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Natural vs. Intentional Disease Outbreaks

With a covert biological agent attack, the most likely first indicator of an event would be an increased number of patients presenting with clinical features caused by the disseminated disease agent. Therefore, health care providers must use epidemiology to detect and respond rapidly to a biological agent attack.

A sound epidemiologic investigation of a disease outbreak, whether natural or human-engineered, will assist medical personnel in identifying the pathogen, as well as instituting the appropriate medical interventions. Documenting the affected population, possible routes of exposure, signs and symptoms of disease, along with rapid laboratory identification of the causative agents, will greatly increase the ability to institute an appropriate medical and public health response. Good epidemiologic information can guide the appropriate follow-up of those potentially exposed, as well as assist in risk communication and responses to the media.

Many diseases caused by weaponized biological agents present with nonspecific clinical features that could be difficult to diagnose and recognize as a biological attack. The disease pattern that develops is an important factor in differentiating between a natural and a terrorist or warfare attack. Epidemiologic clues that can potentially indicate an intentional attack are listed in Table 1. While a helpful guide, it is important to remember that naturally occurring epidemics can have one or more of these characteristics and a biological attack may have none.

Once a biological attack or any outbreak of disease is suspected, the epidemiologic investigation should begin. The conduct of the investigation will not differ significantly whether or not the outbreak is intentional. The first step is to confirm that a disease outbreak has occurred. A case definition should be constructed to determine the number of cases and the attack rate. The case definition allows investigators who are separated geographically to use the same criteria when evaluating the outbreak. The use of objective criteria in the development of a case definition is very important in determining an accurate case number, as additional cases may be found and some cases may be excluded, especially as the potential exists for hysteria to be confused with actual disease. The estimated rate of illness should be compared with rates during previous years to determine if the rate constitutes a deviation from the norm.

Once the attack rate has been determined, the outbreak can be described by time, place, and person. These data will provide crucial information in determining the potential source of the outbreak. The epidemic curve is calculated based on cases over time. In a point-source outbreak, which is most likely in a biological attack or terrorism situation, the early parts of the epidemic curve will tend to be compressed compared with propagated outbreaks. The peak may be in a matter of days or even hours. Later phases of the curve may also help determine if the disease appears to spread from person to person, which can be extremely important for determining effective disease control measures.

Well before any event, public health authorities must implement surveillance systems so they can recognize patterns of nonspecific syndromes that could indicate the early manifestations of a biological warfare attack. The system must be timely, sensitive, specific, and practical. To recognize any unusual changes in disease occurrence, surveillance of background disease activity should be ongoing, and any variation should be followed up promptly with a directed examination of the facts regarding the change.

It is important to remember that recognition of and preparation for a biological attack is similar to that for any disease outbreak, but the surveillance, response, and other demands on resources would likely be of an unparalleled intensity. A strong public health infrastructure with epidemiologic investigation capability, practical training programs, and preparedness plans are essential to prevent and control disease outbreaks, whether they are naturally occurring or otherwise.

Table 1. Epidemiologic Clues of a Biologic Warfare or Terrorist Attack

• The presence of a large epidemic with a similar disease or syndrome, especially in a discrete population

• Many cases of unexplained diseases or deaths

• More severe disease than is usually expected for a specific pathogen or failure to respond to standard therapy

• Unusual routes of exposure for a pathogen, such as the inhalational route for diseases that normally occur through other exposures

• A disease that is unusual for a given geographic area or transmission season

• Disease normally transmitted by a vector that is not present in the local area

• Multiple simultaneous or serial epidemics of different diseases in the same population

• A single case of disease by an uncommon agent (smallpox, some viral hemorrhagic fevers)

• A disease that is unusual for an age group

• Unusual strains or variants of organisms or antimicrobial resistance patterns different from those circulating

• Similar genetic type among agents isolated from distinct sources at different times or locations

• Higher attack rates in those exposed in certain areas, such as inside a building if released indoors, or lower rates in those inside a sealed building if released outside

• Disease outbreaks of the same illness occurring in noncontiguous areas

• Intelligence of a potential attack, claims by a terrorist or aggressor of a release, and discovery of munitions or tampering

(U.S. Army Medical Research Institute of Infectious Diseases, USAMRIID's Medical Management of Biological Casualties Handbook, Fourth Ed., February 2001, Pages 13-14)

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