Conceptual foundations for infectious disease surveillance.

The purpose of this report is to offer concepts for consideration in developing infectious disease surveillance systems, defined here as active, formal, and systematic processes intentionally directed to rapidly seek out and identify infectious disease agents or disease. Performance of surveillance systems can be judged by their accuracy (sensitivity and specificity), precision (repeatability), timeliness, multiple utility, and value. Surveillance system operation and function necessary to achieve high performance are defined in part by characteristics of the specific infectious disease, including disease transition state dynamics, that define probabilities of being in the latent, infectious, or clinical phase of disease. Two key components of surveillance are the sampling scheme, which is intended to maximize the probability of capturing an infected animal or specimen as soon as possible after the herd has been exposed, and the diagnostic assays, which should maximize the probability of detecting the agent, or evidence of the agent, if it is present in the specimen, while minimizing the likelihood of a false-positive result. Proportional risk sampling, targeted sampling, and repeated sampling are strategies that can improve overall surveillance system accuracy and particularly the temporal sensitivity related to early detection. Hierarchical sampling schemes and multiplexed assays can maximize efficiency and improve utility by serving multiple surveillance systems and purposes. Development of the surveillance systems needed to address emerging and foreign animal diseases will necessarily require design and architecture that are highly probability-driven to maximize surveillance sensitivity and specificity and to minimize cost.