Submitted to: Emerging Infectious Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 20, 1999
Publication Date: July 1, 1999
Interpretive Summary: Numerous recent outbreaks of cryptosporidiosis have been associated with the use of recreational water in which chlorine was used as disinfectant. Although several laboratory studies have examined the effects of chlorine exposure on oocyst viability, none have examined such effects under conditions of pH and elevated calcium concentrations required for most community swimming pools. In the present study experiments were conducted in which oocyst survival and infectivity were tested after exposure to clean water and fecally contaminated water that met standards for chlorine, pH, calcium concentration, and temperature for public swimming pools. Oocysts in fecally contaminated water at all exposure times, temperatures, and chlorine concentrations were infectious for mice, indicating that the presence of organic material reduced the killing activity of the chlorine. These data suggest that the presence of fecal material may alter the concentration and time values used in calculations to disinfect swimming pools or other recreational water.
The first experiment in the present study was designed to determine if 2 or 10 ppm chlorine (pH 7.2-7.8) in water at 20 and 30oC altered Cryptosporidium parvum oocyst infectivity. Of oocysts assayed for infectivity in mice at daily intervals for 7 days, only those oocysts exposed to 2 ppm for 1 to 3 days at 20oC were found infective. A second experiment was conducted in which exposure times at the same temperatures and chlorine concentrations were shortened to 6, 12, 24, 48, and 72 hr. At 20o C and 2 ppm oocysts were infectious after water exposure for up to 24 hr; at 10 ppm they were infectious only after 6 hr. At 30oC and 2 ppm oocysts were infectious as long as 12 hr; at 10 ppm none were infectious at any time period except for some exposed for 72 hr. Using the same exposure times and chlorine concentrations, the effect of a simulated fecal accident on C. parvum was determined. Oocysts from all exposure times, temperatures, and chlorine concentrations were infectious for mice, indicating that the presence of organic material reduced the killing activity of the chlorine. These data suggest that the presence of fecal material may alter the Ct values needed to disinfect swimming pools or other recreational water.