The spatio-temporal dynamics of a post-vaccination resurgence of rabies infoxes and emergency vaccination planning

We used a simulation model to study the spatio-temporal dynamics of a poten tial rabies outbreak in an immunized fox population after the termination o f a long-term, large-scale vaccination program with two campaigns per year one in spring and one in autumn. The 'worst-case' scenario of rabies resurg ence occurs if rabies has persisted at a low prevalence despite control and has remained undetected by a customary surveillance program or if infected individuals invade to the control area. Even if the termination of a vacci nation program entails such a risk of a subsequent new outbreak, prolonged vaccination of a wild host population is expensive and the declining cost-b enefit ratio over time eventually makes it uneconomic. Based on the knowled ge of the spatiotemporal dynamics of a potential new outbreak gained from o ur modelling study, we suggest "terminating but observing" to be an appropr iate strategy. Simulating the decline of population immunity without revacc ination, we found that a new outbreak of rabies should be detected by custo mary surveillance programs within two years after the termination of the co ntrol. The time until detection does not depend on whether Vaccination was terminated within the fourth, fifth or sixth years of repeated biannual cam paigns. But it is faster if the program was completed with an autumn campai gn (because next-year dispersal then occurs after a noticeable decrease in population immunity). Finally, if a rabid fox is detected after terminating vaccination, we determine a rule for defining a circular hazard area based on the simulated spatial spread of rabies. The radius of this area should be increased with the time since the last vaccination campaign. The trade-o ff between the number of foxes potentially missed by the emergency treatmen t and the cost for the emergency measures in an enlarged hazard area was fo und. (C) 2000 Elsevier Science B.V. All rights reserved.