Pulmonary Deposition of AerosolizedBacillus Atrophaeusin a Swine Model Due to Exposure from a Simulated Anthrax Letter Incident

Abstract

Dry anthrax spore powder is readily disseminated as an aerosol and it is possible that passive dispersion when opening a letter containing anthrax spores may result in lethal doses to humans. The specific aim of this study was to quantify the respirable aerosol hazard associated with opening an envelope/letter contaminated with a dry spore powder of the biological pathogen anthrax in a typical office environment. An envelope containing a letter contaminated with 1.0 g of dry Bacillus atrophaeus (BG) spores (pathogen simulant) was opened in the presence of an unrestrained swine model. Aerosolized spores were detected in the room in seconds and peak concentrations occurred by three minutes. The swine, located approximately 1.5 m from the source, was exposed to the aerosol for 28 min following the letter opening event and then moved to a clean room for 30 min. A necropsy was completed to determine the extent of in vivo spore deposition in the lungs. The median number of viable colony forming units (CFU) measured in the combined right and left lung was 21,200: the average mass of both lungs was 283 g. In excess of 100 CFU per gram of lung tissue was found at sites within the anterior, intermediate and posterior lobes. The results of this study confirmed that opening an envelope containing spores generated an aerosol spanning the respirable particle size range of 1-10 µm, and that normal respiration of swine led to spore deposition throughout the lungs. The observed deposition of spores in the lungs of the swine is within the LD 50 range of 2,500-55,000 estimated for humans for inhaled anthrax. Thus, there would appear to be a significant health risk to those individuals exposed to anthrax spores when opening a contaminated envelope.

Identifying experimental surrogates for Bacillus anthracis spores: a review

“…Other studies have used stand-ins such as B. thuringiensis to test spore movement in aerial spray [4,110,111], transport and deposition efficiency of spores in ventilation ducts [112], engineered aerosol production [113] and re-aerosolization of spores [114]. B. atrophaeus has been used to reproduce an anthrax letter event, demonstrating how an individual swine located 1.5 m from an opened letter inhaled >21,000 spores [115]. This is a lethal dose for humans exposed to B. anthracis and validates the significant biothreat of passive spore dispersion.…”