Giardiasis
Giardiasis
Disease History, Characteristics, and Transmission
Introduction
Giardiasis (pronounced GEE-are-DYE-uh-sis) is intestinal infection with the protozoan parasite Giardia lamblia (also called Giardia intestinalis or Giardia (gee-ARE-dee-uh). Protozoa are single-celled animals with more complex features and behavior than bacteria, which are also single-celled organisms. Giardiasis is a waterborne disease found almost everywhere in the world. Its symptoms include diarrhea, gas, stomach cramps, fatigue, weight loss, and nausea. Giardiasis is transmitted by ingestion of cysts—extremely small, dormant, seedlike objects—that have been shed in the feces of an infected person or animal. Several drugs can be used to treat giardiasis, but healthy individuals can usually overcome the disease without treatment. The symptoms of untreated giardiasis usually last from two to six weeks.
Disease History, Characteristics, and Transmission
History
Giardiasis has probably been endemic since before modern humans evolved. Giardia were first described by the Dutch scientist Antony van Leeuwenhoek (1632–1723), who significantly improved the microscope and was the first person to observe single-celled organisms. He found Giardia living in his own feces. The organism was originally named Cercomonas intestinalis by the Czech physician Wilhelm Lambl (1814–1895) in 1859. It was renamed Giardia lamblia in 1915 to honor both Lambl and the French physician Alfred Giard (1846– 1908), another early researcher of Giardia. Today, the term Giardia intestinalis is usually preferred by scientists.
In the twentieth century, five species of Giardia were identified. Giardia intestinalis, the species that afflicts humans, can be hosted by mammals, reptiles, and possibly birds.
Characteristics
Giardia is a protozoan flagellate, that is, a one-celled animal that propels itself using tiny, rapidly-waving hairs called flagella. It exists in two forms, the trophozoite and the cyst. A Giardia cyst is a microscopic, oval object about 1 to 12 ?m (millionths of a meter) long and 7 to 10 ?m wide. A Giardia infection occurs when a sufficiently large number of these cysts are ingested by an animal or human.
In the duodenum, the part of the small intestine just below the stomach, each cyst hatches and divides into two trophic individuals or trophozoites. A Giardia trophozoite is shaped somewhat like a limpet with a short, pointed tail. It attaches its flat surface to the cells of the intestinal wall and feeds on them. In a few days, the trophozoites detach from the intestinal wall and divide into two identical individuals. Some are carried downstream through the digestive tract to the large intestine, where feces are formed. The harsh chemical conditions in the large intestine signal the trophozoites to become cysts. Cysts can survive for months in surface waters such as lakes and streams.
Giardia infection usually lasts two to six weeks. In some cases, however, the infection can become chronic or ongoing. Exactly how Giardia cause the symptoms of giardiasis is not known. Between 60% and 80% of people infected with Giardia have no symptoms.
Transmission
During bouts of diarrhea caused by giardiasis, both trophozoites and cysts exit the body in the feces. The trophozoites die outside the body, but the cysts may be ingested by another animal and continue the life cycle. There can be from 1,000,000 to 100,000,000 cysts per gram in stool samples that test positive for Giardia, but many stool samples of infected persons do not contain detectable levels of Giardia at all.
Cysts are almost always transmitted by the fecal-oral route—that is, by the ingestion of fecal material, generally in very small or dilute amount, through the mouth. This may occur through feces-hand-oral contact (common among children or those caring for children) or in drinking water. The drinking water route is common worldwide, but less so in developed countries. A 2001 New Zealand study found that persons who were changing diapers were four times more likely to test positive for Giardia than others. Giardia cysts may also be foodborne, that is, transmitted through contact between food and infected workers or family members.
Whether pets and animals are significant in spreading Giardia is debated. Giardia is common in pets, wild mammals, and farm animals, but there is little evidence that these are important sources of human infection, despite the association in the United States of Giardia with drinking open waters containing fecal matter from beavers. (The disease is sometimes called “beaver fever” in the United States.)
Scope and Distribution
Giardia is present in most surface waters of the world, at rates varying from 0.1 to over 1,000 cysts per 26 gallons (100 liters) of water. About 12% of the groundwater sources in the United States are contaminated either with Giardia or Cryptosporidium, another protozoan parasite.
