voles. In extended rural areas of Germany and Japan, praz- iquantel baits lowered the prevalence of E. multilocularis in foxes (19–21). These results cannot be transferred to the condition of agglomerations and urban areas, where until now no attempt has been made to evaluate an intervention strategy for foxes. The urban cycle of E. multilocularis was studied inten- sively in Zürich, Switzerland (16,17,22). Analyses of fox stomachs indicated that A. terrestris was the most frequent- ly consumed intermediate host (23), and E. multilocularis is highly prevalent (mean 9.1%, maximum 20.9%) in this vole species, which lives predominantly along the city bor- der (22). Accordingly, the prevalence of E. multilocularis in foxes was significantly higher in the urban periphery than in more central areas (17), and the infection risk for alveolar echinococcosis might therefore be concentrated mainly in delimited areas in the urban periphery (16). Since urban inhabitants frequently use the zones of highest con- tamination for recreational activities and their domestic cats and dogs have access to infected voles, the urban periphery may represent a risk for alveolar echinococcosis. In this controlled experimental field study, we investi- gated the effect of anthelmintic baiting in defined urban areas where the organism is highly endemic and tested whether E. multilocularis egg contamination was signifi- cantly reduced. We also examined whether, as an expected consequence, its prevalence in urban intermediate hosts diminished. Methods Study Area The study was conducted in the community of Zürich and its surroundings. Zürich covers 92 km2 and has a pop- ulation of 360,000. Fifty-three percent of Zürich is an urban area with industrial, commercial, and residential buildings; the other portion of Zürich is 24% forest, 17% agricultural, and 6% water. We divided this area into zones: urban, border, and periurban zone. The urban zone is mainly residential with little green space. The periurban zone consists of forests, fields, pastures, and meadows. The border zone, which divides the urban and the periur- ban zone, was defined as extending 250 m from the border of the urban area with buildings into the residential area of the city and 250 m into the periurban surroundings. This zone includes mostly residential areas, allotments, ceme- teries, sports fields, public places, and pastures. The border zone and the periurban zone are used by the public for recreational activities. As far as hunting is concerned, Zürich is organized as a game sanctuary and, compared to the high population den- sity of >10 adult foxes per km2 (24), the hunting bag (foxes shot by game wardens) was relatively low during the RESEARCH course of this study (1.0 shot foxes per km2 and year). Baits Commercial baits were used in the study (Impfstoffwerk Dessau Tornau GmbH, Rosslau, Germany). Each weighed 13.5 g, and the matrix consisted of Altrofox 91 (Impfstoffwerk Dessau Tornau GmbH). This matrix is the same one as in the widely used rabies vaccine bait Rabifox (Impfstoffwerk Dessau Tornau GmbH). The baits contained 50 mg of the anthelmintic praziquantel (Droncit Bayer AG, Leverkusen, Germany), a highly efficient drug against adult cestodes. Experiment Design Along the urban periphery we selected six bait and six control areas of 1 km2 each and an additional bait area of 6 km2. Bait and control areas were separated by at least 600 m to minimize the chance of foxes using two areas (Figure 1). All areas included a similar amount of urban area with buildings, open spaces (public parks, cemeteries, allotment gardens, and meadows), and woodlands in a pat- tern typical for the urban fringe (Figure 1). In baited areas, 50 praziquantel-containing baits per km2 were distributed monthly (intervals of 25 to 35 days) during 19 months from April 2000 through October 2001. Baits were distrib- uted manually at places that were most likely to be fre- quented by foxes (e.g., where fox tracks had been seen, fox dens, and compost heaps) but not by dogs. To avoid olfac- tory contamination, baits were always handled with rubber gloves. Baits were covered with surrounding material to protect them from sun at exposed sites. Figure 1. Study area of the controlled anthelmintic baiting experi- ment in the conurbation of the city of Zürich. 50 Praziquantel-con- taining baits per km2 were delivered monthly in six 1-km2 bait areas and one 6-km2 bait area, that alternated along the urban fringe with six control areas. Black line, Zürich border. Emerging Infectious Diseases • Vol. 9, No. 10, October 2003 1267