RESEARCH Figure 4. Prevalences of Echinococcus multilocularis in Arvicola terrestris and 95% exact binomial confidence intervals in the six 1- km2 bait areas, baited monthly with 50 praziquantel-containing baits per km2, and the six unbaited control areas during the exper- iment. in Zürich showed that approximately half of the baits that disappeared were taken by foxes but the others were con- sumed by hedgehogs, dogs, rodents, and snails (Hegglin et al., unpub. data.). Therefore, bait densities exceeding 20 baits per km2 seem to be appropriate to reach most foxes in urban habitats. In addition, the manual distribution of baits at selected sites attractive for foxes can improve bait uptake of foxes (Hegglin et al., unpub. data). Small-Scale Anthelmintic Baiting Our results show clearly that the E. multilocularis egg contamination in urban areas can be reduced to a low level by manually distributing anthelmintic baits at monthly intervals. This reduction is even possible within defined urban patches of 1-km2 in areas where the organism is highly endemic. Although the initial rate of coproantigen- positive fecal samples was high (38.6%), this rate decreased to 5.4% during the first year of baiting. Additionally, the coproantigen-positive fecal samples in baited areas contained E. multilocularis eggs significantly less frequently. In contrast to our results, a large-scale praziquantel-baiting campaign in a rural area in southern Germany, covering 566 km2, showed a strong effect in the 156-km2 core area, but in the 6- to 10-km border area, the effect was much less pronounced (19). Immigration of young, infected foxes may have caused this border effect. In Hokkaido, Japan, an anthelmintic baiting study was car- ried out in a smaller, rural area of 90 km2, which resulted in a drastic reduction of environmental contamination comparable to our study (21). The main difference between the German study, the Japanese study, and our own may be explained by the different baiting strategies. In Germany, approximately half of the baits were random- ly delivered by aircraft, and the intervals between two bait- ing actions varied from 2 to 4 months. In our study, all baits were delivered manually around places attractive for foxes at monthly intervals. A model for E. multilocularis control indicated that baiting intervals of 4 to 6 weeks would be most efficient (33). The strong local effect in this study shows that in urban areas the population dynamics of E. multilocularis is main- ly determined by factors of very restricted spatial exten- sion. Knowledge about spatial dynamic of fox populations is crucial in understanding the dispersion capacity of E. multilocularis. Urban settings, which provide plentiful food sources, are well-suited to sustain high population densities of foxes (34), who tend to have small home ranges and low dispersing distances (35,36). In addition, urban fox populations are generally organized in family groups, in which predominantly young vixens remain in the parental home range and help rear pups (36,37). Consequently, offspring frequently inherit parental territo- ry and do not have to disperse. Furthermore, a low urban immigration rate, which has been substantiated by genetic microsatellite analyses for the Zürich urban fox population (38), and a low hunting pressure (see Methods) contribute to the moderate spatial dynamics of the urban fox popula- tion, which we assume to be a precondition for the effec- tiveness of the small-scale anthelmintic treatment. Reduction of Infection Pressure During the first year of baiting, when the proportion of E. multilocularis–coproantigen-positive fecal samples had already decreased significantly, no difference was detected in the A. terrestris prevalence of bait and control areas. The significantly lower prevalence of A. terrestris trapped in bait areas during the second year of baiting demonstrates that lower prevalence of E. multilocularis egg contamina- tion resulted in a lower infection pressure for intermediate hosts. Nevertheless, at the end of the baiting study, E. mul- tilocularis egg–containing fecal samples and infected intermediate host could still be detected in the 1-km2 and the 6-km2 bait areas. This finding shows that the life cycle of the parasite in the baited areas was not completely inter- rupted. Dispersing and transient foxes can always contam- inate baited areas, even in much larger areas. Furthermore, eggs of this cestode are stable under suitable environmen- tal conditions (39), infected intermediate hosts can stay infectious over several months (40), and baited foxes can become reinfected just after treatment by consuming an infected intermediate host. In addition, the intervention studies in Germany (19,20) demonstrated that E. multiloc- ularis has the potential to recover from a population break- down in >2 years (T. Romig, pers. comm.; K. Tackmann, pers. comm). Therefore, a baiting strategy that focuses on extinction of the parasite in large areas might fail, and per- manent intervention to lower E. multilocularis egg con- 1270 Emerging Infectious Diseases • Vol. 9, No. 10, October 2003