Bioterrorism is covert by its very nature. It is unannounced and it is hidden — it has to be. Moreover, it can stay hidden — for at least a while — adding to its destructive power. Every minute of every hour that a biological-warfare attack continues without being detected means that much more time to infect that many more victims.

Fundamentally, a bioweapon causes an artificial outbreak of disease. Historically, the accepted method of detecting any outbreak of a disease is to make it reportable by doctors, hospitals or laboratories. On the federal level the process is embodied in the National Notifiable Disease Surveillance System, which depends on doctors, hospitals and laboratories to fill out disease case reports and pass the information forward to public health officials. These well-educated professionals, experts in diagnosing diseases, are licensed and often are regulated by the same public health officials who are tasked with identifying outbreaks.

In practice, the reporting model tracks instances of a known, reportable disease such as the plague. There are some instances of plague every year, so a few cases do not necessarily signify an unusual or unnatural outbreak. However, if three times the annual national average number of cases occur in a city over the course of just a few weeks, it's reasonable to suspect that something unusual is going on.

The diseases caused by traditional bioweapons (anthrax, botulism, brucellosis, cholera, plague, tularemia, and western equine encephalitis and eastern equine encephalitis) are rare enough that a single case can be considered significant depending on its geographic location. A case of smallpox, which has been eradicated from the natural world, will always be considered an act of terrorism or war unless otherwise explained. Similarly, the incidence of naturally occurring anthrax is so low that any and all cases should raise suspicion.

Improvements in data technology have made paper reporting of diseases nearly obsolete; online reporting is now the norm. Reporting professionals can easily add their data to the greater public health picture without leaving their offices. This advance is more than just a convenience, though. Electronic reporting increases the number and improves the quality of reports. The information is then immediately available to public health analysts.

Size, seasons and suspicions

According to the Centers for Disease Control and Prevention, “syndromic surveillance” applies to surveillance “using health-related data that precede diagnosis and signal a sufficient probability of a case or an outbreak to warrant further public health response.” In other words, it refers to the collection of data on clusters of symptoms that do not depend on the final diagnosis.

The New York City Department of Health and Mental Hygiene has established a number of symptom clusters that hospital emergency rooms in New York City report daily. They track eight syndromes: common cold, sepsis, respiratory problems, diarrhea, fever, rash, asthma and vomiting. Of course, the syndromes are defined more by their symptoms than by the actual causes of the symptoms.

A primary goal of syndromic surveillance, therefore, is to recognize that something unusual is going on and to start field investigations early. Among the several factors affecting syndromic surveillance and its usefulness are:

Size

The event has to reach a statistically significant threshold level. If there are 55 cases of a particular disease in an average year, an outbreak of six cases in one month may not be enough to sound the alarm.

Population mobility

A population that moves from one jurisdiction to another means two things to the syndromic surveillance system: Those infected spread a contagious disease across a larger geographic area, and the population of infected victims is now part of a larger overall population pool. In today's commuter age, an attack that originates in New York City could quickly spread to a four-state region just by daily commuters.

Level of suspicion

Health care providers must understand the need to complete reporting procedures both fully and accurately. The index of suspicion within the patient-care community will affect the success of the syndromic surveillance system because the members of that community who are required to report events have to understand both what has to be reported and why the reporting is important.

Season of the year

A bioterrorism attack will take longer, and will be harder to detect, during the seasons when there are natural upswings in disease. For example, asthma cases increase every fall because of seasonal increases in natural irritants. The system has to allow for such known causal relationships. Raw daily tallies of a specific syndrome should not be compared to an annual daily average in any case. Instead, they should be measured against controlled statistics that have been adjusted to remove as many known natural variables as possible.

System design

The source and quality of the provided data will affect the speed and accuracy of any warning that might be issued, and the speed at which data can be received and processed will directly affect the usefulness of the results. A system that provides 100% accuracy but returns results in 30 days would be useless in countering bioweapons, which almost always have a “working life” of two weeks or less. During the 2001 anthrax attacks, the time between infection and onset of specific symptoms was four to six days.

Thresholds for alarm

Like any other alarm, from radiation meters to smoke detectors, the point at which a bioterrorism alarm is sounded is very important. If the threshold is too low, the alarms become routine and will be discounted; if it's too high then the threat will be too advanced when the alarm sounds.

Of course, the human factor always has to be considered. The time when the last false positive was received directly affects the success of the system. The more recently a false positive was received — and the more negative the fallout for those raising the alarm — the more wary those managing the system will be.

Other data

In addition to hospital emergency room data, several other sources of data have been studied and are being used. Two examples are the number of patients admitted to emergency rooms and the volume of calls to EMS units and agencies. An unusual increase in the number of people coming into the emergency room and/or in the number of ambulance runs on any given day may be evidence of a possible outbreak.

Fortunately, EMTs and paramedics collect much of the same information about patients and their reasons for calling an ambulance. It's well within the ability of a qualified paramedic or EMT to describe a patient's symptoms, which means that these patients can be sorted into the same syndromes as ER patients.

However, there are certain factors that might affect the validity of this data source. The first is that the jurisdictions of many public health agencies often are serviced by a large number of EMS agencies, which don't necessarily use the same standardized written reports. Also, they may not be required to turn in their reports. Even if there's a centralized report recipient, there may be a built-in delay or no timely way of entering and analyzing data.

In general, it seems to be the administrative bottlenecks that make data received from EMS units less timely, and therefore less useful. However, there are a number of electronic ambulance reporting systems now available that either record information onto a handheld computer or scan handwritten documents to make them available as electronic data as rapidly as they are scanned.

Laboratory case reports also can be useful. For example, if a certain test is the standard of care for patients with an unidentified rash and is used for little else, then an increase in requests for that test should indicated an increase in unknown rashes. An increase in gross numbers also can tell the disease tracker that something is amiss when the number of requests for lab tests climbs.

The complicated amalgam of data and data sources, laws, regulations, and practical factors that come into play in fighting bioterrorism presents a formidable challenge. Meeting that challenge is mandatory, however, because it presents the best hope for shortening the time between disease attack and disease discovery, and therefore for improving survival rates.

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Joseph Cahill has served as a line paramedic for more than 10 years. He currently works as an emergency planner in the Westchester (N.Y.) County Office of Emergency Management. A longer version of this article can be read at www.domesticpreparedness.com.