Natural dissemination of Bacillus anthracis spores in northern Canada

Abstract

Soil samples were collected from around fresh and year-old bison carcasses and areas not associated with known carcasses in Wood Buffalo National Park during an active anthrax outbreak in the summer of 2001. Sample selection with a grid provided the most complete coverage of a site. Soil samples were screened for viable Bacillus anthracis spores via selective culture, phenotypic analysis, and PCR. Bacillus anthracis spores were isolated from 28.4% of the samples. The highest concentrations of B. anthracis spores were found directly adjacent to fresh carcasses and invariably corresponded to locations where the soil had been saturated with body fluids escaping the carcass through either natural body orifices or holes torn by scavengers. The majority of positive samples were found within 2 m of both year-old and fresh carcasses and probably originated from scavengers churning up and spreading the body fluid-saturated soil as they fed. Trails of lesser contamination radiating from the carcasses probably resulted from spore dissemination through adhesion to scavengers and through larger scavengers dragging away disarticulated limbs. Comparison of samples from minimally scavenged and fully necropsied carcass sites revealed no statistically significant difference in the level of B. anthracis spore contamination. Therefore, the immediate area around a suspected anthrax carcass should be considered substantially contaminated regardless of the condition of the carcass.

FIG. 1.

Distribution of viable B. anthracis spores around carcass site 10 from the 2000 anthrax outbreak. The black circles represent samples from which no anthrax spores could be isolated, and the white circles represent anthrax spore-positive samples. The numbers beside the white circles represent the number of viable anthrax spores per gram at that location.

FIG. 2.

Distribution of viable B. anthracis spores along winter road transects in an area affected by the 2000 anthrax outbreak.

FIG. 3.

Distribution of viable B. anthracis spores around carcasses from the 2001 anthrax outbreak on the Lake Claire Delta. A. Necropsied carcass 44. B. Carcass 49. The numbers represent the number of viable anthrax spores per gram at the center of each square.

FIG. 4.

Analysis of 2001 Wood Buffalo National Park soil samples for the presence of viable B. anthracis spores.