Evolving Epidemiology of Reported Giardiasis Cases in the United States, 1995-2016

Background: Giardiasis is the most common intestinal parasitic disease of humans identified in the United States (US) and an important waterborne disease. In the United States, giardiasis has been variably reportable since 1992 and was made a nationally notifiable disease in 2002. Our objective was to describe the epidemiology of US giardiasis cases from 1995 through 2016 using National Notifiable Diseases Surveillance System data. Methods: Negative binomial regression models were used to compare incidence rates by age group (0-4, 5-9, 10-19, 20-29, 30-39, 40-49, 50-64, and ≥ 65 years) during 3 time periods (1995-2001, 2002-2010, and 2011-2016). Results: During 1995-2016, the average number of reported cases was 19 781 per year (range, 14 623-27 778 cases). The annual incidence of reported giardiasis in the United States decreased across all age groups. This decrease differs by age group and sex and may reflect either changes in surveillance methods (eg, changes to case definitions or reporting practices) or changes in exposure. Incidence rates in males and older age groups did not decrease to the same extent as rates in females and children. Conclusions: Trends suggest that differences in exposures by sex and age group are important to the epidemiology of giardiasis. Further investigation into the risk factors of populations with higher rates of giardiasis will support prevention and control efforts.