Giardiasis

• Reporting criteria
• The disease and its epidemiology
• Case definition

• Laboratory testing
• Case investigation
• Disease control measures

• References
• Contact

What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency

• All specimens with a positive test for Giardia (regardless of specimen source) by DFA, enzyme immunoassays (EIA), polymerase chain reaction (PCR), or other form of testing should be reported to public health within four days (4) of the positive test result.
  • All positive Giardia laboratory tests should be reported to public health, regardless of symptoms. Public health will make the determination of whether the case meets the case definition.
• Cases should be reported using EpiTrax Reportal, fax or telephone to CDPHE or local health departments (telephone and fax numbers are listed at the end of this document).
• All potential foodborne, waterborne, and enteric disease outbreaks should be reported immediately to CDPHE or local health departments and entered into EpiTrax, even if the causative agent is not yet known.
  • In general, an “outbreak” is defined as an increase in the number of illnesses above what is normally expected among a specific population, within a given area, over a certain period of time.
  • Specifically, a foodborne disease outbreak occurs when two or more people get the same illness from the same contaminated food or drink.

Purpose of surveillance and reporting

• To identify cases for investigation and potential outbreaks
• To monitor trends in disease incidence

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Etiologic agent

Giardiasis refers to disease caused by Giardia lamblia (also known as G. intestinalis or G.duodenalis), a protozoan parasite that has two forms: cyst and trophozoite. Infected persons shed both trophozoites and cysts in stool. The cysts are infectious and can remain viable for weeks to months in the environment (e.g., water and soil).

Clinical description

Symptoms of giardiasis are variable but typically include watery, foul-smelling diarrhea. Abdominal cramps and a “bloated” feeling with excess gas often accompany the diarrhea. The diarrhea can be chronic or intermittent and it can be accompanied by fatigue and steatorrhea (fatty or greasy stools). Anorexia combined with malabsorption can lead to significant weight loss, failure to thrive in children, and anemia. However, some infections are asymptomatic. Symptoms often resolve in 2 to 6 weeks, but can last longer. Infected persons may wait to seek care for several weeks after onset because the symptoms can be intermittent.

Reservoirs

Humans are the primary reservoir, but other domestic and wild animals such as dogs, cats, and beavers can be infected. Different genetic assemblages (similar to genotypes) infect certain species, and some assemblages are more commonly seen than others.

Modes of transmission

Giardia is transmitted via the fecal-oral route. The parasite can be spread from person-to-person or through ingestion of water or food contaminated with feces of infected persons or animals.  Giardia spreads easily with only a few cysts needed to cause infection. Although it can infect anyone, children often get sick more often than adults.  

Outbreaks due to person-to-person transmission can occur in child care centers and other institutions. Transmission can also occur through certain types of sexual contact (e.g., oral-anal contact). Localized outbreaks may occur from water contaminated with human and/or animal feces. Untreated recreational water (e.g., streams, rivers, lakes), untreated or poorly maintained well water, and swimming pools/hot tubs have all been documented to be contaminated with Giardia. Eating food contaminated by an infected food handler can be a source, but this has been rarely documented. Transmission from infected animals can also occur.

Incubation period

The average incubation period is about 7–10 days but can range from 3–25 days or longer.

Infectious period

The disease is communicable for as long as the infected person excretes Giardia cysts, which generally begins at the onset of symptoms and can last for months. As is the case with most enteric illnesses, persons are most infectious when they have diarrhea and are usually more infectious than those who are asymptomatic. The infectious dose may be as few as 10 cysts.  

Epidemiology

During 2023, 668 cases were reported in Colorado, with a five-year average of 496 cases per year (2019﹘2023). Cases are reported throughout the year, but are more common in the summer and fall months. 

Giardia is responsible for approximately 6% of domestically-acquired waterborne infections in the United States. It is the most common enteric parasite. In 2022, there were 13,819 giardiasis cases reported in the United States, with a five-year average of 13,064 cases per year (2018﹘2022). The hospitalization rate for Giardia intestinalis is estimated to be 8.8%, with a case fatality rate of 0.1%.

