drawing of trophozoite form of Giardia
from U. C. Publications in Zoology, 1952
Giardia is a protozoan from the phyla Mastigophora. Its form that primarily affects humans goes by the name Giardia lamblia. It is the most common flagellate found in the human digestive tract and it is very contagious, though usually not fatal (McGill University).
The Giardia microorganism was originally discovered by Antony van Leeuwenhoek. He described the Giardia trophozoite from a sample of his own stool in 1681:
“animalcules a-moving very prettily. . .Their bodies were somewhat longer than broad, and their belly, which was flatlike, furnisht with sundry little paws. . . and albeit they made a quick motion with their paws, yet for all that they made but slow progress” (Katz, Despommier, and Gwadz, p. 129-30).
Giardiasis, the symptomology caused by Giardia, has been called “Beaver Feaver”, after an outbreak at
Canadian Banff National Park, which was attributed to beavers infecting stream water which hikers drank (McGill University).
Everyone is at risk for Giardia. In the United States, Giardia most commonly affects those who come into frequent contact with young children, such as their family members or workers at day care centers. It also occurs in people who drink improperly treated surface water (lakes, streams, etc.) and foreign travelers. Giardia is closely associated with recreational water use (i.e., swimming pools and water parks, where epidemics have occurred). Fecal-oral exposure during sex is also a risk factor for Giardia.
The CDC has reported that the two groups in the United States with the highest risk of becoming infected with Giardia are:
-children younger than 5 years of age
-women of childbearing age
Its overall prevalence in the United States ranges from 1.5 to 20 percent.
It is estimated that approximately 200 million people worldwide are currently infected with Giardia (Markell, John, Krotoski, p. 16).
Giardia is a zoonosis that is found in more than 40 animal species, and five strains of the bacteria have been described. Rodents, birds, and reptiles are affected by Giardia muris, Giardia intestinalis affects other mammals, and Giardia agilis affects amphibians (WHO).
It can be transmitted across species (e.g., dogs to beavers, beavers to humans).
Giardia exists in active (trophozoite) and dormant (cyst) stages.
Its active, motile, but noninfective, trophozoite stage takes place after the Giardia cyst has entered the host.
Each cyst undergoes a process called excystation, by which it produces 2 to 4 trophozoites.
The trophozoite has a length from 9 to 21 μm and width of 5 to 15 μm.
Each trophozoite possesses two nuclei and four pairs of flagella.
The anterior side of the trophozoite’s body consists of a sucking disk, which it uses to attach to
the villi of the small intestine.
The posterior side of the sucking disk contains two curved rod-shaped structures of unknown function called median bodies.
Once they have reached the small intestine, the trophozoites can multiply via binary fission.
Giardia trophozoites in the small intestine
From Markell & Voge’s Medical Parasitology
Giardia assumes its dormant cyst state to survive in the environment.
The harsh conditions in the colon cause Giardia to assume the cyst state.
This process is called encystation and protects Giardia for when it enters the environment.
The Giardia cyst is very hardy and able to survive in the environment for long periods of time.
The cysts are oval-shaped and are 8 to 14 μm by 7 to 10 μm.
Each cyst contains four nuclei, four median bodies and 8 flagella, all contained within the cyst’s outer wall.
An infective dose consists of 10 to 100 cysts (Erlandsen, p. 334).
Humans acquire Giardia through ingestion of infective Giardia cysts in contaminated drinking water (or anything else contaminated with infected fecal matter). Excystment occurs when the cyst reaches the small intestine. After traveling to the large intestine Giardia can encyst. The organism can leave the body with the feces in either the cyst or trophozoite form.
from McGill University
The small intestine provides a source of nourishment for Giardia trophozoites, which use their sucking disks to attach themselves to the columnar epithelial cells. Their main food source is glucose. Giardia obtains glucose from the lumen of the small intestine by means of diffusion or pinocytosis.
The best way to diagnose Giardia is through microscopic examination of the stool. Immunoassays such as ELISA (Enzyme-linked Immunosorbent Assay) and DFA (Direct Fluorescence Assay) can also be used.
Diagnosis by examination of the stool can be affected by the fact that the organism presents in the stool in three patterns:
Low : small numbers of parasites present in 40 percent of stool specimens
Mixed: 1 to 3 weeks of a high excretion pattern, that alternates with a short period of low excretion
High: parasites present in almost all stools
(Markell, John, Krotoski, p. 59)
The incubation period is about eight days.
Many people infected with Giardia are asymptomatic.
The symptoms that Giardia causes are lumped under the term giardiasis, which can consist of diarrhea, dehydration, abdominal pain, flatulence, anorexia, steatorrhea, weight loss, and in extreme cases, malabsorption syndrome.
Severe cases of Giardia can cause changes in the intestinal villi that are associated with malabsorption syndrome. Atrophied, or flattened villi is the result:
From Katz, Despommier, Gwadz
Giardiasis can last from 2 to 6 weeks. (CDC)
Lactose intolerance can persist after the eradication of Giardia from the digestive tract (Markell, John, Krotoski, p. 59).
The drug of choice for treating Giardia is the antibiotic Metronidazole (Flagyl). Other antibiotics, such as quinacrine HCl and bacitracin, can also be used. Like many other bacterial infections Giardia has developed resistance to many of drugs used to treat it.
The world will probably not be rid of Giardia anytime soon—the probability of eradication is low due to its global prevalence and multitude of animal reservoirs.
The best way to prevent Giardia is by frequent handwashing.
Giardia is able to survive water filtration and chlorination.
It follows that the best way to prevent giardiasis is to protect water sources from contamination.
Public Health efforts have focused mainly on water filtration and disinfection. E. coliform tests that are routinely done on water supplies are sometimes not able to detect the presence of Giardia, and outbreaks, such as one in Las Vegas, have occurred in which water tests have been negative for Giardia.
Iodine is effective in purifying small supplies of drinking water. A saturated solution must be used double strength with a 20-minute exposure at 20 degrees Celsius (Markell, John, Krotoski, p.62).
Erlandsen, Stanley L., ed. Giardia and Giardiasis: Biology, Pathogenesis, and Epidemiology. Plenum Press: New York, 1984.
Filice, Francis Patrick. “ Studies on the cytology and life history of a Giardia from the laboratory rat.” U. C. Publications in Zoology. v. 57, no. 2. University of California Press: Berkeley, 1952.
“Giardia”. McGill University website. http://martin.parasitology.mcgill.ca/jimspage/biol/giardia.htm
“Giardiasis Infection Fact Sheet.” Centers for Disease Control and Prevention. http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/factsht_giardia.htm
Katz, Michael, Dickson D. Despommier and Robert W. Gwadz. Parasitic Diseases. Second edition. Springer-Verlag: New York, 1989.
Markell, Edward K., David T. John and Wojciech A. Krotoski. Markell and Voge’s Medical Parasitology. Eighth Edition. W. B. Saunders: Philadelphia, 1999.
“Protozoan parasites (Cryptosporidium, Giardia, Cyclospora).” World Health Organization. http://www.who.int/water_sanitation_health/GDWQ/Microbiology/Microbioladd/GDWQMicroAdd4.pdf
by Michelle LaCour, Stanford University, 2003.