The soil and domestic and wild animals (primarily herbivores, including goats, sheep, cattle, horses, and swine) serve as reservoirs.

B anthracis is found in soil in many areas of the world. Ecologic factors (such as abundant rainfall following a period of drought) may enhance spore density in soil, although the exact influence of such factors remains poorly understood.

The organism generally exists in the endospore form in nature; germination of spores outside an animal host may occur when the following conditions are met (see References: WHO 1998):

  • Temperature between 8°C and 45°C
  • pH between 5 and 9
  • Relative humidity >95%
  • Presence of adequate nutrients

Endospores are resistant to drying, heat, ultraviolet light, gamma radiation, and some disinfectants. Spores can persist in soil for decades, as illustrated by biological warfare experiments during World War II on the Scottish island of Gruinard (see References: Manchee 1990). During 1943 and 1944, an estimated 4 x 1014 anthrax spores were dispersed on the island through explosive means. Spores were still detectable more than 40 years later. Disinfection of the island was finally completed in 1987, using a combination of seawater and formaldehyde.

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