Giardiasis NNDSS Summary Report for 2020

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 to categorize 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 2020 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; additionally, the Territories were added as a seventh region.

To examine reporting over time, giardiasis incidence rates were calculated by year (2011 to 2020). To examine changes in giardiasis reporting in 2020 attributed to the COVID-19 pandemic, we calculated the percent change in incidence rate between 2019 and 2020. We also calculated the five-year average-annual incidence rate (2016–2020). Annual giardiasis rates were calculated by demographic variables (e.g., age and sex) and jurisdiction. Rates were not calculated for race or ethnicity due to large proportion of missing data for these variables (26.1% and 39.6% respectively).

Findings

In 2020, 46 jurisdictions reported giardiasis case data to NNDSS, including 41 states, three territories, New York City, and DC. There were 9,501 cases of giardiasis reported to CDC with an average annual incidence of 3.6 cases per 100,000 population (Table 1). Incidence rates of giardiasis in 2020 were lower than those reported in previous years, with a 35.7% decrease in incidence rate from 2019 (incidence rate of 5.6 cases per 100,000) (Table 1). This lower rate may be attributed, in part, to the COVID-19 pandemic, 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. A decrease in incidence rate was observed across a majority of states (Table 1), with the percent decrease in incidence rate ranging from 1.9% to 69.8%. Three states had observed increases in incidence rate between 2019 and 2020: Alaska, North Dakota and Wyoming. Out of the 9,501 reported cases in 2020, 9,187 (96.7%) were confirmed, while 314 (3.3%) were not confirmed. This is similar to the proportion of confirmed cases in 2019 (96.6%), though the proportion of confirmed cases has decreased slightly over time, with 98.8% confirmed in 2011. There were 203 outbreak associated cases in 2020, 57.1% of which were reported by New York City.

Giardiasis is geographically widespread across the United States. By region, incidence of reported giardiasis cases ranged from 2.6 cases per 100,000 population in the South to 4.9 cases per 100,000 population in the Northeast. In 2020, the U.S. territories had an annual incidence of 1.2 cases per 100,000, with Guam, the Northern Mariana Islands, and Puerto Rico reporting cases. By jurisdiction, giardiasis incidence ranged from 0.7 per 100,000 population in Guam to 11.9 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 majority of cases occurred between July and September, with a peak in August (n=613). There was also a peak in reported cases in January (n= 604) (Figure 3). When separated by region, the Northeast had a peak of cases in January, with a second peak later in the year, between July and October (Figure 4). In contrast, reporting was more consistent throughout the year in the South.

During 2020, a total of 5,678 patients were male (59.8%) and 3,788 (39.9%) were female. Data on sex were not reported or not classified as male or female for 35 case reports (0.4%) (Table 2). For cases where race data were available, the majority of classifications included white (54.9%), Black (6.0%), or Other or multi-race (12.0%). The majority of patients for whom data on ethnicity were available were non-Hispanic (53.8%). Data on race were not reported for 24.3% of cases, and data on ethnicity were not reported for 38.9% of cases.

In 2020, the incidence of reported giardiasis cases was highest among ages 25–29 years, 30–34 years, and 55–59 years (incidence rates = 4.2, 4.5, and 4.5 cases per 100,000 population, respectively) (Figure 5). Historically, cases <5 years of age have had higher incidence rates compared to other age groups; however, this was not observed for 2020. In 2020, the incidence rate for those <5 years was 3.8 cases per 100,00 population, similar to other age groups between 24 and 74 years. Rates were highest among males in every age group (Figure 6). The highest incidence of giardiasis was among males ages 30–34 years (5.8 cases per 100,000 population). The difference between males and females was most pronounced among those aged 25–29 years, with males presenting with 3.0 additional cases of giardiasis per 100,000 population compared to females.

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.

Figure 1

Table 1

Number, percentage^*, and incidence^§ of giardiasis cases, by region and jurisdiction — National Notifiable Diseases Surveillance System, United States, 2020 (n=9,501)

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

Figure 2

Figure 3

Figure 4

Table 2

Figure 5

Figure 6