Surveillance of alveolar echinococcosis and control of Echinococcus multilocularis in Switzerland (2000-2017)

In the 1990s, Swiss fox population densities strongly increased and foxes started to colonise urban areas. This phenomenon can be explained by multi- faceted human–wildlife interactions: vaccination against rabies (main cause of fox mortality), elimination of top predators, and changing attitude towards wildlife (feeding, taming) contributed to the development of high fox densities in and around cities (Hegglin et al. 2015). The IPZ investigated parasitological and epidemiological consequences of this development in the framework of a long-term interdisciplinary project (Integrated Fox Project IFP) and demonstrated that E. multilocularis is present in urban habitats and highly prevalent in urban peripheries. In these transition zones, high urban fox densities, which are sustain by rich anthropogenic food resources, overlap with the habitat of the most susceptible intermediate hosts Arvicola scherman and Microtus arvalis that live on meadows and pastures (reviewed Deplazes et al. 2004, Trends Parasitol., Hegglin et al. 2007, Func. Ecology).

Climatic conditions that adversely affect the survival of E. multilocularis eggs and the lack of suitable intermediate hosts are thought to be the primary causes limiting the parasite distribution. In the Canton Grisons, local prevalences in foxes varied significantly between 0 and 40% and correlated with the predation rate on voles by foxes. Our results suggest that the life cycle of E. multilocularis in the Alps is confined to small scale hot spots which can persist for decades. This has been shown in the Canton Ticino, where the parasite has been regularly confirmed in the North of the Canton and did not spread further south during more the 20 years. The endemic area corresponded well with the distribution of the vole Microtus arvalis but not with the distributions of five other vole species (Guerra et al., 2015, Parasitology).

A comparative study in a high endemic region in Zurich confirmed the key role of this intermediate host. Among four frequent and widespread vole species M. arvalis was the species with the highest prevalence and the highest portion of fertile infections (Beerli et al. 2017, Front. Vet. Science). Prevalence and fertility in the water vole A. scherman were lower, but this species was predated with a similar high frequency by foxes as M. arvalis. We therefore postulate that models on the distribution and abundance of M. arvalis and A. scherman, could allow predicting parasite occurrence on a more detailed spatial scale than distribution models for foxes which have a very broad and much more uniform distribution than vole species.

A retrospective study on the incidence of human alveolar echinococcosis (AE) in Switzerland demonstrated an2.6-fold increase within 20 years in the wake of the general increase and urbanisation of foxes (Schweiger et al., 2007, EID). On the other hand, survival analysis revealed a strongly improved prognosis for AE patients over the last 30 years: Mean life expectancy of patients was reduced by around 20 years in 1970ies and roughly three years at the beginning of 2000s (Torgerson et al., 2008, J. Hepatol.). The better survival and the higher incidence caused a steady increase of the prevalence causing an annual burden of disease of approximately 77.6 DALYs and yearly costs per patient approximately €108,762 per patient.

In experimental field studies, the feasibility of E. multilocularis control was tested in urban peripheries. The monthly delivery of Praziquantel-containing baits reduced the parasite egg contamination of defined small urban patches to a very low level (Hegglin and Deplazes, 2008, EID). However, eradication of the parasite is unlikely and long-term baiting campaigns are actually the most effective tool to significantly lower the infection pressure with parasite eggs. Regarding the long latency of 5-15 years of AE, such measures can only be cost effective if they are pursued for several decades and concentrate on restricted areas which are most relevant for the transmission of AE such as highly endemic areas in densely populated zones (Hegglin and Deplazes, 2013, Int. J. Parasitol.).

Considering the high reproduction of E. multilocularis in domestic dogs which live in close contact to humans, a monthly deworming scheme for domestic dogs with access to rodents is likely to be of high importance. This holds true if only low prevalences in domestic dogs are recorded, as high densities of these pets can easily outweigh low infections rates. Thus, in Central Europe their estimated contribution to environmental contamination with E. multilocularis eggs ranges between 4% and 19% (Hegglin and Deplazes, 2013).

Institute members: Daniel Hegglin, Maria Teresa Armua-Fernandez, Diogo Ribeiro Almeida Guerra, Olivia Beerli, Peter Deplazes (Project Leader)

Funding sources: Foundation, Federal Veterinary Office, Grün Stadt Zürich, Stiftung Echinococcose; EU-Project EMIDA (Emerging and Major Infectious Diseases of Livestock)

In collaboration with: PD Dr. Beat Müllhaupt, Klinik für Gastroenterologie und Hepatologie, Universitätsspital Zürich, Switzerland; Dr. S. Gloor and Dr. F. Bontadina, SWILD, Urban Ecology and Wildlife Research, Zurich, Switzerland; Prof. Paul R. Torgerson, Veterinärepidemiologie, VetsuisseFaculty, University of Zürich, Switzerland.