Die-off of Cryptosporidium parvum in soil and wastewater effluents.

Aims:  To determine the effect of biotic and abiotic components of soil on the viability and infectivity of Cryptosporidium parvum, and evaluate the suitability of viability tests as a surrogate for oocyst infectivity under various environmental settings. Methods and Results:  The die-off of C. parvum in saturated and dry loamy soil was monitored over time by immunofluorescence assay (IFA) and PCR to estimate oocysts viability and by cell culture to estimate oocysts infectivity. Pseudomonas aeruginosa activity resulted in digestion of the outer layer of the oocysts, as demonstrated by loss of the ability to react in IFA. Whereas, P. aeruginosa activity did not affect the DNA amplification by PCR. A 1-log reduction in the oocysts infectivity was observed at 30 °C in distilled water and in saturated soil while oocysts viability was unchanged. Incubation for 10 days in dry loamy soil at 32 °C resulted in a 3-log10 reduction in their infectivity while no change of oocysts viability was recorded. Conclusions:  Under low temperature, C. parvum oocysts may retain their infectivity for a long time. Soil desiccation and high temperatures enhance the die-off rate of C. parvum. Significance and Impact of the Study:  Previous die-off studies of C. parvum used viability tests that do not necessarily reflect the oocyst infectivity. Under low temperatures, there was an agreement observed between viability and infectivity tests and oocysts retained their infectivity for a long time. Desiccation and high temperatures enhance the loss of infectivity of C. parvum. The presented die-off data have significant implications on the management of wastewater reuse in warm environments.