Effect of high temperature on infectivity of Cryptosporidium parvum oocysts in water

Abstract

Cryptosporidium parvum oocysts suspended in 0.5 ml of distilled water were pipetted into plastic vials which were inserted into wells in the heated metal block of a thermal DNA cycler. Block temperatures were set at 5°C incremental temperatures from 60 to 100°C. At each temperature setting four vials containing C. parvum oocysts were placed into wells and held for 15 s before time was recorded as zero, and then pairs of vials were removed 1 and 5 min later. Upon removal, all vials were immediately cooled on crushed ice. Also, at each temperature interval one vial containing 0.5 ml of distilled water was placed in a well and a digital thermometer was used to record the actual water temperature at 30-s intervals. Heated oocyst suspensions as well as unheated control suspensions were orally inoculated by gavage into 7- to 10-day-old BALB/c mouse pups to test for infectivity. At 96 h after inoculation the ileum, cecum, and colon from each mouse were removed and prepared for histology. Tissue sections were examined microscopically. Developmental-stage C. parvum was found in all three gut segments from all mice that received oocysts in unheated water and in water that reached temperatures of 54.4, 59.9, and 67.5°C at 1 min when vials were removed from the heat source. C. parvum was also found in the ileum of one of six mice that received oocysts in water that reached a temperature of 59.7°C at 5 min. These data indicated that when water containing C. parvum oocysts reached temperatures of 72.4°C or higher within 1 min or when the temperature was held at 64.2°C or higher for 2 min of a 5-min heating cycle, infectivity was lost.