Giardiasis has a worldwide distribution. Children are infected more frequently than adults. Prevalence is higher in areas of poor sanitation and in institutions with children who are not toilet trained, such as child care centers. 

For more data, see Colorado reportable disease data (Colorado data) and CDC WONDER (National data).

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Clinical description

An illness caused by the protozoan Giardia lamblia (aka G. intestinalis or G. duodenalis) and characterized by gastrointestinal symptoms such as diarrhea, abdominal cramps, bloating, weight loss, or malabsorption.

Laboratory criteria for diagnosis

Laboratory-confirmed giardiasis shall be defined as the detection of Giardia organisms, antigen, or DNA in stool, intestinal fluid, tissue samples, biopsy specimens, or other biological samples.

Case classification

Confirmed

A case that meets the clinical description AND the criteria for laboratory confirmation as described above. When available, molecular characterization (e.g., assemblage designation) should be reported.

Probable

A case that meets the clinical description AND that is epidemiologically linked to a confirmed case.

Note: The Giardia case definition requires that a case has symptoms consistent with the "clinical description”. However, CDPHE requests that public health continue the interview even if a case does not have symptoms consistent with Giardia, and leaves the state case status as the default, “confirmed”.

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• The Colorado State Public Health Laboratory (“State Lab”) provides testing services to support public health investigations.
• The State Lab may test clinical and/or environmental samples when determined to be necessary for public health investigations. These services vary by pathogen but may include:
  • Testing for individuals who have been restricted or excluded from work or child care by public health.
  • Testing for outbreak investigations, with prior approval from CDPHE.
  • Other testing services as discussed with CDPHE.
• The State Lab maintains a test directory, guidance for submitting samples, and other helpful resources on their webpage.
  • Additional questions can be directed to the State Lab coordinators by email at cdphe_labcoordinators@state.co.us (preferred method) or by phone 303-692-3069.

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Interviewing reported cases of giardiasis is optional. Historically, outbreaks of giardiasis are rare and single case investigations seldom lead to public health interventions. Public health interviews can help determine:

• Potential source of infection, and implement disease control measures as appropriate
• If others are ill (i.e., could this be an outbreak?)
• If the case may be a source of infection for others (e.g., a high-risk worker or a diapered child); and if so, prevent further transmission

Local health departments have primary responsibility for interviews of sporadic cases in their jurisdictions. CDPHE is available to assist with case investigations upon request through CDPHE’s Enteric Disease Interview Team (EDIT).

Forms 

If an interview is conducted, use the CDPHE Giardiasis Case Investigation Form for your interview and enter the information into the case’s morbidity record (CMR) in EpiTrax. 

For surveillance purposes, collect exposure histories for the 3-25 days before symptom onset. Determining the exposure period can be difficult for cases who do not have an acute onset of gastrointestinal symptoms. It’s important to do a complete assessment for GI illness, even when the specimen was collected from a source other than stool, to determine if any GI symptoms were present, even if mild. Use the following criteria to determine the case’s exposure period:

1. If the case reports diarrhea or vomiting, then use the onset date of diarrhea/vomiting to determine exposure period.
2. If the case does not report diarrhea or vomiting but reports other clinically relevant symptoms (see Clinical description section above), then use the onset date of other symptoms to determine exposure period.
3. If the case does not report onset of any clinically relevant symptoms (see Clinical description section above), then use the specimen collection date to determine exposure period*.

*Do not enter specimen collection date as “onset date” in EpiTrax (clinical tab); use this date to determine exposure period only. “Onset date” in EpiTrax only applies if the case experienced symptoms of illness. Otherwise, leave this field blank.

Due to the long incubation period of Giardia and its endemicity in the United States, please complete every exposure section of the interview form, even if the case reports international travel.

