What is the prevalence of giardiasis in the US?

Updated: Oct 01, 2018
  • Author: Hisham Nazer, MBBCh, FRCP, DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
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Giardia remains the parasite most commonly identified in stool specimens, causing about 1.2 million annual episodes of illness. [2] From 1964-1984, G lamblia caused at least 90 water-borne outbreaks of diarrhea, affecting more than 23,000 people. These outbreaks typically involved small water systems using untreated or inadequately treated surface water.

Most water-borne outbreaks in the United States have occurred in western mountain regions (eg, Rocky Mountains, Sierra Nevada, Cascades) where giardiasis must be considered endemic. The incidence of giardiasis is high among individuals who camp and backpack in mountainous Western states. Other groups at increased risk for infection include children, homosexual men, and individuals with immunoglobulin deficiency states (inherited or acquired).

Yoder et al reported that the incidence is greatest in northern states, [38] but this may be related to the differences in individual state surveillance systems and may not necessarily reflect an actual higher incidence. [38]  Because water-borne giardiasis outbreaks have been reported in every region in the United States, the diagnosis must be considered anywhere in the country.

Endemic infection occurs most commonly from July through October among children younger than 5 years and adults aged 25-39 years. Carrier rates as high as 30-60% have been documented among children in day care centers, institutions, and on Native American reservations.

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