BACKGROUND: Cryptosporidiosis and giardiasis are nationally notifiable gastrointestinal illnesses caused by the parasites Cryptosporidium and Giardia, respectively. An estimated 748,000 cryptosporidiosis cases and 1.3 million giardiasis cases occur annually in the United States. Cryptosporidium’s extreme chlorine tolerance has facilitated its emergence as the leading cause of waterborne disease outbreaks, particularly those associated with aquatic venues (e.g., swimming pools). Giardiacan be transmitted through contaminated water (drinking or recreational) or through person-to-person or animal-to-person contact.
METHODS: Descriptive analyses of 2011–2015 National Notifiable Diseases Surveillance System (NNDSS) case surveillance data were conducted using SAS 9.3. Annual incidence rates per 100,000 population were calculated using Bridged-Race Census population data. Cases were categorized as confirmed or nonconfirmed based on diagnostic test type using the 2011 and 2012 national case definitions. Regions were defined by the U.S. Census Bureau.
RESULTS: NNDSS captured data on 44,852 cryptosporidiosis cases (annual incidence rates: 2.5–3.0) and 76,734 giardiasis cases (annual incidence rates: 5.5–6.5) for 2011–2015. The yearly proportion of confirmed cryptosporidiosis cases ranged from 62% to 65% of all reported cases; overall >98% of all reported giardiasis cases were classified as confirmed. The highest annual cryptosporidiosis and giardiasis incidence rates were in the Midwest (4–6.3) and Northwest (8.2–9.4), respectively. Cryptosporidiosis showed a marked seasonality, with 46.9% of symptom onset dates occurring in the summer months. Cryptosporidiosis and giardiasis rates were highest in children ages 1–4 years. Annual cryptosporidiosis incidence rates were higher in males among those ages <15 years but higher among females among those ages >15 years; giardiasis rates were typically higher in males across the lifespan.
CONCLUSIONS: Despite laboratory-focused changes to the national case definitions reflecting the changing diagnostic landscape, the proportion of confirmed cryptosporidiosis cases remained consistent from 2011 to 2015. Higher rates of cryptosporidiosis in young children and its marked seasonality likely reflect increased use of communal recreational water in the summer and underscore the need for healthy swimming campaigns targeting parents of young swimmers. Giardiasis disproportionately affects young children, which highlights a continued need for safe water, sanitation, and hygiene education to both children and parents to reduce contamination risk and secondary transmission. Differences seen in incidence rates by region, age, and sex may reflect varying exposures; more complete exposure data, integrated with laboratory data to further characterize cryptosporidiosis and giardiasis, (e.g., molecular characterization of Cryptosporidium through CryptoNet), would increase our understanding of these differences.