After completing the case interview, enter the information into the case’s morbidity record (CMR) in EpiTrax, including the investigation form and conduct any necessary disease control activities. If an outbreak is suspected, create a new outbreak in EpiTrax and contact CDPHE if assistance is needed; this includes requests for additional testing at the State Lab.

Identify and evaluate contacts

Symptomatic contacts

• Contacts of a case with a positive lab test who have clinically compatible symptoms are epi-linked probable cases and are treated the same as cases with a positive lab test for disease control purposes. See Disease control measures below.
  • Complete a case interview with all epidemiologically-linked probable cases.
  • Epi-linked probable cases who have been interviewed should be entered into EpiTrax as a new, probable giardiasis morbidity record (CMR). Link this newly-created CMR to the laboratory-confirmed case’s CMR under the “contacts” tab in EpiTrax.
• Refer symptomatic individuals who have not previously been tested (especially if they are high-risk workers) to their health care provider for stool testing and appropriate medical care.
• If a common source of exposure is suspected (e.g., a shared meal or social event), please create an outbreak record in EpiTrax and notify CDPHE as soon as possible.  

Asymptomatic contacts 

• Ask about high-risk occupations, including food handling, health care, community residential programs, and child care/school attendance.
• Provide information on symptom monitoring and next steps if symptoms develop.
  • Stress the importance of good hand washing, personal hygiene, excluding themselves from work, and notifying their supervisor whenever they have a diarrheal illness.  
  • If an asymptomatic contact develops diarrhea, follow the steps outlined above under symptomatic contacts. If the contact works in a high-risk setting, exclude them from work per the Managing special situations section below, even in the absence of a positive lab test.

Reported incidence is higher than usual/outbreak suspected

If the number of reported cases of giardiasis in your jurisdiction is higher than usual, or if an outbreak is suspected, investigate the situation to determine the source of infection and mode of transmission. Notify CDPHE as soon as possible to discuss potential stool testing of untested, symptomatic contacts and enter the outbreak into EpiTrax. CDPHE staff can assist local public health agencies to investigate outbreaks and determine disease control recommendations to prevent further cases, as well as coordinate surveillance of cases that cross county lines.

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Giardia (Giardiasis) | CDC

Treatment

Symptomatic patients can be treated with a variety of antiparasitic medications. Treatment of asymptomatic persons is not generally recommended. 

In general, individuals experiencing diarrhea are at risk of dehydration due to loss of fluids. If symptoms persist, worsen, or are severe, cases should promptly seek medical care.

Prophylaxis

No prophylactic treatment of close contacts is recommended.

Education

Encourage the case to:

• Avoid preparing food for other individuals until diarrhea has resolved.
• Avoid sex (vaginal, anal, and oral) or use a barrier (condom, dental dam) for two weeks after resolution of diarrhea.
• Avoid swimming for two weeks after resolution of diarrhea (this is due to the extended period of shedding cysts, the low infectious dose, the hearty nature of Giardia cysts, and the fact that waterborne transmission is well documented).
• Always wash hands thoroughly with soap and water for at least 20 seconds before eating or preparing food, after using the toilet, after changing diapers, and after touching pets or other animals.
• After changing diapers, wash your hands and the child’s hands for at least 20 seconds. Clean and sanitize diaper changing stations/areas often.
  • In child care and other institutional settings, dispose of stool and soiled diapers/linens in a sanitary manner.
• Avoid drinking untreated water from streams, lakes, rivers, springs, ponds, streams, or shallow wells. Avoid drinking untreated water or ice made from untreated water while traveling in developing countries or whenever the water quality is unknown.
  • Bringing water to a full, rolling boil is sufficient to kill Giardia or use filters certified for “cyst” or “oocyst” reduction.
  • Refer to CDC guidance for effective water treatment options while hiking, camping, and traveling.
• Avoid swallowing water when swimming. Lakes, streams, other surface waters, and swimming pools may be contaminated with Giardia.
• Adhere to local advisories to boil water.
• Avoid drinking raw milk, other unpasteurized dairy products, or unpasteurized apple cider. 

