This study measured the inactivation rate of bovine genotype A Cryptosporidium parvum oocysts attributable to diurnal oscillations of ambient temperature and solar radiation typical of California rangelands and dairies from spring through autumn. We first measured the relationship between air temperature and the internal temperature of bovine feces exposed to sunlight on commercial operations throughout California. Once maximum air temperature exceeded the mid 20 degrees C, diurnal thermal regimes of bovine fecal material exhibited peaks of over 40, 50, 60, and 70 degrees C. These diurnal thermal regimes were emulated using a thermocycler, with oocysts suspended in distilled water or fecal-water mix. Using oral inoculations of 10(5) C. parvum oocysts per neonatal Balb/c mouse (>1000-fold the ID50), no infections were observed using 1 to 5-day cycles of these thermal regimes. Loss of infectivity induced bythese thermal regimes was primarily due to partial or complete in vitro excystation during the first 24-h diurnal cycle and secondarily to thermal inactivation of the remaining intact or partial oocysts. These results suggest that as ambient conditions generate internal fecal temperatures > or = 40 degrees C via conduction, radiation, and convection, rapid environmental inactivation occurs at a rate of > or = 3.27 log reduction d(-1) for C. parvum oocysts deposited in the feces of cattle.