RESEARCH Anthelmintic Baiting of Foxes against Urban Contamination with Echinococcus multilocularis Daniel Hegglin,* Paul I. Ward,* and Peter Deplazes* In recent years, increases in the urban fox population have been observed in many countries of the Northern Hemisphere. As a result, Echinococcus multilocularis has entered the urban environment. Because of a possible increased risk for alveolar echinococcosis, intervention strategies need to be evaluated. In Zürich, Switzerland, 50 praziquantel-containing baits per km2 were distributed monthly in six 1-km2 bait areas and one 6-km2 bait area from April 2000 through October 2001. The proportion of E. multilocularis coproantigen–positive fox fecal samples col- lected remained unchanged in six control areas but decreased significantly in the 1-km2 bait areas (from 38.6% to 5.5%) and in the 6-km2 bait area (from 66.7% to 1.8%). E. multilocularis prevalence in the intermediate host Arvicola terrestris also decreased significantly in baited areas. This controlled baiting study shows that a pro- nounced reduction of E. multilocularis egg contamination is feasible in urban areas where the organism is highly endemic. Ttypically perpetuated in a wild life cycle, which he zoonotic tapeworm Echinococcus multilocularis is includes foxes (genera Vulpes and Alopex) as definitive hosts and various rodent species as intermediate hosts (1). In addition, eggs are accidentally ingested by humans; the metacestodes enter mainly the liver and cause alveolar echinococcosis, a severe, sometimes fatal disease if left untreated (2,3). Few studies have been performed on the epidemiology of alveolar echinococcosis. Risk factors for alveolar echinococcosis may include occupational and behavioral activities. However, hunters, trappers, and persons work- ing with fur were not at increased risk for alveolar echinococcosis in South Dakota (4). Data from Europe have indicated that farming activities increase the risk for infection (5,6). Contamination of the rural environment with E. multilocularis connected with farming activities was indirectly demonstrated by high prevalences of alveo- *University of Zürich, Zürich, Switzerland lar echinococcosis in sows kept indoors but fed with grass (7). Areas with high water vole (Arvicola terrestris) densi- ties yielded a 10-fold higher risk for human alveolar echinococcosis compared with areas with low densities of this important intermediate host (8). In an area where the organism is highly endemic, up to 39% of A. terrestris and up to 7% of dogs with free access to rodents were infected with E. multilocularis (9), and persons who have kept dogs around dwellings were at higher risk for alveolar echinococcosis on St. Lawrence Island, Alaska (10). Red foxes (Vulpes vulpes) are likely to be the most important final host in many regions (11). In the past 2 decades, foxes have started to colonize in cities around the world (12–14), and evidence of the parasite cycle in urban areas is increasing (13,15,16). In Zürich, Switzerland, one study found that 47% of the urban fox population was infected with E. multilocularis (17). The high number of infected foxes in cities and vil- lages, in close contact with domestic pets and humans, could increase the risk of alveolar echinococcosis (16). The disease has an incubation period of 5 to 15 years; therefore, whether the actual incidence rate of alveolar echinococcosis reflects a continuing stable and low infec- tion risk or whether the increased infection pressure in highly populated areas will lead to a delayed increase in the incidence of alveolar echinococcosis cases in the future is unclear (3). However, ecologic changes resulted in a very high alveolar echinococcosis prevalence of 4.0% in China, which is highly endemic for the organism (18). The high prevalence of E. multilocularis in densely populated areas and the increase of foxes living in close vicinity to humans strongly suggest that evaluating possible interven- tion strategies is prudent. Few field studies focus on anthelmintic treatment of definitive hosts. Rausch et al. (10) demonstrated in a vil- lage that was hyperendemic for the organism (St. Lawrence Island, Alaska) that continual treatment of dogs with praziquantel reduces infection pressure of E. multi- locularis, resulting in lower prevalence in locally trapped 1266 Emerging Infectious Diseases • Vol. 9, No. 10, October 2003