Epidemiology, Infectious Diseases, Parasites

Giardiasis: Why Daycare Centers and Pools May be Spreading the Giardia Parasite

Author Chandana Balasubramanian , 13-Dec-2022

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Giardiasis is an infection in the small intestine caused by a parasite called Giardia lamblia, Giardia duodenalis, or Giardia intestinalis. It is a common waterborne disease that causes stomach cramps, watery diarrhea, bloating, and flatulence. Giardiasis is typically spread through contaminated water or food, or person to person, and is found worldwide. The parasite is hardy and thrives in water, soil, and feces of infected animals. It can lurk in our pools, hot tubs, lakes, water supply, and daycare centers [1,2]. 


Giardia has the dubious distinction of being a leading cause of diarrhea in the world. In the developed world, giardiasis-related deaths are very rare; most people recover in a few weeks. In developing countries, giardiasis infection rates are higher, affecting 2% of adults and 8% of children, and 33% of the population has been infected with giardiasis. In developed countries, international travelers, daycare and nursing home workers, and people who work with human waste are most at risk of getting a giardiasis infection [3]. 


In North America, giardiasis is also known as ‘beaver fever’ after an outbreak at Banff National Park, Canada. At the time, hikers got sick from drinking water contaminated with giardia from beavers [4]. 



The giardia parasite was first discovered in 1681 by Antony van Leeuwenhoek. The Dutch scientist is also known as the ‘Father of Microbiology’ for his contributions to the field. After all, his fascination with lens-making resulted in a handcrafted microscope. He became the first to observe microorganisms after studying his own diarrheal stools. At the time, he named the microorganisms ‘animalcules’ (tiny animals). 

A few centuries later, in 1859, Lambl described giardia in detail and named it Cercomonas intestinalis. However, the parasite was renamed in 1915 to giardia lamblia to recognize Professor Giard (a French professor who also described giardia) and Lambl for their contributions [5].



The giardia parasite can be found all over the world because it can survive even in harsh conditions. Although giardiasis is not serious, it can cause severe dehydration in young children and the elderly [1].

Notable giardiasis outbreaks in developed countries include: 


 – According to the Government of Canada, giardia is Canada’s most commonly identified intestinal parasite. 

 – 1972-1973: An outbreak of 895 cases was reported in the county of Edmonton.

 – In 2016, there were 3,818 giardiasis cases reported [6].

United States: 

 – There were 242 outbreaks reported to the CDC between 1971-2011. Predominantly, these outbreaks were spread by contaminated water.  

 – From 1980-1983, there was a spike in waterborne giardiasis cases in three states, Colorado, Vermont, and Washington.

 – 1995: 1,449 cases in New York State due to lake water contamination [7].


 – August – October 2004: 1300 lab-confirmed cases in the city of Bergen.

 – The majority affected were women aged 20 – 29.

 – Leaking sewage pipes combined with undertreated water were found to be the source of the infection.

 – Giardiasis is not endemic to Norway. Until the 2004 outbreak, 90% of cases were due to travelers [8].


 – November 2018-April 2019: Giardiasis in tap water affected over 200 people in Bologna province (Northeast Italy). This is the first documented giardia outbreak in Italy [9]. 

In the United States, giardiasis is a nationally notifiable disease, which means healthcare providers and labs need to report cases to local or state health departments. These agencies then report to the CDC so that a public health response can be mobilized as required [10].

How is it spread?


Giardiasis spreads mainly through contaminated water, food, or surfaces. It is one of the most common waterborne diseases in the United States [17]. It can also spread from person to person. Drinking unfiltered or untreated water is one of the most common ways giardia spreads. Ingesting water from a shared swimming pool or lake is another source. Even chlorinated pools and water parks are not safe from this parasite. This is because giardia has a hard exterior and can survive for 45 minutes in water. Ingesting even a tiny amount of feces infected with giardia can get us sick [5,11]. 

