Giardiasis surveillance -- United States, 2011-2012.

Problem/condition: Giardiasis is a nationally notifiable gastrointestinal illness caused by the protozoan parasite Giardia intestinalis.

Reporting period: 2011-2012.

Description of system: Forty-four states, the District of Columbia, New York City, the Commonwealth of Puerto Rico, and Guam voluntarily reported cases of giardiasis to CDC through the National Notifiable Diseases Surveillance System (NNDSS).

Results: For 2011, a total of 16,868 giardiasis cases (98.8% confirmed and 1.2% nonconfirmed) were reported; for 2012, a total of 15,223 cases (98.8% confirmed and 1.3% nonconfirmed) were reported. In 2011 and 2012, 1.5% and 1.3% of cases, respectively, were associated with a detected outbreak. The incidence rates of all reported cases were 6.4 per 100,000 population in 2011 and 5.8 per 100,000 population in 2012. This represents a slight decline from the relatively steady rates observed during 2005-2010 (range: 7.1-7.9 cases per 100,000 population). In both 2011 and 2012, cases were most frequently reported in children aged 1-4 years, followed by those aged 5-9 years and adults aged 45-49 years. Incidence of giardiasis was highest in Northwest states. Peak onset of illness occurred annually during early summer through early fall.

Interpretation: For the first time since 2002, giardiasis rates appear to be decreasing. Possible reasons for the decrease in rates during 2011-2012 could include changes in transmission patterns, a recent change in surveillance case definition, increased uptake of strategies to reduce waterborne transmission, or a combination of these factors. Transmission of giardiasis occurs throughout the United States, with more frequent diagnosis or reporting occurring in northern states. Geographical differences might suggest actual regional differences in giardiasis transmission or variation in surveillance capacity across states. Six states did not report giardiasis cases in 2011-2012, representing the largest number of nonreporting states since giardiasis became nationally notifiable in 2002. Giardiasis is reported more frequently in young children, which might reflect increased contact with contaminated water or ill persons, or a lack of immunity.

Public health action: Educational efforts to decrease exposure to unsafe drinking and recreational water and prevent person-to-person transmission have the potential to reduce giardiasis transmission. The continual decrease in jurisdictions opting to report giardiasis cases could negatively impact the ability to interpret national surveillance data; thus, further investigation is needed to identify barriers to and facilitators of giardiasis case reporting. Existing state and local public health infrastructure supported through CDC (e.g., Epidemiology and Laboratory Capacity grants and CDC-sponsored Council of State and Territorial Epidemiologists Applied Epidemiology Fellows) could provide resources to enhance understanding of giardiasis epidemiology.