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Giardiasis Surveillance — United States, 2011–2012

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Giardiasis Surveillance — United States, 2011–2012

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MMWR Surveillance Summaries
Vol. 64, No. SS-3
May 1, 2015

Giardiasis Surveillance — United States, 2011–2012

Surveillance Summaries

May 1, 2015 / 64(SS03);15-25

Julia E. Painter, PhD
Julia W. Gargano, PhD
Sarah A. Collier, MPH
Jonathan S. Yoder, MPH
Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC

Corresponding author: Julia W. Gargano, PhD, National Center for Emerging and Zoonotic Infectious Diseases, CDC. Telephone: 404-718-4893; E-mail:igc5@cdc.gov.


Problem/Condition: Giardiasis is a nationally notifiable gastrointestinal illness caused by the protozoan parasite Giardia intestinalis.
Reporting Period: 2011–2012.
Description of System: Forty-four states, the District of Columbia, New York City, the Commonwealth of Puerto Rico, and Guam voluntarily reported cases of giardiasis to CDC through the National Notifiable Diseases Surveillance System (NNDSS).
Results: For 2011, a total of 16,868 giardiasis cases (98.8% confirmed and 1.2% nonconfirmed) were reported; for 2012, a total of 15,223 cases (98.8% confirmed and 1.3% nonconfirmed) were reported. In 2011 and 2012, 1.5% and 1.3% of cases, respectively, were associated with a detected outbreak. The incidence rates of all reported cases were 6.4 per 100,000 population in 2011 and 5.8 per 100,000 population in 2012. This represents a slight decline from the relatively steady rates observed during 2005–2010 (range: 7.1–7.9 cases per 100,000 population). In both 2011 and 2012, cases were most frequently reported in children aged 1–4 years, followed by those aged 5–9 years and adults aged 45–49 years. Incidence of giardiasis was highest in Northwest states. Peak onset of illness occurred annually during early summer through early fall.
Interpretation: For the first time since 2002, giardiasis rates appear to be decreasing. Possible reasons for the decrease in rates during 2011–2012 could include changes in transmission patterns, a recent change in surveillance case definition, increased uptake of strategies to reduce waterborne transmission, or a combination of these factors. Transmission of giardiasis occurs throughout the United States, with more frequent diagnosis or reporting occurring in northern states. Geographical differences might suggest actual regional differences in giardiasis transmission or variation in surveillance capacity across states. Six states did not report giardiasis cases in 2011–2012, representing the largest number of nonreporting states since giardiasis became nationally notifiable in 2002. Giardiasis is reported more frequently in young children, which might reflect increased contact with contaminated water or ill persons, or a lack of immunity.
Public Health Action: Educational efforts to decrease exposure to unsafe drinking and recreational water and prevent person-to-person transmission have the potential to reduce giardiasis transmission. The continual decrease in jurisdictions opting to report giardiasis cases could negatively impact the ability to interpret national surveillance data; thus, further investigation is needed to identify barriers to and facilitators of giardiasis case reporting. Existing state and local public health infrastructure supported through CDC (e.g., Epidemiology and Laboratory Capacity grants and CDC-sponsored Council of State and Territorial Epidemiologists Applied Epidemiology Fellows) could provide resources to enhance understanding of giardiasis epidemiology.


