Giardia lamblia, The Most Common

Parasitic Infection in Humans Worldwide


by Kista Wnek



Giardiasis is an intestinal infection caused by Giardia intestinalis, also known as Giardia lamblia or Giardia duodenalis. A single–celled eukaryote with a simple mechanism for basic processes, such as DNA replication and RNA processing, (2) Giardia exists in two stages, the trophozoite stage and the cyst stage. The trophozoite is pear shaped with bilateral symmetry. It has a pair of nuclei, when stained under a microscope resemble a pair of eyes looking back at you, and two sucking disc that attach the flagellated protozoan to the intestinal wall.(1) The cyst is oval with four nuclei, and a protective outer shell that is resistant to environmental factors. The genes allow Giardia to evade human immune response by shifting surface proteins, a characteristic not found in other parasites.(2) Giardia is a genus of anaerobic flagellated protozoan parasites.




The Giardia parasite lives in infected humans and animals, transmission is by oral-fecal contamination. Once infected Giardia lives in the intestine and is passed through the body and out in the feces. The cyst protective outer shell allows it to survive for months. Usually a waterborne disease, Giardia can also be found on surfaces, in soil, food, or water that has been infected by human or animal waste. (3) Oral-anal sexual practices also provide transmission of this disease. (1)Ingestion of cyst in contaminated water, food, or by oral-fecal route begins the cycle. When the host consumes the cyst, the acidic stomach environment stimulates excystation. (1) Each cyst produces two trophozoites; they multiply by longitudinal binary fission. In the small bowel they remain where they can be free or attach to the mucosal wall by a sucking discs. Encystation occurs as the parasites move toward the colon. The cyst are infectious when the stool is passed and shortly after, making person-to-person transmission possible. Trophozoites are also passed but do not survive the environment. (4) Giardia is not spread by contact with blood. (5)



 Signs & Symptoms:

Acute Giardiasis may develop after an incubation period of 1-14 days. Symptoms may include:


·                    Diarrhea

·                    Abdominal pain

·                    Bloating

·                    Nausea

·                    Vomiting

·                    Malaise

·                    Flatulence

·                    Cramping

·                    Greasy stools that tend to float

·                    Asymptomatic

Symptoms may last 2 to 6 weeks or longer.

Chronic Giardiasis may follow, which is difficult to eliminate. Bouts of loose stool and steatorrhea, followed by malabsorption, fatigue, weight loss and dehydration. (1,3&4)




 Identification of cysts or trophozoites in stool samples provide the diagnosis. Basic guidelines require at least three stool samples, to be tested over a few days, because cyst excretion occurs intermittently all samples may not contain the parasite. Direct mounts as well as concentration procedures may be used. Samples of duodenal fluid or biopsy may reveal trophozoites.(4) Direct fluorescent antibody technique is used to detect low numbers of cyst.  Enzyme immunoassay is used to detect antigens of Giardia in stool samples. Both DFA & EIA  require futher confirmation if detection is borderline positive or a questionable negative.(6) Stool studies for ova, and parasites continue to be the most common diagnostic procedure.(1)




 G. intestinalis was first noted by Anthony van Leeuwenhook, making this observation while examining his own feces under a microscope.

“All the particles aforesaid lay in a clear transparent medium, wherein I have sometimes also seen animalcules a-moving very prettily; some of ‘em a bit bigger, others a bit less, than a blood globule, but all of one and the same make.  Their bodies were somewhat longer than broad, and their belly was flatlike, furnisht with sundry little paws, where with they made such a stir in the clear medium and among the globules that you might e’en fancy you saw a pissabed (type of bug) running up against a wall; and albeit they made a quick motion with their paws, yet for all that they made but slow progress.”(4)

However, he did not make the connection between the creature and a disease process.(7) Two hundred years after, in 1859, Dr. Vilem Lambl observed the parasite in children with diarrhea believing it to be commensal, not responsible for disease. The parasite was named Giardia lamblia after Dr. Lambl. In the 1950s,R.C. Rendtorff  identified infection after the introduction of small numbers of Giardia cyst. J D Karapetyan, was able to culture G.intestinalis for the first time in the 1960s. However, it was not until the late 1970s that Giardia was confirmed as a pathogen.(7)


Giardia in U.S. & the World:

 Giardiasis is widespread geographically in the U.S. Incidences may be higher in the northern states. Five classes of people who are at risk include:


1) People in cities whose drinking water originates from streams or rivers and whose water treatment process does not include filtration, or filtration is ineffective because of malfunctioning equipment. 2) Hikers/campers/outdoors people. 3) International travelers 4) Children who attend day-care centers, day-care center staff, and parents and siblings of children infected in day-care centers. 5) Homosexual men. Animals may infect rivers or streams that seem clean and unpolluted. The illness usually begins in early summer when outdoor activity increases. All age groups are affected but children 1-9 and adults 35-44 who care for them, have the highest rate of infection. Oral–fecal contamination is more likely in children who pass it on to caregivers. Investigation of a Giardiasis outbreak in a day care facility determined that 47% of sick children transmitted the disease to more than one family member. In 2003, two community wide outbreaks of Giardiasis were traced to wading pools, one in Massachusetts, the other in Iowa. The disease spreads to the community via secondary transmission, more than 150 people were affected.(8) Giardia is a worldwide problem, and prevalence is higher in populations with poor sanitation. Africa, Asia and Latin America are highly infected. Approximately 200 million people have symptomatic Giardiasis. Although not life threatening (2) in the U.S., Giardiasis can lead to severe complications such as poor nutrient absorption, weight loss and dehydration. Complications that children and the elderly in underdeveloped countries may find hard to live with.(8)




 Prevention begins with good hygiene;


·        Wash hands thoroughly with soap & water


1.      After using the bathroom

2.      After changing a diaper

3.      After handling animals or their belongings

4.      After touching soil, trash cans, cleaning outside items

·        Be aware of animals or sewage overflows getting into drinking water

·        Polluted storm water runoff & agriculture runoff

·        Do not allow children to swim if they have diarrhea

·        Avoid water that may be contaminated, do not drink from shallow wells ,lakes ,rivers or ponds

·        Do not swallow recreational water.

·        Do not drink water or use ice from communities that have been hit by large storms, had broken water lines or outbreaks of disease caused by the water supply.

·        Avoid food that may be contaminated. Wash all foods to be eaten raw, peel, or cook

·        Avoid eating raw foods or drinking water in countries while traveling.

·        Avoid fecal exposure during sexual activity.(3)


Always consider outside water infected, 10 to 25 cysts can cause human illness

several prescription drugs are available to treat Giardiasis. Notify your health-care provider if you suspect ingestion .People with compromised immune system, infants and the elderly

should be especially cautious. If you are concerned about your drinking water, Giardia can be

killed by heating water to a rolling boil for at least one minute. Water should be stored in the refrigerator in a clean container with a lid.(5)Home water filters are also available.

·        Reverse osmosis

·        Absolute pore size of 1 micron or smaller

Work Cited


1. Giardia. 2008.  (accessed March 14, 2009).


2. Genome of highly infectious parasite, Giardia Lamblia Unlocked. October 3, 2007.

  (accessed March 14, 2009).


3. Giardia Infection. giardia.htm  (accessed March 14, 2009).


4. Causal Agent. December 5, 2008.  (accessed March 14, 2009).


5. Fact Sheet. December 29, 2008. (accessed March 14, 2009).


6.Giardia intestinalis.  February 13, 2009.   (accessed March 14, 2009).


7.History. February 15, 2009.  (accessed March 14, 2009).