Repeated collection of information from the same selected sites or groups of animals (e.g. veterinary practices laboratories herds or animals) to identify changes in the health status of a specified population over time. These sentinels should act as a proxy for the larger population of interest; they may be selected on the basis of risk but can also be selected randomly or on the basis of convenience or compliance. Sentinel surveillance is frequently used for early detection of vector-borne disease and can also be used for estimation of prevalence of endemic disease.