Giardiasis NNDSS Summary Report for 2022

Giardiasis is an illness caused by the protozoan parasite Giardia duodenalis (formerly called G. lamblia or G. intestinalis), which causes gastrointestinal symptoms such as diarrhea, abdominal cramps, bloating, weight loss, or malabsorption (1, 2). Each year in the United States, it is estimated that Giardia causes more than 1.1 million illnesses (3).

Giardiasis is a nationally notifiable disease, with the first full year of reporting in 1993 National data are collected through passive surveillance. Healthcare providers and laboratories that diagnose confirmed giardiasis cases report to the local, state, or territorial health departments. The 50 states, District of Columbia (DC), New York City, and territorial public health agencies, in turn, voluntarily notify CDC of cases via the National Notifiable Disease Surveillance System (NNDSS). Although tribes do not report directly to CDC via NNDSS, the inclusion of case data submitted by states for American Indian and Alaskan Native individuals residing on tribal lands may vary based on laboratory and case reporting practices in each jurisdiction, the location where healthcare services were delivered, and other relevant factors. The number of health departments that submit data vary from year to year depending on which jurisdictions have designated giardiasis as reportable, as well as their capacity to report data to CDC.

Public health agencies voluntarily notify CDC of giardiasis outbreaks via the National Outbreak Reporting System (NORS). NORS data are not presented here; however, summaries of data on waterborne disease outbreaks are published elsewhere.

The definition of a confirmed case of giardiasis has changed over time; the first national case definition was published in 1997 (4), and a revised case definition was published in 2011 (5). The current (2011) case definition differs from the 1997 definition in clarifying that clinical symptoms are necessary for categorizing giardiasis cases as confirmed.

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 (5). Non-confirmed 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. A national case definition for suspected cases of giardiasis does not exist; the definition varies by state. Unknown cases are those not classified as confirmed or probable.

National giardiasis surveillance data for 2022 were analyzed using R version 4.4.1. Data cleaning processes included case deduplication and the verification of case status (e.g., confirmed, non-confirmed). Numbers, percentages, and incidence rates (cases per 100,000 population) of giardiasis were calculated in aggregate for the United States and separately for each reporting jurisdiction. Rates were calculated by dividing the number of giardiasis cases by each year's mid-year census estimates (6, 7) and multiplying by 100,000.

U.S. Census Bureau data were obtained using their Application Programming Interface and the R "tidycensus" package (8–9). Regional and total population estimates included only jurisdictions that reported (Figure 2). In addition to analyzing data nationally and by reporting jurisdiction, data were analyzed by region (Northeast, Midwest, South, West, and Territories), as defined by the U.S. Census Bureau (10). 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 were calculated by year (2013 to 2022). To examine changes in giardiasis reporting, we calculated the percent change in incidence rate between 2021 and 2022. We also calculated the five-year average-annual incidence rate (2018–2022). Annual giardiasis rates were calculated by demographic variables (e.g., age and sex) and jurisdiction. Incidence rates were not calculated for race or ethnicity due to large proportion of missing data for these variables (28.4% and 37.9% respectively).

In 2022, 47 jurisdictions reported giardiasis case data to NNDSS, including 43 states and three territories, and New York City. There were 13,829 cases of giardiasis reported to CDC with an average annual incidence of 5.2 cases per 100,000 population (Figure 1). Estimates of giardiasis in 2022 were higher than in 2021, with a 18.2% increase in incidence rate from 2021 (incidence rate of 4.4 cases per 100,000) (Table 1). This higher rate may be attributed, in part, to the COVID-19 pandemic in 2020 and 2021, when health care providers and local, state, and territorial health departments in the United States may have had lower capacity to detect, investigate, and report cases. An increase in incidence rate was observed across 22 out of 47 reporting states (Table 1), with the percent increase ranging from 6.0% to 117.4%. Twenty-three states had no change or an observed decrease in incidence rate between 2021 and 2022. Out of the 13,829 reported cases in 2022, 13,097 (94.7%) were confirmed, while 732 (5.3%) were not confirmed. This is a slight decrease from the proportion of confirmed cases in 2021 (95.1%). The proportion of confirmed cases has been gradually declining over time, with 97.7% confirmed in 2013. There were 102 outbreak-associated cases, 78.4% of which were reported by Florida (n=80).

