Effects of low and high temperatures on infectivity of Cryptosporidium muris oocysts suspended in water.

Cryptosporidium muris oocysts suspended in 200 microl of water were pipetted into plastic microcentrifuge tubes which were stored at 4 degrees C or frozen at -5 degrees C for 1, 3, 5, 7, and 10 days and at -20 degrees C for 1, 3, 5, and 8h, respectively. Other samples of C. muris oocysts suspended in water were heated in the metal block of a thermal DNA cycler. Block temperatures were set at 5 degrees C incremental temperatures from 40 to 70 degrees C. At each high temperature setting microcentrifuge tubes containing C. muris oocysts were exposed for 1 min. Both, frozen and heated oocyst suspensions as well as untreated control oocyst suspensions were then inoculated into each of four ICR mice by gastric intubation. Untreated, freeze-thawed or heated oocysts were considered infectious when oocysts of C. muris were found microscopically in the faeces of mice after inoculation. All inoculated mice that received oocysts frozen at -5 degrees C for 3, 5, 7, and 10 days and -20 degrees C for 1, 3, 5, and 8h had no oocysts in faeces. In contrast, C. muris oocysts frozen at -5 degrees C for 1 day remained infective for inoculated mice. Our results also indicated that when water containing C. muris oocysts was exposed at a temperature of 55 degrees C or higher for 1 min, the infectivity of oocysts was lost.