Giardia intestinalis (also known as G. lamblia and G. duodenalis), a flagellated protozoan, is the most common intestinal parasite of humans identified in the United States (1), and a common cause of outbreaks associated with untreated surface and groundwater (2,3). Annually, an estimated 1.2 million cases occur in the United States (4); and hospitalizations resulting from giardiasis cost approximately $34 million (5).
Giardiasis is generally a self-limited illness typically characterized by diarrhea, abdominal cramps, bloating, weight loss, and malabsorption; asymptomatic infection also occurs frequently (6–8). Case reports and epidemiologic studies have associated giardiasis with the development of chronic enteric disorders, allergies, chronic fatigue, and reactive arthritis (9–14).
Giardia infection is transmitted by the fecal-oral route and results from the ingestion of Giardia cysts through the consumption of fecally contaminated food or water or through person-to-person (or, to a lesser extent, animal-to-person) transmission (15). The cysts are environmentally hardy, moderately chlorine tolerant, and infectious immediately upon being excreted in feces (16). The infectious dose is low; ingestion of 10 cysts has been reported to cause infection (16). Infected persons have been reported to shed 108–109 cysts in their stool per day and to excrete cysts for months (16–18). Effective therapies are available for patients with symptomatic giardiasis, including metronidazole, tinidazole, nitazoxanide, paromomycin, furazolidone, albendazole, and quinacrine (19).
Giardia is primarily transmitted through ingestion of infected human waste (20,21). Drinking untreated water from lakes and rivers, swimming, having contact with some animal species, and sexual practices involving fecal contact might increase risk for giardiasis (22). Giardiasis is often detected in international travelers (23,24) and among internationally adopted children (25). Transmission to close contacts of infected persons can also occur, including to children in child-care settings and their caregivers (18,26) or persons with occupational exposure to human waste (2021).
CDC recommends that health-care providers consider giardiasis in their differential diagnosis when a patient experiences diarrhea lasting >3 days. Routine examination of stool for ova and parasites does not always include testing for Giardia (27); thus, health-care providers should specifically request Giardia testing. Cyst excretion can be intermittent. Because the parasite might not be detected in a given stool specimen, three stool specimens collected on separate days should be examined before considering test results to be negative (28). Direct fluorescent antibody (DFA) testing is an extremely diagnostically sensitive and specific detection method and is considered the standard in Giardia testing (29).
In the United States, Giardia has been reported since 1992 and became a nationally notifiable disease in 2002. Surveillance data for 1992–2010 have been published previously (3034). This report summarizes national giardiasis surveillance data for 2011–2012. Federal, state, and local public health agencies can use these giardiasis surveillance data to better understand the epidemiology of giardiasis in the United States, design efforts to prevent the spread of disease, and establish research priorities.


Case Definition
The first national case definition was published in 1997 (35), and a revised case definition was published in 2011 (36). The current (2011) case definition differs from the 1997 definition in clarifying that clinical symptoms are necessary for categorizing giardiasis cases as confirmed.
Giardiasis is an illness caused by the protozoan Giardia intestinalis (also known as G. lamblia or G. duodenalis) and characterized by gastrointestinal symptoms (e.g., diarrhea, abdominal cramps, bloating, weight loss, or malabsorption). A confirmed case of giardiasis is defined as a case that meets the clinical description and the criteria for laboratory confirmation. Laboratory-confirmed giardiasis is defined as the detection of Giardia organisms, antigen, or DNA in stool, intestinal fluid, tissue samples, biopsy specimens, or other biological samples (36).
Nonconfirmed cases of giardiasis include probable, suspected, and unknown cases. A probable case of giardiasis meets the clinical description and is epidemiologically linked to a confirmed case (36). A national case definition for suspected cases of giardiasis does not exist; the definition varies by state. If cases are not classified as confirmed, probable, or suspected, then they are considered unknown.
Forty-four states, the District of Columbia (DC), New York City (NYC), the Commonwealth of Puerto Rico, and Guam voluntarily reported cases of giardiasis to CDC through the National Notifiable Diseases Surveillance System (NNDSS) in 2011 and 2012. Giardiasis was not reportable in Kentucky, Mississippi, North Carolina, Oklahoma, Tennessee, and Texas. Reports include the patient's place of residence (state), age, sex, race, ethnicity (Hispanic or non-Hispanic), date of symptom onset, and whether the case is outbreak-associated. As has been done historically, the criteria for CDC's annual summary of notifiable diseases were used to classify case status (37). Because data in this report were finalized at a different time, the number of cases differs slightly from the number reported in CDC's annual summary of notifiable diseases.
National giardiasis surveillance data for 2011–2012 were analyzed using statistical software. Numbers, percentages, and incidence rates (cases per 100,000 population) of giardiasis were calculated in aggregate and separately for the United States and territories. Rates were calculated by dividing the number of reported new giardiasis cases by each year's mid-year census estimates for the reporting jurisdictions and multiplying by 100,000 (38). In addition to analyzing data nationally and by reporting jurisdiction, data were analyzed by region (Northeast, Midwest, South, and West regions), as defined by the U.S. Census Bureau (39). To account for differences in the seasonal use of recreational water, the West region was further subdivided into Northwest and Southwest. To examine reporting over time, giardiasis rates per 100,000 population were calculated by year (from 1993 to 2012) and case type (confirmed or nonconfirmed). To assess current patterns in reporting, average annual giardiasis rates per 100,000 population were calculated by demographic variables (e.g., age and sex) and by month of symptom onset across 2011–2012 combined. This was performed by summing all cases occurring in the 2-year period, and then dividing by the sum of the number of persons in reporting jurisdictions in each year, and multiplying by 100,000. Rates could not be calculated for some variables (race and ethnicity) because of a large percentage of reports missing data (>20%) for these variables.


