Cryptosporidiosis NNDSS Summary Report for 2021

Background

Cryptosporidiosis is a gastrointestinal illness caused by protozoa of the genus Cryptosporidium, a leading cause of U.S. waterborne disease outbreaks (1) and U.S. zoonotic enteric illness (2,3). An estimated 823,000 cryptosporidiosis cases occur annually; this means <2% of cases are nationally notified (4). Cryptosporidium infection can be symptomatic or asymptomatic. Immunocompetent patients can experience frequent, non-bloody, watery diarrhea typically lasting one to two weeks. Additional symptoms can include vomiting, nausea, abdominal pain, fever, anorexia, fatigue, and weight loss. Immunocompromised patients can experience profuse watery diarrhea lasting weeks to months or even life-threatening malnutrition and wasting (5).

Cryptosporidiosis is a nationally notifiable disease; the first full year of reporting was 1995. National data are collected through passive surveillance. Healthcare providers and laboratories that diagnose cryptosporidiosis are mandated to report cases to the local, state, or territorial health department. The 50 states, District of Columbia (DC), New York City (NYC), 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. Some jurisdictions conduct enhanced molecular surveillance of cryptosporidiosis through participation in CryptoNet; CryptoNet data are not presented here.

State, territorial, local, and tribal health agencies voluntarily notify CDC of cryptosporidiosis outbreaks via the National Outbreak Reporting System (NORS). NORS data are not presented here; however, summaries of data on waterborne disease outbreaks are reported elsewhere.

Methods

Case definition

The definition of a confirmed case of cryptosporidiosis has changed over time. The first national case definition was published in 1995 (6); the current case definition was published in 2012 (7). The pre-2011 case definitions classified a case with any laboratory evidence of Cryptosporidium infection as a confirmed case.

The 2012 confirmed case definition requires evidence of Cryptosporidium organisms or DNA in stool, intestinal fluid, tissue samples, biopsy specimens, or other biological sample by certain laboratory methods with a high positive predictive value (e.g., direct fluorescent antibody [DFA] test, polymerase chain reaction [PCR], enzyme immunoassay [EIA], or light microscopy of stained specimen).

A probable case of cryptosporidiosis is defined as 1) having supportive laboratory test results for Cryptosporidium spp. infection using a screening test method, such as immunochromatographic card or rapid card test, or a laboratory test of unknown method or 2) meeting clinical criteria (diarrhea and one or more of the following: diarrhea duration of >72 hours, abdominal cramping, vomiting, or anorexia) and being epidemiologically linked to a confirmed case.

A suspect case is defined as having a diarrheal illness and being epidemiologically linked to a probable case. Cases not classified as confirmed, probable, or suspect are classified as unknown.

Analysis

National cryptosporidiosis surveillance data for 2021 were analyzed using R version 4.4.1. Data cleaning processes included case deduplication and the verification of case status (confirmed or nonconfirmed). Numbers, percentages, and incidence rates (cases per 100,000 population) of cryptosporidiosis were calculated in aggregate for the United States and separately for each reporting jurisdiction. Rates were calculated by dividing the number of cryptosporidiosis cases by each year's mid-year census estimates (8,9) and multiplying by 100,000.

U.S. Census Bureau data were obtained using its Application Programming Interface and the R "tidycensus" package (10,11). Region and total population estimates included only jurisdictions that reported (Supplemental Table 1). 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 (12). 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, cryptosporidiosis rates were calculated by year (2012 to 2021). To examine changes in cryptosporidiosis reporting in 2021 attributed to the COVID-19 pandemic, we calculated the percent change in incidence between 2020 and 2021. We also calculated the five-year average-annual incidence (2017-2021). Annual cryptosporidiosis rates were calculated by demographic variables (age and sex) and jurisdiction. Rates were not calculated for race or ethnicity due to large proportion of missing data for these variables (12.7% and 23.5% respectively).

Findings

In 2021, 53 jurisdictions reported cryptosporidiosis case data to NNDSS, including 50 states, 1 territory, DC, and New York City. There were 9,110 cases of cryptosporidiosis reported to CDC with an average annual incidence of 2.7 cases per 100,000 population (Figure 1, Table 1). Reported cases of cryptosporidiosis in 2021 were higher than those reported in 2020, with a 17.4% increase in incidence from 2020 (incidence rate of 2.3 cases per 100,000) (Table 1). This higher rate is 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 was observed across a majority (71.7%) of reporting jurisdictions (n=38/53) (Table 1), with the percent increase in incidence ranging from 2.0% to 225.0%. Fifteen jurisdictions had an observed decrease or no change in incidence between 2020 and 2021. Of the 9,110 reported cases in 2021, 7,145 (78.4%) were confirmed, while 1,965 cases (21.6%) were non-confirmed. This is a slight increase in the proportion of confirmed cases compared with 2020 (76.5%), with the proportion of confirmed cases increasing slightly over time, with 65.5% confirmed in 2012. There were 192 outbreak-associated cases reported in 2021.,

Cryptosporidiosis is geographically widespread across the United States. By region, incidence of reported cryptosporidiosis cases ranged from 1.7 cases per 100,000 population in the Southwest to 4.7 cases per 100,000 population in the Midwest. By jurisdiction, cryptosporidiosis incidence ranged from 0.0 per 100,000 population in Puerto Rico and 0.6 in Hawaii to 14.2 per 100,000 population in South Dakota (Table 1, Figure 2). Differences in incidence might reflect differences in risk factors or mode of transmission of cryptosporidium; the magnitude of outbreaks; or the capacity or requirements to detect, investigate, and report cases.

For cases with a reported symptom onset date, most cases occurred between July and September, with a peak in July (n=1,071) (Figure 3). When separated by region, the Northeast and Midwest had the most distinct pattern of July and August peaks (Figure 4). The South and Southwest cases were higher in the second half of the year, and the Northwest exhibited a more consistent number of cases throughout the year (Figure 4).

During 2021, a total of 4,440 cases were male (48.7%) and 4,614 (50.6%) were female (Table 2). Most cases for which data on race were available occurred among the classifications White (71.3%), Black (7.2%), or other or Multi-race (6.5%). Most patients for whom data on ethnicity were available were non-Hispanic (65.8%). Data on race were not included for 12.7% of cases, and data on ethnicity were not reported for 23.5% of cases.

With respect to age, the incidence of reported cryptosporidiosis cases was highest in persons aged <5 years (5.4 cases per 100,000 population). The highest incidence of cryptosporidiosis was among boys ages <5 years (6.2 cases per 100,000 population) (Figure 5). Rates were similar between male and female patients for every age group, although male patients <5 years had a higher rate than female patients <5 years, and women 20-29 years of age had higher rates than men in the same age group (Figure 6).

Acknowledgements

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

Tables and figures