The development of an exposure risk index as a rational guide for biosecurity programs.

Population disease risk can be ranked using an exposure risk index that uses quantitative data on exposure risk factors and the proximity of susceptible animals to disease reservoirs. The reservoir represents the available microbial load (a quantity), derived from the mass of the contaminant, the percentage available for dissemination, the initial microbial content of the contaminant, and its age and half-life. The proximity measurement uses distance from the reservoir to calculate an area over which the microbial exposure might be spread. Dividing the reservoir by the proximity measurement, one obtains an exposure risk measurement that is significantly correlated with veterinarians' perceptions of risk. This exposure risk ranking can be used to rank population disease transmission risks associated with events and practices in animal production. The advantages of the exposure risk index are that it derives an exposure risk measurement from available objective information, it provides a way to compare disparate sources of disease exposure risk, it can be modified for specific diseases, and it provides a foundation for developing and evaluating mitigation strategies. From the exposure risk measurement, mitigation strategies and available resources can be focused appropriately to prevent or control disease. Biosecurity programs and disease control measures can be directed at those areas of greatest risk for spreading disease.