During 2011–2012, all jurisdictions in the United States where giardiasis is reportable (including 44 states, DC, and NYC) voluntarily reported giardiasis cases to CDC through the National Notifiable Diseases Surveillance System (NNDSS). Among United States territories, Puerto Rico reported cases in 2011–2012, and Guam reported cases in 2012. A total of 16,868 giardiasis cases were reported in 2011 (98.8% confirmed), and 15,223 cases were reported in 2012 (98.8% confirmed) (Table 1). The rates of reported cases were 6.4 per 100,000 population in 2011 and 5.8 in 2012. This represents a slight decline from the relatively steady rates observed during 2005–2010 (range: 7.1–7.9 cases per 100,000 population), and a further decline from the peak of 13.84 cases per 100,000 population reported in 1995 (Figure 1). Approximately 99% of cases were confirmed for both 2011 and 2012, which is consistent with previous years. Of all cases reported for 2011 and 2012, 1.5% (251 of 16,868) and 1.3% (200 of 15,223) were reported to be associated with a detected outbreak (Table 1).
By region, the rates of reported giardiasis cases per 100,000 population ranged from 4.8 in the Southwest to 9.4 in the Northwest in 2011 and 4.6 in the Southwest and South to 8.5 in the Northwest in 2012 (Table 1). By state, giardiasis rates were lowest in Arizona (2.1 in 2011 and 1.7 in 2012) and highest in Vermont (35.6 in 2011 and 29.2 in 2012); 10 jurisdictions had rates higher than 10 per 100,000 population in 2012 (Figure 2Table 1). The categories delineated in Figure 2 were initially used in the 1998–2002 giardiasis surveillance summary (31). In this and previous surveillance summaries they have remained the same to allow for comparison over time.
Surveillance data displayed a bimodal age distribution, with the largest number and rate of reported cases occurring among young children aged 1–9 years, with a smaller, flatter peak among middle-aged adults aged 40–49 years (Figure 3). In both 2011 and 2012, the largest number of cases was reported in children aged 1–4 years followed by those aged 5–9 years and adults aged 45–49 years. During 2011–2012, the rate of reported giardiasis per 100,000 population was highest in children aged 1–4 years (16.4) and 5–9 years (8.4) followed by adults aged 40–44 years (6.3) and 45–49 years (6.2). Rates were lowest among adults aged ≥80 years (2.7 per 100,000 population).
During 2011–2012, a total of 18,437 (57.7%) patients were male and 13,354 (41.8%) were female; 190 (0.6%) were missing data on sex (Table 2). The majority of cases for which data on race were available occurred among whites, followed by blacks, and Asians/Pacific Islanders (Table 2). However, data on race were not included for 41.2% of total annual case reports. Although 6.5% (2,074 of 16,590) of patients were identified as Hispanic, data on ethnicity were lacking for 48.1% of total annual case reports.
Analysis of rates by age and sex showed that giardiasis rates were higher among males in almost every age group (Figure 4). This difference was most pronounced among males aged 45–49 years. However, in persons aged 65–69 years, rates were slightly higher among females than males (5.1 versus 4.9 per 100,000 population).
Date of symptom onset was reported for 17,105 (53.5%) of the 31,981 cases during 2011–2012. The number of cases by symptom onset peaked in late July to early August (n = 1,075), which was 2.2 times higher than the lowest number of cases by symptom onset in February (n = 480) (Figure 5).


