Veterinary Public Health Institute

Completed Projects

Risk-based herd sampling

Development and evaluation of a model for risk-based herd sampling to substantiate freedom from disease

Contact person: Sarah Blickenstorfer
Duration:  2009 - Dez. 2010

Documented freedom from disease is the basis for international free trade of animals and animal products. It requires reliable information on the actual disease situation in a well-defined animal population. The implementation of surveys at regular intervals is an effective and often prescribed tool to gain the required information. But such surveys are very costly and conflict the decreasing resources in veterinary service. Therefore, an international requirement exists to develop new scientific methods providing the opportunity to decrease surveillance costs without jeopardizing the quality of the statement.
In Switzerland, surveys are conducted annually for the documentation of freedom from IBR, EBL, Brucella melitensis, Aujeszky’s disease and PRRS. Currently, the herds that have to be sampled are extracted randomly from the AGIS or TVD database. But random sampling of herds does not take into account uneven distribution of disease risk. In fact, some herds have higher probability for infection than others due to different risk factors, e.g. excessive animal movement. Testing those high-risk strata is assumed to offer a higher potential of detecting disease if it would be present. Conversely, if herds are selected randomly for a survey, this sampling does not offer the highest probability of detecting disease. Therefore, the sampling should be conducted in herds with the highest risk for infection and therefore the highest probability of detecting disease to optimize the information content of a survey. Consequently, we assume that the necessary sample size for such a targeted surveillance will be smaller than for random sampling. This will reduce the costs of the regular surveys to document freedom from disease.
In this project, the application of targeted surveillance shall be worked out for the case studies of infectious bovine rhinotracheitis (IBR) and enzootic bovine leucosis (EBL) in Swiss cattle population. A quantitative model will be developed providing the number of required herds out of a specific high-risk stratum for the documentation of freedom from disease for a certain level of confidence and the optimal combination of herds from different risk strata will be tested.
The results of this project and the developed model will provide the Swiss veterinary service with a cost-efficient surveillance tool for future surveillance strategies.