Terrorist incidents in the United States and elsewhere involving bacterial pathogens (Török et al. 1997), nerve gas (Okumura et al. 1998), and a lethal plant toxin (i.e., ricin) (Tucker 1996) have demonstrated that the United States is vulnerable to biological and chemical threats as well as traditional explosive weapons. Recipes for preparing “homemade” agents are readily available (Fester 1997), and reports of arsenals of military bioweapons (Davis 1999) raise the possibility that terrorists might have access to highly dangerous agents that have been engineered for mass dissemination as small-particle aerosols. Agents such as the variola virus, the causative agent of smallpox, are highly contagious and often fatal. Responding to large-scale outbreaks caused by these agents will require the rapid mobilization of public health workers, emergency responders, and private healthcare providers. Large-scale outbreaks will also require rapid procurement and distribution of large quantities of drugs and vaccines, which must be immediately available.
In the past, most planning for emergency response to terrorism has been concerned with overt attacks, such as bombings. Chemical terrorism acts are also likely to be overt because the effects of chemical agents absorbed through inhalation or through the skin or mucous membranes are usually immediate and obvious. Such attacks elicit immediate response from police, fire, and emergency medical services (EMS) personnel.
In contrast, attacks with biological agents are more likely to be covert. They present different challenges and require an additional dimension of emergency planning that involves the public health infrastructure. Because the initial detection of and response to a covert biological or chemical attack will probably occur at the local level, disease surveillance systems at local hospitals and state and local health agencies must be capable of detecting unusual patterns of disease or injury, including those caused by unusual or unknown threat agents. Epidemiologists at state and local health agencies must have expertise and resources for responding to reports of clusters of rare, unusual, or unexplained illnesses.
Covert dissemination of a biological agent in a public place will not have an immediate impact because of the delay between exposure and onset of illness (i.e., the incubation period). Consequently, the first casualties of a covert attack will most likely be identified by physicians or other primary healthcare providers. For example, in the event of a covert release of the contagious variola virus, patients will appear in doctors’ offices, clinics, and emergency rooms during the first or second week after release, complaining of fever, back pain, headache, nausea, and other symptoms of what initially might appear to be an ordinary viral infection. As the disease progresses, these persons will develop the papular rash characteristic of early-stage smallpox, a rash that physicians might not recognize immediately. By the time the rash becomes pustular and patients begin to die, the terrorists would be far away and the disease disseminated through the population by person-to-person contact.
Only a short window of opportunity will exist between the time the first cases are identified and a second wave of the population becomes ill. During that brief period, public health officials will need to determine that an attack has occurred, identify the organism, and avoid more casualties through prevention strategies (e.g., mass vaccination in the case of smallpox or prophylactic treatment in the case of anthrax). In the case of smallpox, as person-to-person contact continues, successive waves of transmission could carry infection to other worldwide localities, similar to what occurred in the SARS (severe acute respiratory syndrome) outbreak in spring 2003. These issues might also be relevant for other person-to-person transmissible etiologic agents (e.g., plague or certain viral hemorrhagic fevers).
Certain chemical agents can also be delivered covertly through contaminated food or water. In 1999, the vulnerability of the food supply was illustrated in Belgium, when chickens were unintentionally exposed to dioxin-contaminated fat used to make animal feed (Ashraf 1999). Dioxin, a cancer-causing chemical that does not result in immediate symptoms in humans, was probably present in chicken meat and eggs sold in Europe as early as 1999, because the contamination was not discovered for months. This incident underscores the need for prompt diagnoses of unusual or suspicious health problems in animals as well as humans, a lesson that was also demonstrated in New York City by the winter 1999 outbreak of mosquito-borne West Nile virus first diagnosed in birds and humans. The dioxin episode also demonstrates how a covert act of food-borne biological or chemical terrorism could affect commerce and human or animal health.
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