The detection of Cryptosporidium oocysts and Giardia cysts in cistern water in the US Virgin Islands

Most homes and public facilities in the U.S. Virgin Islands use a roof catchment system to obtain drinking water. Because water is so scarce throughout the islands, every building (except those federally owned) are required to have a cistern. Rainwater is collected in the cisterns and is subject to contamination from enteric pathogens found in the environment. The objective of this study was to determine the occurrence and concentrations of human enteric protozoa in cisterns originating from animal fecal contamination. Volumes of 4001 of water were filtered from nine private and four public cisterns four times over a 1-year period for a total of 44 samples. After processing the Alter, the entire volume was examined using Cryptosporidium and Giardia specific antibodies and epifluorescence microscopy to determine levels of Cryptosporidium oocysts and Giardia cysts. One or both of the protozoa were found in 81% of the public cisterns and this was statistically significant (P = 0.005) when compared to the private cisterns where 47% of the samples were positive. Cryptosporidium was found statistically more often in the 44 samples than Giardia. In addition, the use of a polyclonal antibody for Cryptosporidium which is genera-specific, also detected oocysts statistically more often than a monoclonal antibody which was more species-restrictive to C. parvum, which is associated with disease in humans, suggesting that non-mammalian oocysts were found more frequently in cistern waters. Levels ranged from 1 to 10 organisms/1001 with one sample at 70 oocysts. These levels are associated with estimated daily risks of 10(-2) to 10(-4) and are well above acceptable guidance as described for safe drinking water in the United States. On occasion high levels of heterotrophic bacteria (9.9 x 10(5) CFU/ml) and total coliforms (> 2000 CFU/100 ml) were also detected in these waters. A statistically significant correlation was found between the detection of Cryptosporidium spp. and Giardia spp. (r = 0.47853, P = 0.0008). The results of this study show that Cryptosporidium and Giardia, as well as bacteria, are present in these waters at levels which may involve significant public health risks. Public cistern systems are of particular concern because of the high percentage which were contaminated and the greater number of people exposed.