Globally, about 200 million people, about 3% of the world's population, are infected with Giardia at any one time. Giardiasis is particularly common in children in poor countries. In industrialized countries, giardiasis is most common among children one to four years old, in adults caring for small children, and in those who have traveled recently to the developing world. It can also be contracted by people who drink untreated lake or stream water while visiting wilderness areas. However, drinking such water does not usually result in infection. The body can usually fend off infection if it has ingested only a small to moderate number of cysts.
WORDS TO KNOW
TROPHOZOITE: The amoeboid, vegetative stage of the malaria protozoa.
Treatment and Prevention
The primary public-health approach to preventing giardiasis is to keep Giardia cysts out of drinking water supplies. This is accomplished by keeping water sources used for drinking water from Giardia contamination by sewage or livestock waste, and by treating drinking water before distributing it. Because Giardia and other protozoan cysts are resistant to chemicals such as chlorine at the levels ordinarily used to treat water, the primary means of treating water is filtering. Portable, hand-operated filters can be used by persons in wilderness areas to filter out not only Giardia cysts, but other parasites and bacteria. The U.S. Centers for Disease Control (CDC) recommends not drinking recreational water, untreated surface water, or untreated ice or drinking water while traveling in developing countries.
For healthy individuals, treatment for giardiasis is usually not necessary, as the body is capable of freeing itself from the infection. Where treatment is needed or desired, a number of drugs are available, including albendazole, furazolidone, metronidazole, nitazoxanide, and quinacrine.
Impacts and Issues
Infection with Giardia intestinalis is the most commonly reported protozoan parasite infection worldwide. In developing countries, about 20% of patients with diarrhea are positive for Giardia (the range is 5% to 43%). In developed regions such as the United States and Europe, about 3% of diarrhea patients have Giardia. Because untreated diarrhea can cause severe dehydration (loss of water and electrolytes from the body), it can be life endangering in persons with little access to medical care and with compromised immune systems, especially small children. Diarrhea causes 4% of all deaths worldwide. In 1998, for example, diarrhea killed some 2.2 million people, most of them children under the age of five. It is not known how many of these deaths are due to Giardia infection. Chronic Giardia infection may also cause failure to thrive in children due to impaired uptake of fats and vitamins A and B12.
IN CONTEXT: REAL-WORLD RISKS
The Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Parasitic Diseases states that, “Anyone can get giardiasis. Persons more likely to become infected include:
- Children who attend day care centers, including diaperaged children
- Child care workers
- Parents of infected children
- International travelers
- People who swallow water from contaminated sources
- Backpackers, hikers, and campers who drink unfiltered, untreated water
- Swimmers who swallow water while swimming in lakes, rivers, ponds, and streams
- People who drink from shallow wells.”
- “Contaminated water includes water that has not been boiled, filtered, or disinfected with chemicals. Several community-wide outbreaks of giardiasis have been linked to drinking municipal water or recreational water contaminated with Giardia.”
SOURCE: Centers for Disease Control and Prevention (CDC)
See AlsoCryptosporidiosis; Parasitic Diseases; Water-borne Disease.
BIBLIOGRAPHY
Books
Erlandsen, Stanley, and Ernest Meyer. Giardia and Giardiasis, Biology Pathogenesis, and Epidemiology. New York: Springer, 2001.
Parker, James N., and Philip M. Parker. The Official Patient's Sourcebook on Giardiasis. San Diego, CA: Icon Health Publications, 2002.
Periodicals
Casemore, David P. “Foodborne Illness: Foodborne Protozoal Infection.” Lancet. 336 (1990): 1427–1433.
Hawralek, Jason. “Giardiasis: Pathophysiology and Management.” Alternative Medicine Review. 8.2 (2003): 129–143.
Hoque, M. Ekramul, et al. “Nappy Handling and Risk of Giardiasis.” Lancet. 357 (2001): 1017.
Meng, Tze-Chiang, et al. “Inhibition of Giardia lamblia Excystation by Antibodies against Cyst Walls and by Wheat Germ Agglutinin.” Infection and Immunity. 64 (1996): 2151–2157.
Reiner, David S., et al. “Identification and Localization of Cyst-Specific Antigens of Giardia lamblia.” Infection and Immunity. 57 (1989): 963–968.
Vogel, Gretchen. “Searching for Living Relics of the Cell's Early Days.” Science. 277 (1997): 1604.
Web Sites
Centers for Disease Control (U.S. Government). “Parasitic Disease Information: Giardiasis.” September 14, 2004 <http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/> (accessed February 1, 2007).