NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by the American Animal Hospital Association, the Association of American Veterinary Medical Colleges, the American Veterinary Medical Association, the Centers for Disease Control and Prevention under Contract No. 200-2000-00629 (Task Order No. 28), the National Association of Federal Veterinarians, and the National Center for Research Resources and the National Institute of Environmental Health Sciences of the National Institutes of Health under Contract No. N01-OD-4-2139 (Task Order No. 140). Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the organizations or agencies that provided support for the project.