Other common ways to get a giardia infection are:

  • Traveling to regions with poor sanitary practices
  • Swallowing infected food
  • Fecal-oral contact through sexual activity
  • Being in close contact with an infected individual
  • Touching contaminated surfaces in daycares, like diapers, toys, changing tables


Since giardiasis can be transmitted through oral or anal sex, it is considered a sexually-transmitted infection (STI or STD) [12].  

Although giardia can infect animals, the chance of getting giardia from pets is low, according to the CDC [13]. 

Biology of the disease


Giardia is a flagellated, eukaryotic microorganism. The parasite is believed to have had ancestors from a billion years ago [14]. When a person ingests giardia cysts, the cysts make their way through the digestive tract. Once they reach the small intestine, they release trophozoites (the growing or vegetative phase of a giardia’s lifecycle). Giardia’s trophozoites begin absorbing nutrients from the host. 

Eventually, the trophozoites split in two, move to the host’s colon, and turn into cysts. Some of these cysts are excreted through the stool and infect anyone else who comes in contact with them. In feces, they can survive for a long time in soil and water [3,5]. 

The rest of the giardia cysts, which remain in the small intestine, burrow into the epithelial lining. They interfere with how the body absorbs fats and carbohydrates from food, resulting in bloating, diarrhea, and other symptoms [1,3,5].



Many people with giardia have no symptoms. Infected individuals can recover within 2 to 6 weeks [15]. 

Symptoms of giardia are:

  • Diarrhea
  • Bloating and gas
  • Greasy feces
  • Stomach pain and cramps
  • Nausea, vomiting
  • Dehydration 
  • Fatigue [15]

The poop with giardia is watery with a foul smell. Stools may be greasy and float on top. 

Some people may develop a fever, itchiness, and swelling in the eyes and joints (inflammatory response). Children with severe giardiasis are susceptible to physical and mental developmental issues and malnutrition [3, 15].



Laboratory testing on stool samples is the best way to diagnose giardiasis. Doctors may ask for several stool samples over a few days since giardia is not always excreted in every bout of diarrhea [16].



Most people who develop symptoms of giardiasis can recover on their own in a few weeks. Infected individuals must drink a lot of water to counter the effects of dehydration [17]. 

For severe cases of giardiasis, medications include:  

  • Metronidazole (Flagyl) is the most common antibiotic for giardiasis
  • Tinidazole (Tindamax) 
  • Nitazoxanide (Alinia)


Doctors may advise an anti-inflammatory diet that avoids fatty foods, acidic food like tomatoes or lemons, spicy food, and raw vegetables until recovery.



The best way to minimize your risk of getting infected with giardia is to:

  • Avoid drinking untreated water in pools, lakes, rivers
  • Wash hands regularly with soap, especially before eating, after gardening (giardia can be found in soil), and after changing diapers 
  • Sanitize toys and other household surfaces
  • Teach your children not to drink the water in pools, water parks, and lakes
  • Avoid direct contact with animal poop. If unavoidable, ensure you wash and clean your hands with soap and water before touching your eyes or face
  • Avoid uncooked or under-cooked food if traveling to countries where you may not be sure of their hygienic practices
  • Practice safe sex since giardia can last in feces for weeks [18]


Lowering the number of giardiasis outbreaks in developing nations requires targeted public health policies and education. According to WHO, at least 2 billion people worldwide drink water contaminated with feces. Apart from water, contaminated water can cause cholera, dysentery, polio, typhoid, and other waterborne diseases. WHO estimates that over 800,000 people, including children, die each year from diarrhea because of unsanitary drinking water [19].

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Teaching? Download our FREE lesson plan on Giardiasis


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[1] “Giardia,” Cdc.gov, 19-May-2022. [Online]. Available: https://www.cdc.gov/parasites/giardia/index.html. [Accessed: 22-Nov-2022].

