Abstract
(Englisch)
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A red fox (Vulpes vulpes) mediated rabies epizootic reached central and western Europe after WW II and reached the Atlantic cost in the mid-1980s. In the western European countries, building upon early experiments in Switzerland, rabies control strategies by means of oral vaccination of the vector population have been developed and successfully implemented until 1990. In the early 1990s, however, several serous re-infections and repeated outbreaks of rabies in areas believed to be rabies-free put the entire principle of oral rabies immunisation into question. In 1997, research teams of several western European countries (B, CH, D, F, and I) started a scientific project to evaluate the wildlife vaccination against rabies in difficult and emergency situations under the aspects of ecology, epidemiology, and immunology. Switzerland, who had experienced the last rabies case in a wild animal in fall 1996 and discontinued the rabies vaccination programme after the spring 1998 campaign, concentrated on the ecology of the vector population and on the consequences of the strong increase of the fox population along and after effective rabies control programmes. Switzerland had seen a general 3-4-fold increase of the fox population from 19985-95, which was regionally even stronger. Besides the obvious fact that fox populations recover after the strong decline as a consequence of a rabies outbreak, we have disclosed a long-term growth of the fox population indicating a general increase in the carrying capacity and an adaptation of the red fox to a human altered environment. One consequence of this development was the invasion of Swiss cities such as Zurich, where the fox density today is higher than in the rural surroundings.
The dynamic of the vector population has important implications for prolonged rabies control measures and in regard to future possible outbreaks of any zoonoses.
1. A fox population will show a logistic recovery from a rabies-induced low when a vaccination campaign becomes effective for 3-7 years after the low, possibly leading to much higher fox abundance than it was known before the first outbreak of rabies. This increase will also affect the distribution and behaviour (social structure) of the vector species.
2. Any rabies vaccination programme should be planned to reach the herd immunity needed to defeat rabies before the phase of the fastest population increase is reached. If a prolonged control programme is required, the vaccination strategy (e.g. density vaccine baits, period and pattern of vaccination campaigns) must be adapted to both, the higher fox abundance and the behavioural changes.
3. Prolonged rabies control programmes require an adapted strategy in regard to the spatial and temporal precautions. High fox abundance will lead to an increased probability that individuals cross artificial and natural barriers. Changes in behaviour may lead to a basic change in dispersal and migration patterns and hence change the traditional pattern of the spread of rabies. Consequently, rabies vaccination campaigns should be expanded in space (e.g. beyond barriers to fox movement which traditionally formed the boundary of a vaccination campaign) and in time (e.g. from one to two or more years after the discovery of the 'last known case').
4. The colonisation of urban habitat has consequences for the control of epizootics in the future, as the foxes there live in close vicinity of humans (in regard to zoonoses) and of pet animals (alternative vector species and potential competitors for vaccine baits). Furthermore, control measures in cities require improved public awareness campaigns.
The positive and negative experiences of European countries with and after the defeat of rabies can help to design tailor-made, cost-effective control programmes for central and eastern European countries, and studies of the ecology and epidemiology of urban fox populations can help to improve control programmes of the dog-mediated urban rabies which is a problem in many third-world cities.
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