National giardiasis surveillance data are critical in assessing the disease prevalence and epidemiologic characteristics of giardiasis in the United States. Following a gradual decline in case reports during 1996–2001, the number of case reports and disease rates stabilized during 2002–2010, coinciding with the disease becoming nationally notifiable in 2002 (Figure 1) (3034). For the first time in 10 years, giardiasis rates appear to be decreasing. Possible reasons for the decreased rates during 2011–2012 might include changes in transmission of disease caused by Giardia, a decreased emphasis on giardiasis surveillance in public health agencies, the 2011 change in case definition clarifying that clinical symptoms are necessary for categorizing giardiasis cases as confirmed, increased uptake of strategies to reduce waterborne transmission (e.g., implementation of EPA's Ground Water Rule to address contamination of public ground water systems (40), or a combination of these factors.
The last national surveillance data were published in 2009–2010 (34). Since publication of that data, rates have declined across all regions. Giardiasis rate reductions were most pronounced in the Midwest, where rates declined from 10.3 and 11.4 per 100,000 population in 2009 and 2010 to 6.6 and 5.8 in 2011 and 2012. As in previous years, rates were highest in northern states, and Vermont reported the highest rate for the last 7 years. The geographic differences might suggest actual regional differences in giardiasis transmission, or they might reflect variation in surveillance capacity across states. Although giardiasis is a nationally notifiable disease, six states did not report giardiasis during 2011–2012. The number of states that do not report giardiasis cases has increased from four to six states over the past 2 years (34). This represents the largest number of nonreporting states since giardiasis became nationally notifiable in 2002(3134), which is a concerning trend, given that giardiasis is the most frequently identified enteric parasite in the United States (1). However, the nonreporting states do not explain the declines in national rates because rates declined in most states that consistently reported cases.
Giardiasis rates varied by age and sex. The rate of reported giardiasis was higher in males than females in almost all age groups, particularly among adults aged 45–49 years. Compared with previous years, giardiasis rates declined across most age groups and both sexes in 2011–2012. Among males and females, rates were highest among children aged 1–9 years, which is consistent with previously published reports (3034). Higher rates in children might be related to increased recreational water exposures, poor hygiene skills, close contact with other potentially infected children in child-care settings, and lack of previous exposure toGiardia, which could render them more susceptible to infection and illness (41,42). Giardia has been identified frequently as the cause of diarrhea among children examined in outpatient clinics (43), and transmission from ill children to household contacts has been documented in outbreak investigations (44,45). The sharpest declines were seen in this age group as well. The rate among children aged 1–4 years declined from 23.5 per 100,000 population in 2009–2010 to 16.4 in 2011–2012, and the rate among children aged 5–9 years dropped from 12.5 per 100,000 population in 2009–2010 to 8.4 in 2011–2012 (34). No national efforts to prevent person-to-person transmission in child-care settings have occurred that would explain the rate reductions in young children. Interventions to reduce drinking water-associated transmission of Giardia (e.g., EPA's Ground Water Rule) might have a larger impact on the young, because older persons have had more opportunities to be previously infected with Giardia, which could confer partial protection from reinfection or symptomatic infection (46,47). Reducing the presence of this parasite in the water might, in turn, prevent person-to-person transmission in settings that foster increased risk for infection, such as child-care centers (18,26).
During 2011–2012, a twofold increase in giardiasis reporting occurred during summer compared with winter months, with a peak in late July and early August. This finding is consistent with temporal patterns observed previously in the United States (3034) and Canada (41), and similar to the seasonal profile of other parasitic and bacterial enteric diseases (e.g., cryptosporidiosis and vibriosis) (48,49). The summer peak coincides with increased outdoor activities (e.g., camping and swimming) that likely increase exposure to contaminated water. Transmission associated with outdoor activities is facilitated by the substantial number ofGiardia cysts that can be shed by a single person (17), the environmental hardiness of the organism (50), the extended periods of time that cysts can be shed(18), and the low infectious dose for infection (16).
