Water disinfection strategies for Cryptosporidium

Cryptosporidium has been designated by the Centers for Disease Control and Prevention (CDCP) as an emerging infection with important public health consequences [1]. The first human case was detected in 1976. Serologic studies demonstrated that unrecognized infection was common [2], but subsequent clinical reports were scarce until 1982 when the AIDS epidemic created a large group of immunocompromised persons who were particularly susceptible to enteritis from Cryptosporidium. However, clinical cases have increased in immunocompetent persons as well [3], leading to the question of whether this increase is the result of increased surveillance, the prevalence of the organism in the environment, a change in its mammalian reservoir, or an alteration of the virulence of the organism. Infection can be transmitted through direct person-to-person or animal-to-person contact or through ingestion of fecally contaminated water or food. The water-borne route appears to be the most important, accounting for large outbreaks from ingestion of both treated or untreated water [4]. Surface water and even inadvertent ingestion of recreational water have been implicated in outbreaks of cryptosporidiosis [5]. Research has found that a median infectious dose in healthy volunteers was 132 oocysts; one in five volunteers who ingested only 30 oocysts became ill [6]. Environmental studies in the United States have detected Cryptosporidium oocysts in a high percentage of surface water samples, including pristine sources that have little or no human activity, agricultural runoff, or sewage contamination [7]. The oocysts have also been found in filtered and chlorinated water; they are known to be quite resistant to chlorine. Cryptosporidium therefore appears to be a prevalent, significant human pathogen with an animal reservoir and commonly transmitted via water-borne route, all of which would make it a concern to wilderness and foreign travelers who must ingest surface water or tap water of questionable quality. Iodine is one of the major disinfectants used for field water treatment; therefore, the study by Gerba et al. in this issue of the Journal provides important information to help formulate effective measures for assuring potability of surface water. The experimental protocol for this disinfection study was well designed. In fact, their data show that Cryptosporidium oocysts can be killed by iodine but not in doses that are practical for field use. To compare efficacy, disinfectant lethality for water treatment is commonly designated as a Ct constant: the product of the concentration (C) of disinfectant (in mg/L) and time (t) (in minutes) which is required for death of a given percentage of organisms, in this case 99.9%, under specified water conditions of temperature, pH, and turbidity. Disinfection follows a first-order chemical reaction with an inverse relationship between concentration of halogen (in this case, iodine) and time in contact with microorganisms; lower concentrations require longer contact times. The disinfection reaction is slowed by cold temperatures. The results in this study for iodine destruction of Giardia cysts are consistent with previously published disinfection studies that found a wide range of Ct constants averaging 50-150 in warm, clear waters to 250-650 or more in cold, cloudy water [8]. I am not aware of previous studies for iodine against Cryptosporidium. Other investigations have documented the susceptibility of Cryptosporidium to chlorine dioxide and ozone [9], disinfectants that are used in municipal water treatment plants but are not feasible for field use. The CDCP has reported a Ct constant of 9600 for hypochlorite with Cryptosporidium [10]. If the data from this study are extrapolated, it is likely that 99.9% inactivation would have been achieved in a few more hours, giving a Ct constant less than chlorine, but one that could not be used reliably in the field. However, the data suggest that the equivalent of two iodine tablets in good-quality surface water, left for at least 8 hours overnight, would result in a 3-log reduction of Cryptosporidium. Fortunately, other techniques are available to remove or inactivate Cryptosporidium oocysts. Like Giardia. these oocysts are very susceptible to heat. Two minutes of 65°C heat has been shown to inactivate oocysts [11]. Inocula held at 45°C for 20 minutes inactivated the oocysts, demonstrating that food pasteurization procedures are effective for Cryptosporidium. Oocysts are also susceptible to filtration [12]. Although smaller than Giardia cysts, Cryptosporidium oocysts, measuring 2-3 fJ-m in