Theoretical distribution of John's disease stages in individual herds and simulation of consequences to risk management options
2006
Favre, D. | Muehlemann, M. | Schaellibaum, M. (Agroscope Liebefeld-Posieux (ALP), Posieux (Switzerland))
The present work presents a model of the propagation of Johnes disease (JD) in individual herds in order to allow for the simulation of consequences of herd management options for disease control or eradication. Sources for model parameterization were a qualitative description of JD based on 4 distinct disease stages, literature data and expert estimates. Modelling was based on the Susceptibility-Exposition- Infection-Retrieval (SEIR) model combined with the Leslie population model. The model comprises an iterative process with matrices for initial conditions and transition between stages. The initial conditions matrix depends on the actual health status of the herd and is derived from the modelling of the distribution of JD and analytical data. The transition matrix depends on country and the specific expert knowledge of herd parameters, i.e. death and birth rates, contamination at birth, and duration of stages. The corresponding estimates may easily be reexamined. For a shorter duration than any JD stage duration, the matrix describes the changes in animals classes. Without this condition, the model is redefined between iterations. An upper limit of 3 stage III animals, 9 stage II animals and 27 stage I animals and a lower limit of 2 stage III, 4 stage II and 8 stage I animals were implemented in order to estimate the initial stage values. For that reason, simple hypotheses concerning duration of stages and considered intervals of time were also implemented in order to test the combined model. Moreover, parameters such as births, contaminations and deaths were set as uniformly distributed. Output may be expressed as a user-friendly spreadsheet in which different scenarios could be tested.
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