Review of Disinfection and Associated Studies on Cryptosporidium

DWI0805

Executive Summary

The project here reported was undertaken to review the published literature on disinfection studies on Cryptosporidium. The purpose was to assess the variations in experimental models and the effect of such variables on the results. A wide variety of disinfectants have been used to study the inactivation of Cryptosporidium parvum for clinical, veterinary and water treatment purposes. Test parameter variables include, pH, time of exposure, temperature, presence or absence of organic material, use of a neutralizing step, and measurement of disinfectant exposure and of post exposure residual.

Of particular note is that such studies have employed oocysts of often ill defined characteristics, from different sources, which have been cleaned using different protocols and stored, with or without preservatives, under different conditions for different lengths of time and temperature. All studies have been conducted using animal-derived oocysts, which are thus probably all genotype 2. No studies have used the apparently human-specific genotype 1, which has been responsible for a number of major waterborne outbreaks. Oocysts have proved inherently difficult or impossible to preserve in the way that other inocula for conventional disinfection studies can be preserved.

The assessment of disinfection efficacy for water treatment purposes presents special difficulties, and has been done using various methods. These include, excystation by varying protocols, with and without measurement of sporozoite ratios, by dye inclusion/exclusion, and by infectivity assessment using tissue culture or animal inoculation methods; in some cases additional disinfectant treatment has been a part of the excystation protocol. In disinfection studies for other potential pathogens it has not been the practice to demonstrate infectivity as opposed to viability.

These factors have led to poor standardisation of disinfection study protocols and render the comparison of different research projects and their findings almost impossible. In order to provide consistent and comparable test data, these variables need to be standardised. Whilst many research papers describe the test parameters in some detail, there is little critical appreciation of the fundamental importance of the need particularly for standardisation of oocyst preparations for use in disinfection studies.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.