Giardiasis surveillance--United States, 2003-2005.

PROBLEM/CONDITION: Giardiasis, a gastrointestinal illness, is caused by the protozoan parasite Giardia intestinalis.

REPORTING PERIOD: 2003-2005.

DESCRIPTION OF SYSTEM: State, commonwealth, territorial, and two metropolitan health departments voluntarily reported cases of giardiasis through CDC's National Notifiable Disease Surveillance System.

RESULTS: During 2003-2005, the total number of reported cases of giardiasis remained relatively stable. Reporting increased from 20,084 for 2003 to 20,962 for 2004 and then decreased to 20,075 for 2005. A total of 49 jurisdictions reported giardiasis cases; the number of areas reporting >15 cases per 100,000 population increased from four areas in 2003 to seven in 2005. Compared with other age groups, a greater number of case reports were received for children aged 1-9 years and for adults aged 30-39 years. Incidence of giardiasis was highest in northern states. Peak onset of illness occurred annually during early summer through early fall.

INTERPRETATION: Transmission of giardiasis occurs throughout the United States, with increased diagnosis or reporting occurring in northern states. State incidence figures should be compared with caution because individual state surveillance systems have varying capabilities to detect cases. The seasonal peak in age-specific case reports coincides with the summer recreational water season and might reflect increased outdoor activity and exposures such as use of communal swimming venues (e.g., lakes, rivers, swimming pools, and water parks) by young children.

PUBLIC HEALTH ACTION: Giardiasis surveillance provides data to educate public health practitioners and health-care providers about the epidemiologic characteristics and the disease burden of giardiasis in the United States. These data are used to improve reporting of cases, plan prevention efforts, and establish research priorities.