Human Alveolar Echinococcosis after Fox Population Increase, Switzerland

Abstract

We analyzed databases spanning 50 years, which included retrospective alveolar echinococcosis (AE) casefi nding studies and databases of the 3 major centers for treatment of AE in Switzerland.A total of 494 cases were recorded.Annual incidence of AE per 100,000 population increased from 0.12-0.15during 1956-1992 and a mean of 0.10 during 1993-2000 to a mean of 0.26 during 2001-2005.Because the clinical stage of the disease did not change between observation periods, this increase cannot be explained by improved diagnosis.Swiss hunting statistics suggested that the fox population increased 4-fold from 1980 through 1995 and has persisted at these higher levels.Because the period between infection and development of clinical disease is long, the increase in the fox population and high Echinococcus multilocularis prevalence rates in foxes in rural and urban areas may have resulted in an emerging epidemic of AE 10-15 years later. H uman alveolar echinococcosis (AE), a hepatic disor-der that resembles liver cancer, is a highly aggressive and lethal zoonotic infection caused by the larval stage of the fox tapeworm, Echinococcus multilocularis (1).The parasite is widely distributed in the Northern Hemisphere; the disease-endemic area stretches from North America through Europe to central and east Asia, including northern parts of Japan (1-3).Parasite eggs are shed into the environment in the feces of canid defi nitive hosts that harbor the adult parasite in their intestines.In addition, some of these eggs contaminate the fur of infected defi nitive hosts.Humans and intermediate host animals acquire the infection by ingesting E. multilocularis eggs in contaminated food or water or by having close physical contact with infected fox-es, dogs, or host feces.In Europe, E. multilocularis exists predominantly in a cycle of wild animals that includes red foxes (Vulpes vulpes) as main defi nitive hosts and several vole species as intermediate hosts.In the United States and Canada, the coyote (Canis latrans) has also been shown to be a suitable host for this parasite; in Arctic regions, the artic fox (Alopex lagopus) is the principal defi nitive host (1).Domestic dogs are also highly susceptible defi nitive hosts (4); in some areas, such as Alaska (5), People's Republic of China (6), and Europe (7), they can play an additional or even the dominant role as an infection source for humans.The documented area of E. multilocularis endemicity in Europe has recently increased (3,7,8).However, whether this increase results from a true extension of the geographic range or simply increased detection in populations of wildlife not previously investigated is still unclear (1).Similarly, new distribution ranges have been reported in North America.In addition to being found in the tundra zone of Alaska and Canada, the parasite has now been recorded in 3 Canadian provinces and an additional 11 contiguous US states (9).Important changes have occurred in the population dynamics of foxes in central Europe.Between 1970 and the mid-1980s, fox populations decreased during an epidemic of rabies.After the successful establishment of anti-rabies vaccination programs, fox populations increased almost 4-fold (10).At the same time, fox habitat extended into urban areas and is still progressing.For example, large fox populations have now become established in all major cities and towns in Switzerland.In the largest city, Zurich, the number of foxes shot or found dead within the city 878