Designing serological surveillance programmes to document freedom from disease with special reference to exotic viral diseases of pigs in Denmark

Surveillance programmes based on laboratory screening tests are increasingl y used to document freedom from disease in order to facilitate trade, The f ollowing aspects must be considered when designing such programmes: disease s to be selected; epidemiology of the diseases; unit of analysis (animal or herd); target age group (or target farm type); test characteristics and sa mple size. Issues related to these aspects are discussed and illustrated us ing the example of serological surveillance for exotic viral diseases in th e pig population of Denmark, Sampling designs based on individual animal sa mples are compared with herd-based sampling (two-stage sampling). While the latter is likely to require a larger sample size, the increased level of i nformation and the reliability of the results obtained are considered to be worth the expense. Issues related to the development of international stan dards for declaring freedom from disease are discussed. The authors conclud e that international standards are desirable, providing that these standard s represent scientifically valid principles.