Summary
The author describes the characteristics of surveillance systems and the use of surveillance in the three following scenarios: absence of infection, appearance of an exotic/emerging infection and endemic infections. In a population free from an infection, surveillance is used mainly to protect, by means of early detection systems, the population itself from the introduction of the infectious agent from other populations, and to document the health status of the population for international trade purposes. When an exotic infection enters a country, the information required to foresee its possible spread and to plan control and eradication activities is very often missing. As an example of the use of surveillance to collect the information needed to plan control and eradication activities, the author describes the response to the incursion into Europe of bluetongue in the early 2000s. The European brucellosis programme (from 1964 to the present) is taken as an example of the use of surveillance to monitor the control/eradication activities and to steer the control/eradication programme. Finally, the principal challenges currently faced by animal health surveillance professionals are discussed, namely: the methods for gathering information from the wild animal populations, and the methods used to evaluate the equivalence between different surveillance systems based on structured non-random activities and random surveys.

Keywords
Control, Early detection systems, Eradication, Freedom from infection, Notification systems, Surveillance.