Anthrax is an acute infectious disease caused by the spore forming bacterium Bacillus anthracis. Anthrax most commonly occurs in wild and domestic lower vertebrates (cattle, sheep, goats, camels, antelopes, and other herbivores), but it can also occur in humans when they are exposed to infected animals, tissue from infected animals, or any other source of anthrax spores. Human anthrax infection can occur in three forms: cutaneous (skin), inhalation, and gastrointestinal. Bacillus anthracis spores can live in the soil for many years, and humans can become infected with anthrax by handling products from infected animals or by inhaling anthrax spores from contaminated animal products. Anthrax can also be spread by eating undercooked meat from infected animals. If left untreated, anthrax in all forms can lead to septicemia and death.
By analogy with similar spore forming bacteria, a tough protective coat and a variety of other specific protective mechanisms including the presence of dipicolinic acid (possible as a complex with Ca++), specific DNA stabilizing proteins, and an efficient DNA repair system allow anthrax bacteria to survive as spores for decades. Such spores are particularly dangerous when present in a state in which they can be easily aerosolized (dry and present as particles under about 5 microns in size).
Recent terrorism attacks in the U.S. and other countries have involved anthrax spores sent through the mail and have resulted in several deaths. The initial terrorist-related anthrax cases occurred among persons with known or suspected contact with opened letters containing anthrax spores. Subsequent anthrax cases have been confirmed among U.S. postal workers and others who have had no known contact with contaminated opened letters. This suggests that sealed envelopes containing anthrax spores passing through the postal system may be the source of anthrax exposure. The number of anthrax-contaminated mail pieces passing through the U.S. postal system to date is not known. It has been surmised that automated sorting and handling equipment utilized by postal services may have damaged mail pieces containing anthrax spores causing the release of anthrax spores into postal environments, or that sealed mail may be permeable to anthrax spores causing the release thereof into postal environments.
The U.S. Postal Service is currently investigating various strategies to address the risk of anthrax exposure among workers involved in mail handling. These strategies include providing workers with protective suits. Unfortunately, protective suits can be cumbersome and awkward to the wearer and may cause the wearer difficulties in performing mail handling duties.
In addition, various methods have been proposed for neutralizing anthrax spores contained within mail pieces. These include irradiation with electron beams, gamma rays, X-rays, and ultraviolet (UV) light. Unfortunately, these irradiation techniques may require direct and prolonged exposure to anthrax spores to effectively neutralize them. As such, issues such as costs, personnel safety, damage to mail and mail contents, and mail handling efficiency may limit widespread application of these irradiation techniques.
Methods for heating biological materials for various reasons with single frequency microwave energy are known. For example, U.S. Pat. No. 4,250,139 to Luck et al. discloses a method of exposing dried protein to a lethal dose of single frequency microwave radiation for a time sufficient to provide a desired degree of decontamination. U.S. Pat. No. 5,073,167 to Carr et al. discloses a method of uniformly heating liquid blood and other intravenous fluids using single frequency microwave energy. The use of single frequency microwaves to inactivate spores and bacteria is described by Jeng et al. in Mechanism of Microwave Sterilization in the Dry State, Applied and Environmental Microbiology, September, 1987 53: 2133-2137, and by Latimer et al. in Microwave Oven Irradiation as a Method for Bacterial Decontamination in a Clinical Microbiology Laboratory, Journal of Clinical Microbiology, October, 1977 6:340-342.
Unfortunately, it can be difficult to achieve uniform distribution of microwave energy within a microwave furnace using single frequency microwave radiation. Hot spots may develop within a microwave furnace cavity which can damage an article being processed. In addition, repeatability of treatment time and results may not be achievable using single frequency microwave radiation without positioning an article in the same position and orientation as a previous article within a microwave furnace cavity.
Single frequency microwave radiation may also cause conductive elements to arc and spark. As such, conductive articles within envelopes and packages, such staples, paper clips, and the like, may arc when exposed to microwave energy, which may damage envelopes and packages and their contents.
U.S. Pat. No. 6,268,200 to Tucker et al., describes attenuating viruses contained within a lyophilized biotherapeutic sealed within a microwave permeable container without harming the biotherapeutic and without exposing the biotherapeutic to additional viruses, by subjecting the container and biotherapeutic therewithin to variable frequency microwave energy.