidone and paromomycin.
Kathleen Meyer, How to Shit in the Woods: An Environmentally Sound Approach to a Lost Art, Ten Speed Press (1989). A practical guide to wilderness waste disposal to prevent giardiasis.