Drinking water is a well-documented vehicle for Giardia transmission. G. intestinalis was the single most frequently identified pathogen in all drinking water outbreaks reported in the United States during 1971–2006, responsible for 28% of all outbreaks with an identified etiology (3). Untreated drinking water has been identified as a risk factor for sporadic giardiasis in the United States (51,52) and New Zealand (24). Groundwater can be particularly risky if acquired from poorly constructed or maintained wells that might have been subject to surface water contamination.
Both treated and untreated recreational water also have been implicated as vehicles of giardiasis transmission. During 1999–2008, Giardia was identified as a causal agent of eight (3.5%) of 228 reported recreational water-associated gastroenteritis outbreaks (53). In studies of sporadic giardiasis, swallowing water while swimming and recreational contact with fresh water were both risk factors for contracting Giardia (22,24). Giardia can be frequently detected in fecal material in pools (54), and transmission has been documented among diapered children who use swimming venues regularly (45,55,56).
Reported foodborne outbreaks of giardiasis have generally been caused by direct contamination by an infected food handler (57,58) or animal contamination of food (59). However, foodborne outbreaks of giardiasis are infrequently reported in the United States. During 2000–2010, <1% of foodborne outbreaks with an identified etiology was attributed to Giardia (59). Infections from contamination of widely distributed foods (e.g., fresh produce) might be difficult to detect. A recent study of Canadian produce showed that 1.8% of precut salad and leafy green samples were contaminated with Giardia (60), and a study of sporadic giardiasis in England identified eating lettuce as a risk factor for giardiasis (22). The use of reclaimed wastewater for irrigation is associated with the finding ofGiardia cysts on fresh produce (61), highlighting the importance of using noncontaminated irrigation water to prevent foodborne disease.
Person-to-person transmission of Giardia also occurs. Persons attending or working in child-care settings or those who have close contact with persons with giardiasis are at increased risk for being infected (51,52,62,63). Exposure to feces through handling diapers and poor hygiene, particularly after toileting, in child-care settings might contribute to increased risk (20,55).
Although G. intestinalis infects both humans and animals, the role of zoonotic transmission to humans and the importance of animal contamination of food and water are being reexamined in light of advances in molecular epidemiology. Giardia has been detected in nearly all classes of vertebrates, including domestic animals and wildlife (64), but molecular characterization of G. intestinalis has identified relatively species-specific genetic assemblages. Humans are only infected with assemblages A and B, which can sometimes be found in other animals. However, animals are usually infected with other species-specific assemblages (64). Epidemiologic data implicating wildlife, cattle, and pets as sources of human-pathogenic Giardia assemblages are limited, and findings from molecular studies of G. intestinalis assemblages and subtypes suggest that the risk of zoonotic transmission is not as high as previously thought (15). No molecular data are reported to CDC surveillance systems, limiting the ability to understand the role of zoonotic transmission.
Strategies to reduce the incidence of giardiasis have focused on reducing waterborne and person-to-person transmission (Box). The low infectious dose of Giardia, protracted shedding of cysts, and moderate chlorine tolerance make it ideally suited for transmission through these pathways. The Environmental Protection Agency (EPA) enacted a series of rules designed to prevent pathogens in surface water sources from contaminating drinking water systems (65,66,67,68). These regulations might have contributed to a decrease in the number of giardiasis outbreaks associated with community drinking water systems (3). In 2006, EPA finalized the Ground Water Rule to address contamination of public ground water (well) systems, which is likely to reduce the number of groundwater-associated outbreaks of giardiasis (40). For recreational water, proper pool maintenance (i.e., sufficient disinfection, filtration, and recirculation of water) and excluding children with diarrhea from pools should decrease transmission through treated recreational water. Person-to-person transmission of Giardia is difficult to interrupt in a systematic fashion, particularly in child-care settings (63). Adherence to appropriate infection control policies (e.g., exclusion of children ill with diarrhea, hand washing, diaper changing, and separation of ill children from well children) is recommended for controlling giardiasis and other enteric pathogens in these group settings (69).


