Giardia - 249-037-13


A Common Cause of Diarrhea

by Steven S. Bhimji, MD, PhD and Catherine J. Swift, MT(ASCP)
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This course is credentialed for:
Laboratory Technology (1.00 Contact Hr)

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One common organism associated with diarrhea is Giardia intestinalis (alsoknown as Giardia lamblia). Giardia was first identified in human stool in the early 1820s, but its role as a human pathogen was not recognized until the late 1970s. During the last 40 years, there have been several large community outbreaks of diarrhea in travelers who have returned from endemic regions, and by the late 1970s, it was clear that Giardia was not a harmless commensal organism of the intestine but a common protozoan known to cause “travelers’ diarrhea.” 1,2
Giardia is the most common cause worldwide of diarrhea in travelers; in the United States, Giardia is frequently identified as the most common cause of waterborne diarrhea.3 The protozoa have been found in close to 80% of raw water supplies from streams, lakes, ponds, rivers and mountain water. In addition, more than 15% of filtered water has also been found to contain Giardia.3,4
Giardia is a very simple organism with a two-stage life cycle: cyst and trophozoite. When the cyst is ingested from contaminated water or food, excystation occurs within minutes in the stomach and the duodenum. Excystation is the escape of the organism from the cyst once inside the host. The presence of acid and pancreatic enzymes plays a role in excystation. The trophozoites then enter the small bowel, where they rapidly multiply with a doubling time of less than 12 hours. This is why most people develop symptoms within seven to 10 days after ingestion of dirty water. The trophozoites have the ability to attach to small bowel enterocytes with adhesive disks.5
Unlike other parasites, there are no intermediate steps for transmission of Giardia. As the trophozoites pass into the large bowel, encystation occurs in the presence of neutral pH and secondary bile salts. Encystation is the forming of new cysts or capsules. Cysts are passed into the environment via feces, and the cycle repeats itself.1
The trophozoite form of Giardia has a very typical teardrop shape composed of microtubules and microribbons. The trophozoite has four pairs of flagella that help with forward motion. The organism has two symmetrical and very prominent nuclei that produce a face-like image on stained preparations.4
How Giardia causes diarrhea is not fully understood. Possible mechanisms include damage to small bowel brush border, release of enterotoxins, induction of immune mechanisms, inhibition of disaccharidase activity and altered gut motility. It is believed that the excess watery diarrhea is most likely due to hypersecretion via increased stimulation of the enzyme adenyl cyclase. Biopsies have revealed that in severe cases of Giardia, there is loss of the small bowel brush border surface area and flattening of villi.6
In the United States, Giardia is one of the most common intestinal parasites associated with diarrhea. Sudden outbreaks are common in the summer months. People at highest risk of acquiring these protozoa include travelers to endemic areas, campers, children in daycare centers, homosexual men and people with immunodeficiency (e.g., HIV). The incidence of Giardia is most common in the northeastern United States, and the peak infection times are late spring through the end of summer. However, transmission of giardiasis occurs throughout the United States, with more diagnosed cases occurring in northern states. The majority of U.S. outbreaks have occurred in populations where there is close physical contact, poor sanitation, lack of personal hygiene and low socioeconomic means.7
State incidence figures should be compared with caution because individual state surveillance systems have varying capabilities to detect cases. The seasonal peak in age-specific case reports coincides with the summer recreational water season and likely reflects increased outdoor activities and exposures such as camping and use of swimming venues (e.g., lakes, rivers, swimming pools and water parks) by young children. More information, including annual giardiasis surveillance, is available at the Centers for Disease Control and Prevention website.
Giardia has a global distribution and it commonly affects children in developing countries. In Europe and the United Kingdom, Giardia is the most common intestinal parasite associated with diarrhea.8 Worldwide data reveal that Giardia does not favor any particular race or culture; however, in North America the highest carrier rates are seen in native North Americans.
Giardiasis occurs in all ages but is most common in early childhood, possibly through exposure at daycare centers.8 The majority of epidemics reported over the past two decades have originated at daycare centers. A few studies indicate that prevalence of infection based on presence of cyst passage in stools may be as high as 25% in children younger than age 3.2,4
