Pulmonary Deposition of Aerosolized Bacillus Atrophaeus in 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 LD₅₀ 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.

This work was carried out as part of Task Plan No. T-750B under the USA/Canadian Agreement on Cooperative Research and Development Concerning Counterterrorism signed 23 June 1995 between The Secretary of Defense of the USA and The Solicitor General of Canada. The authors would like to extend their appreciation to the Medical Therapy Group for the extensive groundwork, resources and expertise lent to this effort which contributed to the successful completion of this study. In particular, the authors would like to thank L. Schofield for his expertise and assistance with the animal use protocol and lung sampling methodology, T. Sawyer for discussions concerning methodology, N. Mazuryk, R. Hilsen and L. Stadnyk for technical assistance carrying out the study, and C. Davidson and K. Weatherby for their technical assistance with the animal model during the experiments. G. Schnell's work designing and constructing the exposure cage is gratefully acknowledged. The authors would like to thank P. Lundy for the opportunity to utilize minute ventilation and respiration rate data from control experiments on anesthetized swine of the same age and body weight as the animals in this study.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.