The findings in this report are subject to at least five limitations. First, case reports lack data on exposure history and often have incomplete data on race and ethnicity; thus, it was not possible to evaluate the contributions of exposures or identify racial or ethnic groups at increased risk for giardiasis. Second, incomplete data on symptom onset date could have led to an inaccurate representation of the seasonal distribution of cases. Third, incidence of giardiasis is likely to be underestimated by these national surveillance data because of underreporting (e.g., not all persons infected with Giardia are symptomatic, persons who are symptomatic do not always seek medical care, health-care providers do not always include laboratory diagnostics in their evaluation of nonbloody diarrheal diseases, and case reports are not always completed for positive laboratory results or forwarded to public health officials). Fourth, the 2011 case definition clarification that symptoms should be present for a case to be confirmed might limit direct rate comparisons with previous years. Finally, giardiasis is not a reportable disease in all states, which can lead to an incomplete picture of its geographic distribution and an underestimation or overestimation of national incidence rates.
Future Directions
Although giardiasis is the most common enteric parasitic infection in the United States, gaps in understanding of its epidemiology still exist. Methods to improve reporting include encouraging health-care providers to consider and specifically request testing for Giardia in the workup of gastrointestinal illness, and encouraging health-care providers and laboratories to improve reporting of cases to jurisdictional health departments. Improved case investigations, geospatial studies, serosurveys, and the use of molecular tools would enhance understanding of the epidemiology of giardiasis. The majority of data on giardiasis transmission comes from outbreak investigations; however, the overwhelming majority of reported giardiasis cases occur sporadically. During 2011–2012, <2% of reported giardiasis cases was associated with outbreaks. Many giardiasis outbreaks associated with drinking water occur (3), but the relative contributions of waterborne, foodborne, person-to-person, and animal-to-person transmission are not well understood, especially for sporadic cases. Whether the geographic variability noted in this report reflects actual differences in transmission patterns and disease burden versus diagnosis and reporting artifacts is unclear; however, the sharp decline in rates in the Midwest is likely because of a regional decrease in transmission.
Future research is needed to help elucidate the sources of nonoutbreak associated giardiasis infections. Ecologic studies could characterize the potential contributions of private wells, septic systems, land application of biosolids (organic matter recycled from sewage), and agricultural operations in giardiasis transmission. Infected persons can shed Giardia for several weeks, and symptoms are variable; however, until recently, no reliable serologic assays for Giardiahave been available, and no population studies of Giardia seroprevalence have been conducted. With recent laboratory advances (70), such studies might now be feasible and would contribute substantially to understanding of the prevalence of giardiasis in the United States. Enhanced genotyping methods would increase knowledge of the molecular epidemiology of Giardia, including elucidating the importance of zoonotic transmission. Molecular methods also could be used to assist public health officials in linking cases sharing common transmission routes, which could lead to increased outbreak detection. These tools, combined with traditional epidemiology and surveillance, would improve understanding of giardiasis risk factors and inform future prevention strategies. Although recent studies indicate a potential for chronic sequelae from giardiasis (9–14), additional research is needed to further improve understanding of the prevalence and scope of these conditions.


For the first time since 2002, giardiasis rates appear to be decreasing. Despite this decrease, giardiasis remains the most commonly reported intestinal parasitic infection in the United States. National surveillance data can be used to guide the revision, updating, and expansion of health communication efforts and other public health interventions to prevent and control giardiasis. Federal, state, and local health agencies can use giardiasis surveillance data to help elucidate the epidemiology of giardiasis in the United States, establish public health priorities for giardiasis prevention, target health communication messages, and design public health interventions to prevent the transmission of Giardia. Additional information about giardiasis is available at http://www.cdc.gov/parasites/giardia/.


This report is based, in part, on contributions by Michele C. Hlavsa, epidemiologist, Division of Foodborne, Waterborne, and Environmental Diseases, and jurisdiction surveillance coordinators Ruth Ann Jajosky, DMD, and Willie Anderson, Office of Surveillance, Epidemiology, and Laboratory Services, CDC.


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