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Concern is growing in Europe about alveolar echinococcosis (AE) with the increase in grassland rodent and red fox populations, intermediate and definitive hosts for Echinococcus multilocularis,respectively. The objective of this study was to assess the influence of rodent densities on human AE distribution. Spatial Poisson regression analyses were performed with geomorphologic features, landscape composition, climatic characteristics, and water vole density as independent variables. The outcome consisted of AE cases diagnosed over the period 1980-1992. High vole density yielded a 10-fold risk (relative risk (RR ) 5 10.34, 95% confidence interval (CI) 5 2.78-38.39), and the first plateau (400-700 m altitude) compared with the plain (200-400 m) was associated with a large increase in risk (RR 5 7.10, 95% CI 5 1.30-38.63). These results confirm that human AE is strongly influenced by the densities of arvicolid species. Foxes feeding almost exclusively on grassland rodents when the latter expand could mediate this relation. Alveolar echinococcosis (AE) is a very rare and severe zoonosis due to the hepatic development of the larval ces- tode Echinococcus multilocularis. The latter is largely re- stricted to wild animals, involving rodent intermediate hosts, and primarily red foxes as definitive hosts. Humans can be- come exposed after ingesting tapeworm eggs through direct contact with definitive hosts, or by food contaminated by feces of these infected carnivores. 1 The human disease risk appears relatively low within the distribution range of the parasite, but several hotspots have been observed in North America, Euro-Siberia, and Asia. In eastern France, such an area of high endemicity exists. For the period 1971-1989, 85 cases were identified in the Fran- che-Comteregion, corresponding to an annual diagnostic in- cidence rate of 0.4 per 100,000 inhabitants. 1 This estimate is much lower than that observed in St. Laurent Island, Alas- ka (annual incidence rate of 65 per 100,000), or in Gansu Province in China, one of the most highly endemic areas for AE. 2 Nevertheless, concern is growing in this area (just as in all of Europe), especially with the increase in grassland rodent and red fox populations. Little is known about the transmission patterns and pro- cesses involved in this zoonotic disease, which is cycled syl- vatically rather than synantropically. Several characteristics of the E. multilocularis life cycle represent many obstacles to transmission studies. This parasite affects a low propor- tion ( , 1%) and is overdispersed between rodent interme- diate host species. More than 40 species of rodents can serve as intermediate hosts, among which 6 have been found to be infected in France: common vole (Microtus arvalis), earth vole (Microtus subterraneus), red-backed vole (Clethriono- mys glareolus), fossorial water vole (Arvicola terrestris scherman), muskrat (Ondatra zibethicus), and house mouse (Mus musculus). 1 Moreover, population structure of wild in- termediate and definitive hosts is highly variable both in time and space, and is hardly accessible to investigations. Finally, the human latency period, corresponding to the development of the larval stage, is long (5-20 years), impeding follow- up studies if migration occurs. As a result, one cannot ex- trapolate findings dealing with short time span or small spa- tial scales for animal hosts, or easily identify individual oc- cupational or recreational risk factors for humans. Therefore, transmission patterns should be investigated on the basis of ecologic exposure variables relevant to large scales and wide time spans. Such an approach has been recently carried out in the light of an ecologic theory linking landscape composition and in- terannual variations of small mammals densities, namely, the Ratio of Optimal to Marginal Patch Area (ROMPA) hypoth- esis. According to this hypothesis, population dynamics of small mammals would depend on the ratio of their optimal habitat in a landscape (e.g., grassland for A. terrestris). At a low ratio, the landscape matrix (ploughed fields, forest, permanent grassland, uncultivated land plus areas without forests) serves as a large dispersal sink in which young an- imals cannot disperse successfully and the population den- sities remain constantly low in the optimal habitats. At a high ROMPA, production of young cannot be compensated by unsuccessful dispersion and rodent density reaches a very high level. 3-6 Landscape composition would therefore drive both the population dynamics of arvicolid species and the host-parasite interaction (foxes feed almost exclusively on grassland arvicolid species, with their diet becoming more diversified only during decreases in rodent populations). 7,8

WATER VOLE (ARVICOLA TERRESTRIS SCHERMAN) DENSITY AS RISK FACTOR FOR HUMAN ALVEOLAR ECHINOCOCCOSIS