Managing special situations

Food handlers 

• Food handlers with giardiasis must be excluded from work until at least 24 hours after diarrhea has resolved without the use of anti-diarrheal medications and adequate hygiene can be maintained, ideally as verified by environmental health. "Exclude" means to prevent a person from working as an employee in a food establishment or entering a food establishment as an employee.
  • While individual circumstances may vary, cases are generally not required to provide two consecutive negative stool tests to return to work.
  • If a case has questionable hygienic practices or there are other concerns, consider excluding the case from work until the case has obtained two consecutive negative stool tests collected at least 24 hours apart and at least 48 hours after completion of antimicrobials, if antimicrobials are given.
• In an outbreak situation, longer exclusion and/or return to work testing may be required to return to food handling.
• The Colorado Retail Food Establishment Regulations may require additional restrictions or disease control measures for the case, their close contacts, and/or their employer.
  • Please consult with environmental health to determine if the case’s place of employment should enact additional disease control measures to be in compliance with the regulations. 

Child care centers/preschools

Refer child care providers to the CDPHE Infectious Disease in Child Care and School Settings for an overview of Giardia infections.

• Children or staff with giardiasis should be excluded until at least 24 hours after diarrhea has resolved without the use of anti-diarrheal medications.
  • Parents of cases should be counseled not to take their children to another child care center during this period of exclusion in order to avoid possible transmission of giardiasis at a new location.
  • Since many child care center staff assist with food preparation and/or feeding children, it is very important that staff with Giardia infection follow this guidance. See Disease control measures section (Food handlers), above.
  • If there are concerns about hygienic practices at the child care center, consider excluding the case from care until the case has obtained two consecutive negative stool tests collected at least 24 hours apart and at least 48 hours after completion of antimicrobials, if antimicrobials are given.
  • Parents and staff should be reminded that cases should not swim or wade in pools for at least two weeks after resolution of diarrhea.
• Determine whether additional children or staff are (or have recently been) ill with diarrheal illness.
  • Other children or staff with diarrhea should be excluded and referred to their health care provider for stool testing and appropriate medical care.
  • If other cases in the center are identified, initiate an outbreak investigation and consider sending a letter home to parents.
    • In an outbreak situation, longer exclusion and/or return to work/child care testing may be required. Contact CDPHE for assistance.
  • If the case is the only person in the classroom or facility who has been ill, no further action is indicated for other children/staff in that classroom or center.
• Reinforce the importance of meticulous hand washing, proper sanitizing and disinfection, and proper diaper changing technique with child care center staff. If possible, this should be verified by environmental health.
• Consult with CDPHE for instances in which children attend drop in style care at gyms, churches, etc.

Schools 

Refer school personnel to the CDPHE Infectious Disease in Child Care and School Settings for an overview of Giardia infections. 

• Students or staff with giardiasis should be excluded until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications.
  • If there are concerns about the case’s hygiene (e.g., the case has developmental disabilities and wears diapers) consider obtaining two consecutive negative stool tests collected at least 24 hours apart and at least 48 hours after completion of antimicrobials, if antimicrobials are given, before a case returns to school.
• Students or staff who handle food for others must not prepare food until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications. See Disease control measures section (Food handlers) above.
• In an outbreak situation, longer exclusion and/or return to work testing may be required.
• Parents and staff should be reminded that cases should not swim or wade in pools for 2 weeks after resolution of diarrhea.

Community residential programs (including facilities serving the developmentally disabled)

Actions taken in response to a case of giardiasis in a community residential program will depend on the type of program and the level of functioning of the residents. In general: 

• Staff members with giardiasis should be excluded from work until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications.
• For staff members who provide direct patient care (e.g., feed patients, give mouth or denture care, or give medications), follow guidelines for staff in health care facilities below.
• While individual circumstances may vary, in general, staff members are not required to provide two negative stool tests to return to work.
• Residents with giardiasis should be placed on contact precautions until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications.
  • If the resident has questionable hygiene, is incontinent, or there are other concerns, the resident should remain on contact precautions until they have obtained two consecutive negative stool tests collected at least 24 hours apart and at least 48 hours after completion of antimicrobials, if antimicrobials are given.
• Residents and staff with giardiasis must be excluded from handling or preparing food for others until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications. See Disease control measures section (Food handlers) above.
• In an outbreak situation, longer exclusion and/or return to work testing may be required.