[2] L. Cernikova, C. Faso, and A. B. Hehl, “Five facts about Giardia lamblia,” PLoS Pathog., vol. 14, no. 9, p. e1007250, 2018.

[3] N. Dunn and A. L. Juergens, “Giardiasis,” in StatPearls [Internet], StatPearls Publishing, 2022.

[4] Gov.on.ca. [Online]. Available: https://www.health.gov.on.ca/en/public/publications/disease/giardiasis.aspx. [Accessed: 22-Nov-2022].

[5] L. A. Bartelt and R. B. Sartor, “Advances in understanding Giardia: determinants and mechanisms of chronic sequelae,” F1000Prime Rep., vol. 7, p. 62, 2015.

[6] Canada.ca. [Online]. Available: https://www.canada.ca/en/public-health/services/diseases/giardia-infection/health-professionals.html. [Accessed: 22-Nov-2022].

[7] E. A. Adam, J. S. Yoder, L. H. Gould, M. C. Hlavsa, and J. W. Gargano, “Giardiasis outbreaks in the United States, 1971-2011,” Epidemiol. Infect., vol. 144, no. 13, pp. 2790–2801, 2016.

[8] K. Nygård et al., “A large community outbreak of waterborne giardiasis-delayed detection in a non-endemic urban area,” BMC Public Health, vol. 6, no. 1, p. 141, 2006.

[9] D. Resi et al., “A large outbreak of giardiasis in a municipality of the Bologna province, north-eastern Italy, November 2018 to April 2019,” Euro Surveill., vol. 26, no. 35, 2021.

[10] Cdc.gov. [Online]. Available: https://www.cdc.gov/parasites/giardia/audience-health-professionals.html. [Accessed: 22-Nov-2022].

[11] “Facts About Giardia and Swimming Pools,” Cdc.gov. [Online]. Available: https://www.cdc.gov/healthywater/pdf/swimming/resources/giardia-factsheet.pdf. [Accessed: 22-Nov-2022].

[12] A. A. Escobedo, P. Almirall, M. Alfonso, S. Cimerman, and L. Chacín-Bonilla, “Sexual transmission of giardiasis: a neglected route of spread?,” Acta Trop., vol. 132, pp. 106–111, 2014.

[13] “Giardia and pets,” Cdc.gov, 02-Mar-2021. [Online]. Available: https://www.cdc.gov/parasites/giardia/prevention-control-pets.html. [Accessed: 22-Nov-2022].

[14] J. Steenhuysen, “Human parasite genome reveals long past,” Reuters, Reuters, 27-Sep-2007.

[15] “Illness and symptoms,” Cdc.gov, 02-Mar-2021. [Online]. Available: https://www.cdc.gov/parasites/giardia/illness.html. [Accessed: 22-Nov-2022].

[16] “Diagnosis and treatment,” Cdc.gov, 02-Mar-2021. [Online]. Available: https://www.cdc.gov/parasites/giardia/diagnosis.html. [Accessed: 22-Nov-2022].

[17] “Giardia infection (giardiasis),” Mayoclinic.org, 08-Nov-2022. [Online]. Available: https://www.mayoclinic.org/diseases-conditions/giardia-infection/diagnosis-treatment/drc-20372790. [Accessed: 22-Nov-2022].

[18] “Prevention and control,” Cdc.gov, 10-Aug-2021. [Online]. Available: https://www.cdc.gov/parasites/giardia/prevention-control.html. [Accessed: 22-Nov-2022].

[19] “Drinking-water,” Who.int. [Online]. Available: https://www.who.int/news-room/fact-sheets/detail/drinking-water. [Accessed: 22-Nov-2022].

Chandana Balasubramanian

Chandana Balasubramanian is an experienced healthcare executive who writes on the intersection of healthcare and technology. She is the President of Global Insight Advisory Network, and has a Masters degree in Biomedical Engineering from the University of Wisconsin-Madison, USA.

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