As part of the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) safeguarding mission, the agency has prepared a factsheet that explains the criteria used to describe "high-consequence" disease incidents and includes a list of serious foreign animal diseases and pests that do not currently exist in the United States.
The factsheet is available at http://www.aphis.usda.gov/publications/animal_health/2013/fs_hc_diseases.pdf