DETERMINING GIARDIASIS PREVALENCE BY EXAMINATION OF SEWAGE

Raw sewage samples were collected monthly for one year from 11 wastewater treatment plants located across the United States. Giardia cyst concentrations in the raw sewage were determined by direct count using criteria of size, shape and the presence of two or more internal morphological characteristics for identification. The data were adjusted based on percentage of industrial wastewater reported processed by each plant. Although differences in the adjusted annual geometric mean cyst concentrations were noted among the sites (the highest was 3750 cysts/L and the lowest was 683 cysts/L), there was no correlation of cyst concentration with geographical location as determined by either latitude or longitude. However, when the three southermost sites were grouped and compared against the group of all remaining sites, there was a significant difference in the annual geometric mean cyst concentration with the southermost sites being higher. There was no significant association of geometric mean cyst concentration with size of the wastewater treatment plant as represented by mean daily flow. Attempts were made to obtain and correlate giardiasis cases with the monthly cyst levels at each of the sites. Due to reporting problems, low number of cases or unavailability of data, case information was deemed usable for only four of the 11 sites. A significant positive correlation with cases in the community was found at one site. Lack of correlation at other sites was believed to be due to deficiencies in case reporting. The results suggest that sewage examination may be useful for surveillance of Giardia infections in the community.