Evaluation of the surveillance system for MAP infection in Swedish ...

Proceedings of 10ICP - 2009MATERIALS AND METHODSA stochastic scenario-tree model was used to estimate the probability of freedom from MAPinfection. This method has previously been described by Martin and co-workers (Martin et al.,2007a; Martin et al., 2007b) and allows information from several different sources, e.g.random or non-random surveillance data as well as documentation of differences in risk, tocontribute to the quantitative estimation of surveillance sensitivities and probability ofdisease. In this type of model, factors relating to the structure of the study population and tothe probability of detection, or infection, are included as nodes for which input proportions orprobabilities are given. One example of the tree-like structure of the model is given in Fig. 1which shows the risk categories and relevant detection pathways relating to paratuberculosisinvestigations in fallen stock.INTRODUCTIONSlow riskhigh riskPARATB‐FREE HERDnon‐certifiedcertifiedHERD STATUSinfecteduninfectedANIMAL ORIGINdomesticimportANIMAL STATUSinfecteduninfectedREGIONRegion 1 Region 2 Region kANIMAL SENTTO LABnoyesDIAGNOSISnegativepositiveSymbol explanationNODE Node nameRisk category nodeInfection nodeDetection category nodeDetection nodePositive outcomeNegative outcomeFigure 1. Structure of the scenario-tree model for evaluation of MAP surveillance in Sweden.This example illustrates the surveillance component covering investigations of fallen stock.Empty arrows indicate corresponding branches.In brief, the scenario-tree model is used to calculate the probability of freedom based on thedifferent probabilities of detecting each type of infected unit, in case infection is present in thepopulation. By this approach more weight is given to investigations in herds and animal ofknown risk categories.The probability of detection at the animal level, i.e. the animal-level sensitivity, is usedto calculate the herd-level sensitivity. In our model, the herd sensitivity was calculatedseparately for each existing herd of cattle in Sweden and was based on the type of herd andthe actual number of individuals included in a specific surveillance activity. The numbers ofherds and animals included for 2008 are presented in Table 1.212