Risk-based design of repeated surveys for the documentation of freedom from non-highly contagious diseases

The documentation of freedom from disease requires reliable information on the actual disease status in a specific animal population. The implementation of active surveillance (surveys) is an effective method to gain this information. For economical reasons, the sample size should be as small as possible but large enough to achieve the required confidence level for a targeted threshold. When conducting surveys repeatedly, various information sources about the disease status of the population can be taken into account to adjust the required level of confidence for a follow-up survey (e.g. risk assessments regarding disease introduction and results of previous surveys). As a benefit, the sample size for national surveys can be reduced considerably. We illustrate this risk-based approach using examples of national surveys conducted in Switzerland. The sample size for the documentation of freedom from enzootic bovine leucosis (EBL) and Brucella melitensis in sheep and in goats could be reduced from 2325 to 415 cattle herds, from 2325 to 838 sheep herds and from 1975 to 761 goat herds, respectively.