Deliberate and accidental events
The burden of infectious disease in the community is significant – at any one time many individuals in any human population will be experiencing symptoms and exhibiting signs of infectious disease. A key goal of health surveillance is to identify cases of infectious disease as early as possible. Coupled with health intelligence – the processing, analysis and interpretation of health surveillance data – health systems then use these data to make decisions on how to adequately manage an infectious disease event.
Given the pre-existing burden of infectious disease, most health systems are designed to be able to manage cases of infectious disease that fall within expected disease types, case incident rates and treatment paradigms. The pre-existing burden of infectious disease in populations around the world is highly variable. Factors such as socio-economic status, access to health care, nutrition, vulnerability factors and the presence of disease vectors (e.g. mosquitoes, herbivorous animals, domesticated animals) or contact with zoonotic disease cycles are important influences of the burden of infectious disease in any population.
The emergence of a new or unexpected infectious disease event – such as due to a bioterrorism or biosafety event – can present a health system with significant practical challenges. Recognition of new or unexpected infectious diseases cases can take place at a number of points in a health system. These are:
- Recognition of an unusual or unexpected disease by front line clinicians
- The identification of unusual or unexpected pathogens on routine clinical testing in pathology labs
- A sudden increase in clinical cases significantly above the normal burden of infectious disease in a community
- An increase in mortality
- A change in the utilisation of certain medical resources
Often these factors are sufficient for a health system to identify the occurrence of an outbreak, or a new or unexpected disease event.
However, as you might have noticed, there can be a significant delay between the occurrence of infectious disease cases and the recognition through the mechanisms outlined above. For certain high risk biological agents this delay in recognition could result in the disease spreading faster than efforts to prevent, contain and control an outbreak. Equally such detection mechanisms are not able to differentiate between a deliberate and accidental outbreak event. Understanding if an event is accidental or deliberate provides valuable information to scientists, clinicians, and criminal investigators and assists in the identification of likely biological agents that are causing an outbreak in its early phases.
Factors associated with unusual outbreaks, bioterrorism events or biosecurity events are:
- An unusual case presentation recognised at a front line clinical facility (e.g. clear diagnosis of bioterrorism related disease in an Emergency Department)
- Higher morbidity or mortality than expected for a particular disease
- Disease occurring outside of geographic areas where the disease is normally found, particularly in the absence of a vector that is normally required for infection
- A distribution of cases in a “downwind” pattern
- Simultaneous epidemics of the same disease widely separated, and without clear linkage between them
- Serial epidemics, without clear linkage between them
- High attack rates in normally resistant populations (e.g. high attack rates in young adults)
- Disease occurring in humans before animals (reverse spread), particularly where the disease naturally occurs in animals before humans
- Direct evidence, admission/confession or a release event (e.g. explosion)
A number of researchers have combined these and other factors into sophisticated scoring systems (see related links) that attempt to provide at the earliest stage in an unusual or unexpected infectious disease event whether it is deliberate or natural. Coupled with health surveillance, these scoring systems can improve the accuracy of health intelligence processes and potentially provide additional early warning to decision makers, first responders and health systems.
After reading this article, discuss your answer to the following question in the comments section below:
Why do you think it is so difficult to differentiate between disease caused by high risk biological agent events in individuals, compared with disease caused by seasonal outbreaks or other common infectious diseases?
© UNSW Sydney