Giardiasis surveillance--United States, 1992-1997

Giardia intestinalis, the organism that causes the gastrointestinal illness giardiasis, is the most commonly diagnosed intestinal parasite in public health laboratories in the United States. In 1992, the Council of State and Territorial Epidemiologists assigned giardiasis an event code that enabled states to begin voluntarily reporting surveillance data on giardiasis to CDC.

This report includes data that were reported from January 1992 through December 1997.

The National Giardiasis Surveillance System includes data about reported cases of giardiasis from participating states. Because most states were already collecting data on occurrence of giardiasis, the assignment of an event code to giardiasis has allowed voluntary reporting of these data to CDC via the National Electronic Telecommunications System for Surveillance.

Since 1992, the number of states reporting cases of giardiasis to CDC has risen from 23 to 43. The annual number of giardiasis cases reported has ranged from 12,793 in 1992 to 27,778 in 1996. In 1997, cases per 100,000 state population ranged from 0.9 to 42.3, with 10 states reporting >20.0 cases per 100,000 population and a national average of 9.5 cases per 100,000 population. In 1997, New York State, including New York City, reported the highest number of cases (3,673, or 20.3 cases per 100,000 population), accounting for 14.5% of cases nationally; however, Vermont reported the highest incidence rate in 1997 (42.3 cases per 100,000 population). Both states have active surveillance systems in place for giardiasis. Cases have an approximately equal sex distribution. Nationally, rates were the highest among children aged 0-5 years, followed closely by persons aged 31-40 years. In these two age groups, most cases were reported during late summer and early fall--an indication that transmission occurred during the summer.

This report documents the first nationwide look at epidemiologic parameters and disease burden estimates for giardiasis in the United States. Transmission occurs in all major geographic areas of the country. The seasonal peak in age-specific case reports coincides with the summer recreational water season and might reflect the heavy use by young children of communal swimming venues (e.g., lakes, rivers, swimming pools, and water parks)--a finding consistent with Giardia's low infectious dose, the high prevalence of diaper-aged children in swimming venues, the extended periods of cyst shedding that can occur, and Giardia's environmental resistance. Estimates based on state surveillance data indicate that as many as 2.5 million cases of giardiasis occur annually in the United States.

Giardiasis surveillance provides data to educate public health practitioners and health-care providers about the scope and magnitude of giardiasis in the United States. These data can be used to establish research priorities and to plan future prevention efforts.