Health Stream Literature Summary - Issue 56 - December 2009

Effect of the radiation intensity, water turbidity and exposure time on the survival of Cryptosporidium during simulated solar disinfection of drinking water.
Gomez-Couso, H., Fontan-Sainz, M., McGuigan, K.G. and Ares-Mazas, E. (2009) Acta Tropica, 112(1); 43-48.

Solar water disinfection (SODIS) has been endorsed by the World Health Organisation as effective point-of-use technique for the treatment of drinking water that is environmentally sustainable and low cost for households in developing countries. This study assessed the combined effects of solar radiation intensity, water turbidity and exposure time on the survival of C. parvum oocysts during simulated SODIS exposure using a multi-factorial mathematical model design.

Cryptosporidium oocysts were collected from a naturally infected calf and identified as C. parvum. Soil was mixed into distilled water to prepare turbid water samples with turbidity levels of 5, 100 and 300 NTU. A 100 W xenon arc lamp solar simulator was used to provide global radiation intensities (320 nm to 10 micro m) of 200, 600 and 900 W/sq m which is approximately equivalent to the intensities of solar radiation described for equatorial areas and corresponding to overcast, hazy sunlight and strong sunlight, respectively. Turbid water samples in 30 ml transparent containers were inoculated with suspensions of 5 x 106 purified C. parvum oocysts. After shaking, the containers were placed for different lengths of time (4, 8 or 12h) under the solar simulator at the different radiation intensities. Samples were maintained at a constant temperature of 30 degrees C by being immersed in a water bath so the lower half of the sample container was in contact with the water bath and the upper half was outside the water exposed to the simulated sunlight. Control samples were prepared in the same way with the same inoculum and then wrapped in aluminium foil to prevent light falling on the suspension. Experiments were performed in triplicate.

After exposure the containers were centrifuged and the sediments obtained were used to determine the viability and infectivity of C. parvum. The potential viability of C. parvum oocysts was determined by inclusion/exclusion of fluorogenic vital dye propidium iodide (PI) and a further modification that includes an immunofluorescence antibody test to verify oocyst identification. The potential infectivity of C. parvum oocysts was determined by inoculating Swiss mice with a 0.1 ml of a suspension containing 2.5 x 104 C. parvum oocysts from each combination of turbidity level, exposure time and radiation intensity. After 7 days the mice were sacrificed and oocysts in homogenised intestine samples were counted by haemocytometer. Statistically significant effects on potential viability and infectivity were identified by analysis of variance (ANOVA). The effects of the factors shown to be significant (p less than 0.05) by ANOVA were modelled by means of least-squares multiple regression.

It was found that as the intensity of radiation and the time of exposure increased, the percentage of PI-negative (potentially viable) oocysts and the infectivity decreased. The largest decreases were seen in the water samples with the lowest level of turbidity (5 NTU). However even after exposure at 900 W/sq m for 12 hours at 5NTU, around 54% of oocysts remained viable when assessed by PI exclusion and 30.3% remained infective when assessed by inoculation of mice. All three factors (turbidity, radiation intensity and exposure time) had statistically significant independent effects on the potential viability and infectivity of C. parvum oocysts, as did the interactions of turbidity level with intensity of radiation and intensity of radiation with exposure time (p less than 0.05). Regression of the potential viability results and infectivity results on the variables with statistically significant effects in ANOVA resulted in two equations and these were valid for predicting the potential viability and infectivity of C. parvum oocysts during simulated SODIS procedures carried out under the range of conditions assayed. The levels of potential viability and infectivity calculated from both equations were highly correlated, however the levels of potential viability obtained by inclusion/exclusion of the fluorogenic vital dye PI overestimate the infectivity when the infectivity is less than 100%. Further studies are needed to extrapolate these equations to field conditions which take into account the temperatures reached during SODIS experiments, which may vary according to the ambient temperature, topography and microclimate, period of the year and water turbidity.

Comment The authors note that water temperature also has a strong influence on pathogen inactivation during SODIS treatment but because high temperatures (45 degrees C or more) cannot be consistently reached, their experimental conditions were limited to 30 degrees C. The results presented here suggest SODIS is much less effective against Cryptosporidium than bacterial pathogens.

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