Giardiasis NNDSS Summary Report for 2021

Background

National Giardiasis case surveillance

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. In turn, state and territorial health departments, as well as local health departments including the District of Columbia (DC), and the New York City health departments, voluntarily notify CDC of cases via the National Notifiable Disease Surveillance System (NNDSS). 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.

Methods

Case definition

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.

Analysis

National giardiasis surveillance data for 2021 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 (2012 to 2021). To examine changes in giardiasis reporting, we calculated the percent change in incidence rate between 2020 and 2021. We also calculated the five-year average-annual incidence rate (2017-2021). 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 (24.0% and 36.9% respectively).

Findings

In 2021, 46 jurisdictions reported giardiasis case data to NNDSS, including 43 states, one territory, New York City, and DC. There were 11,665 cases of giardiasis reported to CDC with an average annual incidence of 4.4 cases per 100,000 population (Figure 1, Table 1). Reported cases of giardiasis in 2021 were higher than in 2020, with a 22.2% increase in incidence rate from 2020 (incidence rate of 3.6 cases per 100,000) (Table 1). This higher rate may be attributed, in part, to the COVID-19 pandemic during 2020, 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 a majority (78.3%) of reporting jurisdictions (n=36/46) (Table 1), with the percent increase in incidence rate ranging from 4.3% to 83.3%. Nine jurisdictions had an observed decrease in incidence rate between 2020 and 2021. Out of the 11,665 reported cases in 2021, 11,096 (95.1%) were confirmed, while 569 (4.9%) were not confirmed. This is a slight decrease from the proportion of confirmed cases in 2020 (96.7%). The proportion of confirmed cases has gradually declined over time, with 98.9% confirmed in 2012. There were 90 outbreak associated cases in 2021, with the majority reported out of Florida (n=49), Idaho (n=20), and California (n=10).

Giardiasis is geographically widespread across the United States. By region, incidence of reported giardiasis cases ranged from 3.1 cases per 100,000 population in the South to 6.1 cases per 100,000 population in the Northeast. In 2021, among territories, only Puerto Rico reported cases and had an annual incidence of 0.6 cases per 100,000. By jurisdiction, giardiasis incidence ranged from 0.6 per 100,000 population in Puerto Rico to 10.2 per 100,000 population in Maine (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 majority of cases occurred between June and October, with peaks in July and August (n=800, 798) (Figure 3). These patterns primarily held across regions, although the Northeast and Northwest regions had a more pronounced increase in reported cases between July and September (Figure 4). In contrast, reporting was more consistent throughout the year in the South region.

During 2021, a total of 6,952 patients were male (59.6%) and 4,666 (40.0%) were female. Data on sex were not reported or not classified as male or female for 47 case reports (0.4%) (Table 2). For cases where race data were available, the majority of classifications included white (54.5%), Black (6.1%), or Other or multi-race (12.6%). The majority of patients for whom data on ethnicity were available were non-Hispanic (54.2%). Data on race were not reported for 24.0% of cases, and data on ethnicity were not reported for 36.9% of cases.

In 2021, the incidence of reported giardiasis cases was highest among ages <5 years, 25-29, and 60-64 years (incidence rates = 5.8, 5.0, and 5.2 cases per 100,000 population, respectively) (Figure 5). The higher incidence among the <5 years age group is consistent with historical trends. Rates were highest among males in every age group (Figure 6). The highest incidence of giardiasis among males occurred for cases 25-29 years of age (6.8 cases per 100,000 population). The difference in incidence rate between males and females was most pronounced for middle-age groups, between 20 and 64 years, with a less stark difference between sexes observed for individuals <20 years and >65 years.

Acknowledgements

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.

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