Giardiasis outbreaks in the United States, 1971-2011

Abstract

Giardia intestinalis is the leading parasitic aetiology of human enteric infections in the United States, with an estimated 1·2 million cases occurring annually. To better understand transmission, we analysed data on all giardiasis outbreaks reported to the Centers for Disease Control and Prevention for 1971-2011. The 242 outbreaks, affecting ~41 000 persons, resulted from waterborne (74·8%), foodborne (15·7%), person-to-person (2·5%), and animal contact (1·2%) transmission. Most (74·6%) waterborne outbreaks were associated with drinking water, followed by recreational water (18·2%). Problems with water treatment, untreated groundwater, and distribution systems were identified most often during drinking water-associated outbreak investigations; problems with water treatment declined after the 1980s. Most recreational water-associated outbreaks were linked to treated swimming venues, with pools and wading pools implicated most often. Produce was implicated most often in foodborne outbreaks. Additionally, foods were most commonly prepared in a restaurant and contaminated by a food handler. Lessons learned from examining patterns in outbreaks over time can help prevent future disease. Groundwater and distribution system vulnerabilities, inadequate pool disinfection, fruit and vegetable contamination, and poor food handler hygiene are promising targets for giardiasis prevention measures.

Keywords: Community outbreaks; Giardia lamblia; foodborne infections; waterborne infections; zoonoses.

Fig. 1.

Giardiasis outbreaks, by year and mode of transmission, United States, 1971–2011 (n = 242). The timeline below the figure outlines key changes in disease surveillance activities and systems. ^a Waterborne outbreaks reported from Colorado, Vermont, and Washington; these states received 2-year funding from the U.S. Environmental Protection Agency to support intensive waterborne disease surveillance.

Fig. 2.

Giardiasis outbreaks, by mode of transmission and month of first reported illness, United States, 1971–2011 (n = 241). Information on the month of first illness was missing for one outbreak associated with animal contact transmission.

Fig. 3.

Waterborne giardiasis outbreaks, by year, water vehicle, and outbreak deficiency, United States, 1971–2011. (a) Number of waterborne outbreaks, by year and implicated water vehicle (n = 181). ^a Waterborne outbreaks reported from Colorado, Vermont, and Washington; these states received 2-year funding from the U.S. Environmental Protection Agency to support intensive waterborne disease surveillance. (b) Deficiencies identified during drinking water outbreak investigations, by year, and timeline of key drinking water regulations and surveillance activities; each deficiency represents one outbreak (n = 135). ^b One outbreak was assigned an additional distribution system deficiency. ^c One outbreak was assigned an additional plumbing deficiency.