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Chapter 13.4 : Outbreak Investigations: Laboratory and Epidemiologic Concepts
Epidemiologic investigations may be performed in response to a change in the rate of an event or outcome compared with a community or facility's baseline or to evaluate a rate that is considered unacceptable compared with published rates in similar populations ( 20 ). The degree of departure from baseline needed to identify an outbreak is variable, depending on factors such as the population in which it occurs and the organism or agent responsible. A single case of a rare or virulent infection, e.g., West Nile encephalitis, rabies, measles, or cholera, may be enough to herald an outbreak. For more common or endemic organisms, like methicillin-resistant Staphylococcus aureus (MRSA), an outbreak may mean a significant increase above baseline rates of clinical isolates or illness. Thus, specific thresholds for concern vary from case to case. If a particularly fragile population is involved, e.g., low-birth-weight infants in the neonatal intensive care unit or patients in a burn unit, the threshold for investigation may be lower. In addition to the pathogen and the population involved, outbreaks may be defined by a change in prevalence of certain organism characteristics, e.g., antimicrobial susceptibility or toxin production. In each of the above cases, meaningful surveillance data against which to measure a change are a critical component of outbreak investigation ( 12 ).