Giardiasis is geographically widespread across the United States. By region, incidence of reported giardiasis cases ranged from 4.2 cases per 100,000 population in the South to 6.8 cases per 100,000 population in the Northeast. In 2022, the U.S. territories had an annual incidence of 0.4 cases per 100,000, with Puerto Rico, Northern Mariana Islands, and the Virgin Islands reporting cases. By jurisdiction, giardiasis incidence ranged from 0.3 per 100,000 population in Puerto Rico to 13.3 per 100,000 population in Alaska (Table 1, Figure 2). Differences in incidence might reflect differences in risk factors or mode of transmission of Giardia; the magnitude of outbreaks; or the capacity or requirements to detect, investigate, and report cases.

For cases with a reported symptom onset date, the highest number of cases occurred between July and September, with a peak in August (n=983) (Figure 3). These patterns primarily held across regions, although the Northeast, Northwest, and Midwest regions had a more pronounced increase in reported cases between July and September (Figure 4). In contrast, cases were more consistently reported throughout the year in the South region.

During 2022, a total of 8,508 patients were male (61.5%) and 5,268 (38.1%) were female (Table 2). Data on sex were not reported or not classified as male or female for 53 case reports (0.4%). For cases where race data were available, the majority of classifications included white (53.6%), Black (6.1%), or Other or multi-race (8.6%). The majority of patients for whom data on ethnicity were available were non-Hispanic (51.1%). Data on race were not reported for 28.4% of cases, and data on ethnicity were not reported for 37.9% of cases.

In 2022, incidence of reported giardiasis cases was highest among ages <5 years and 30–34 years (incidence rates = 7.0 and 6.6 cases per 100,000 population, respectively) (Figure 5). Compared to 2020 and 2021, incidence rates were higher among younger age groups. Rates were highest among males in every age group (Figure 6), a pattern that has remained consistent over time. The highest incidence of giardiasis was observed for males ages 25–29 and 30–34 years (8.4 and 9.0 cases per 100,000 population, respectively). The difference in incidence rate between males and females was most pronounced for middle-age groups, between 20 and 64 years of age, with a less stark difference between sexes observed for individuals <20 years and >65 years.

This report is based on contributions by state and local epidemiologists and microbiologists. The authors gratefully acknowledge Zainab Salah for their assistance in reviewing code, and thank Amanda MacGurn, Vince Hill, and Jeremy Sobel for their assistance in publishing this annual report.

^* Cases per 100,000 population

^§ Overall, 137,430 cases were confirmed (96.9%); 4,359 cases were non-confirmed (3.1%). In 2022, 13,097 cases were confirmed (94.7%); 732 cases were non-confirmed (5.3%)

Abbreviation NR = Non-Reporting Jurisdiction

^*Percentages might not total 100% because of rounding

^§ Cases per 100,000 population

^¶New York State and New York City data are mutually exclusive

^*Cases per 100,000 population

^±Non-continental jurisdictions (i.e., Alaska, Hawaii, and U.S. territories) are not shown to scale and are not meant to depict their true geographic location.

^§Non-reporting jurisdictions included Illinois, Mississippi, North Carolina, Oklahoma, Tennessee, Texas, and Vermont, DC, Guam, and American Samoa.

^¶New York State and New York City data are mutually exclusive

^*Symptom onset date was available for 57.4% of cases (n=7,932/13,829)

Midwest

Northeast

Northwest

South

Southwest

Territories

^*Symptom onset date was available for 57.4% of cases (n=7,932/13,829)

^§ Note that the y-axes vary for each region

^*Percentages might not total 100% because of rounding

Figure 5. Incidence^* of giardiasis cases, by age group — National Notifiable Diseases Surveillance System, United States, 2022(n = 13,817^§)

^*Cases per 100,000 population

^§ Age data was available for 99.9% of cases (n=13,817/13,829)

^*Cases per 100,000 population

^§ Age and sex data were available for 99.5% of cases (n=13,766/13,829)