Patients and staff in health care facilities (including hospitals, medical and dental clinics, skilled nursing and long-term care facilities)

Hospitals and skilled nursing/long-term care facilities generally have written infection control policies and procedures for handling cases of communicable disease among patients and staff members. 

If a facility does not have such policies in place, provide the following recommendations: 

• Health care workers with giardiasis who provide direct patient care or handle medications (e.g., pharmacists) should be excluded from work until at least 24 hours after diarrhea has resolved without the use of anti-diarrheal medications.
  • While individual circumstances may vary, in general, health care workers are not required to provide two negative stool tests to return to work.
• Patients with giardiasis should be placed on contact precautions until at least 24 hours after their diarrhea has resolved without the use of anti-diarrheal medications.
  • If the patient has questionable hygiene, is incontinent, or there are other concerns, the patient should remain on contact precautions until they have obtained two consecutive negative stool tests collected at least 24 hours apart and at least 48 hours after completion of antimicrobials, if antimicrobials are given.  
• In an outbreak situation, longer exclusion and/or return to work testing may be required.

Environmental measures

• Implicated food items and food prepared by cases must be removed from the environment.
• If a private drinking water source (e.g., well) is implicated, CDPHE recommends that the owner test for Giardia and can direct the owner to appropriate commercial laboratories to perform this testing.
• If a municipal drinking water source or other public supply is implicated, CDPHE will work with the Water Quality Control Division to ensure appropriate testing and follow up.
• If a diarrheal event from an infectious case occurred in a swimming pool, the LPHA should work with their environmental health team to implement remediation steps, such as hyperchlorinating the pool.

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About giardia infection. (2024, May 3). Giardia. https://www.cdc.gov/giardia/about/index.html 

American Academy of Pediatrics. Red Book: 2024-2027 Report of the Committee on Infectious Diseases, 33rd Edition. Illinois, Academy of Pediatrics, 2024.  

Estimate of waterborne disease burden in the United States. (2025, May 29). Waterborne Disease and Outbreak Surveillance Reporting. 
https://www.cdc.gov/healthy-water-data/waterborne-disease-in-us/results.html

Giardiasis 2011 Case Definition | CDC. (n.d.). 
https://ndc.services.cdc.gov/case-definitions/giardiasis-2011/

Giardiasis NNDSS Summary Report for 2019. (2024, May 22). Waterborne Disease and Outbreak Surveillance Reporting.
https://www.cdc.gov/healthy-water-data/documentation/giardiasis-nndss-summary-report-for-2019.html#:~:text=Since%202011%2C%20the%20incidence%20of,confirmed%20and%203.4%25%20nonconfirmed%29

Heymann DL, ed. Control of Communicable Diseases Manual, 21st Edition. Washington, DC, American Public Health Association, 2022.

National Center for Emerging and Zoonotic Infectious Diseases & Division of Foodborne, Waterborne, and Environmental Disease. (n.d.-a). Estimated annual number of hospitalizations and deaths caused by 31 pathogens transmitted commonly by food, United States. In National Center for Emerging and Zoonotic Infectious Diseases (Report No. CS228412).
https://www.cdc.gov/foodborneburden/pdfs/scallan-estimated-hospitalizations-deaths-foodborne-pathogens.pdf

CDPHE Communicable Disease Branch

• Phone: 303-692-2700 or 800-866-2759
• Fax: 303-782-0338
• After hours: 303-370-9395

CDPHE Lab Coordinators: 

• Email (preferred): cdphe_labcoordinators@state.co.us  
• Phone: 303-692-3480

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