Fortunately, the majority who ingest Giardia are asymptomatic. Even those who develop an infection are not seriously ill for any extended period of time. However, a few patients may develop chronic diarrhea with malabsorption. These episodes often last several weeks to months. Death is very rare after Giardia infection and usually occurs in malnourished people or very young children. In developing countries, Giardia has been implicated as a major cause of failure to thrive.8
Giardia is usually acquired through ingestion of infectious protozoa from either contaminated water or food or through person-to-person contact (although acquisition of Giardia from food is atypical). In families who live close together, person-to-person transmission is common and reinfection rates are high. Giardia can also be acquired via oral-anal or oral-genital contact.4,9
The number of organisms required to produce disease is as low as 10 cysts. Ingestion of 25 or more organisms usually means a 100% probability of clinical infection. Giardia is a hardy organism and retains viability in cool temperatures for as long as eight to 12 weeks.5
Over the years, several strains of Giardia have been identified with varying ability to cause disease. Sometimes multiple strains may be present in the same patient. Giardia is also pathogenic in animals; the protozoa have been found in the stools of beavers, dogs, cats, squirrels, sheep, rodents and cattle.
When Giardia is acquired, not everyone develops a symptomatic infection. The attack rate is quite variable and depends on the environment and comorbid factors. Analysis of stool samples has revealed the presence of Giardia in healthy people living in endemic areas. These people become chronic carriers and continue to pass cysts in stools.10,11
Illustration courtesy of CDC/Alexander J. da Silva, PhD/Melanie Moser. 2002.
Removing Giardia Cysts From Water Sources
Removing Giardia cysts before they reach your water tap usually involves disinfection to inactivate cysts and filtration through a fine medium to remove cysts from the water. (BecauseGiardiacysts are resistant to disinfection, filtration is often required.) Giardia cysts are large in comparison to bacteria and viruses (ranging from 7 to 10 microns in diameter); therefore, they are more easily removed by filtration.
Both disinfection and filtration processes are used in combination at municipal plants to protect water supplies and ensure high-quality drinking water.12 Home water treatment techniques and devices are also available to help effectively remove Giardia. Techniques such as superchlorination, boiling and manufactured treatment devices are available to homeowners and outdoor enthusiasts.
Because normal chlorine levels used to kill bacteria in water supplies will not inactivate Giardia cysts, superchlorination is required. Not only is a higher concentration of chlorine in the water necessary, but increased contact time is required. Superchlorination can effectively eliminate one-time contamination events. The strong smell and taste of chlorine will make the water unpalatable for a period of time. Boiling is also effective against Giardia cysts.
A disinfection process widely used in European water treatment plants is now emerging as a home treatment technology in the United States. Ozone, a strong oxidizing gas, is injected into water and kills bacteria and Giardia cysts with less contact time than other disinfection processes. Unlike chlorination, ozone leaves no aftertaste or residue and has been found to be more effective at Giardia deactivation than chlorine.12
Risk Factors
The majority of U.S. outbreaks have occurred in populations where there is close physical contact, poor sanitation, lack of personal hygiene and low socioeconomic means.7 Outbreaks also occur in areas where camping and outdoor activities bring people into contact with contaminated streams or other water sources.
Although Giardia can be acquired by anyone, clinical disease does not manifest in all people. Predisposing factors that contribute to clinical symptoms include a variety of immunodeficiency syndromes (e.g., HIV), extremes of age, hypochlorhydria, malnutrition, cystic fibrosis and having blood group A.4
Once Giardia is acquired, the incubation period varies from seven to 14 days. Most people tend to develop symptoms somewhere between days five and 12. The average duration of the illness varies from two to 10 weeks.1,4
Giardiasis is a mild infectious disorder. The majority of people who develop Giardia infection will complain of watery, foul-smelling stools that appear greasy. A few patients may present with abrupt explosive watery diarrhea, abdominal cramps, fever, vomiting and general malaise. These symptoms usually last several days, which is then followed by a subacute phase of watery diarrhea for a few more days. Weight loss due to water loss is seen in more than 50% of patients. In developing countries, long-standing Giardia infections may be associated with failure to thrive.5,8
Other symptoms may include excessive belching, bloating, acid indigestion and heartburn. Blood in stools is very rare and should raise suspicion of another type of colitis.
Some people who develop giardiasis later complain of lactose intolerance; others may develop symptoms of functional bowel disease. Rarely presenting features include erythema multiforme, reactive arthritis, wheezing and biliary colic.10

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