Cost-efficient vaccination of foxes (Vulpes vulpes) against rabies and theneed for a new baiting strategy

In this study, ecological models, optimisation algorithms and threshold ana lysis were linked to develop oral-vaccination strategies against rabies in fox populations. It is important that such strategies are cost-efficient an d resistant to environmental conditions which would lessen their success. The model validation shows that the ecological models used are suited to pr edict the proportion of tetracycline- (TC) marked foxes in the course of ti me. This figure indicates the proportion of foxes which had at least one co ntact to vaccine baits, and is based on the design of the vaccination strat egy (i.e. the number and timing of vaccination campaigns and the number of baits used per square kilometre and campaign). The design of a vaccination strategy also determines the costs. It is the combination of ecological models and optimisation algorithms that helped us to design a vaccination strategy which is capable of achieving a continuous rate of >70% of TC-marked foxes within an analytical horizon of 3 years at low costs. Compared to the standard strategy (baseline comparat or), the improved strategy incurs just over half of the cost while almost d oubling the number of weeks during which the proportion of TC-marked foxes is >70%. In the improved strategy, June is recommended as the time for bait distribu tion. The standard strategy, however, avoids summer months (because high te mperatures reduce the durability of the baits) which again leads to a reduc tion of the bait intake by the foxes. Using threshold analysis, we examined the effect of a reduced durability of the baits on the design of the impro ved vaccination strategy. We concluded that distribution of baits in June w as optimal given that the durability of baits is above a threshold of 7 day s. (C) 2001 Elsevier Science B.V. All rights reserved.