Studies on sylvatic echinococcosis. V. Factors influencing prevalence of Echinococcus multilocularis Leuckart 1863, in red foxes from North Dakota, 1965--1972.

A total of 1,153 red foxes, Vulpes vulpes, was examined for strobilae of Echinococcus multilocularis from 2 geographic regions in North Dakota during 1965 through 1972. Echinococcus multilocularis was found in 184 (16%) of these foxes. The data including date and location of collection, age and sex of the host, and presence or absence of the cestode were used to assess the relative influence of collection interval (sampling period), climatic season, geographic location, and age and sex of the host on the prevalence of the adult cestode. The rate of infection varied significantly with collection interval, which was probably attributable to annual changes in the overall environment. Seasonally, the highest prevalences were observed during summer (32.4%) followed by spring (25.3%), autumn (13.7%) and winter (6.4%). Differences in prevalence between adult and juvenile foxes and that between male and female foxes were not significant. A 3-factor analysis of variance showed the order of importance of the significant variables to be collection interval, season, and the interaction of collection interval and season. A 4-factor ANOVA, which in addition to the above variables included geographic location, showed that the collection interval-location interaction had significant effects on prevalence. Also, evidence is presented that suggests that density of the definitive host may in part be responsible for annual fluctuation in prevalence.