An interactive map to assess the potential spread of Lymnaea truncatula and the free-living stages of Fasciola hepatica in Switzerland

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Abstract

The intermediate host of Fasciola hepatica is Lymnaea truncatula in Switzerland. The snail and the free-living stages of the parasite require a moderate climate and moisture for survival, reproduction, and transmission. In Switzerland, these conditions are present in many regions, resulting in a mean prevalence of bovine fasciolosis from 8.4 to 21.4%. An interactive map was created in order to demonstrate the relative risk of transmission by modelling the environmental conditions that promote the survival and reproduction of the larval stages of the parasite and the parasite's intermediate host. The map is based on temperature and rainfall data, soil conditions including ground water and forest cover in Switzerland. Extensive information on the free-living stages of F. hepatica and the intermediate host L. truncatula and how the development of these are affected by these environmental factors was used to create the interactive risk map.

Introduction

Fasciolosis, caused by Fasciola hepatica, is a serious parasitic disease in Switzerland. Transmission depends on susceptible definitive hosts (e.g. sheep and cattle) and appropriate habitats for development of both the parasite larvae and the intermediate host Lymnaea truncatula. This snail requires a moderate climate and moisture for survival and reproduction (Thomas, 1883). In Switzerland, these conditions are present in many regions, resulting in a mean prevalence from 8.4 to 21.4% in cattle (Eckert et al., 1975, Ducommun and Pfister, 1991, Schweizer et al., 2003, Rapsch, 2005, Rapsch et al., 2006).

As a result of the widespread distribution of bovine fasciolosis, significant economic losses occur. This is due to confiscated livers, reduced milk yield, reduced fertility and reduced meat production. Median financial loss due to bovine fasciolosis in Switzerland is approximately €52 million (95% CI €22–92 million) per annum, which represents a median loss of €299 per infected animal (Schweizer et al., 2005). Suitable control strategies, such as pasture management strategies (Boray, 1971, Boray, 1972), could help to avoid some of these losses. Geographical information systems such as risk maps could help identify areas where disease monitoring should be established. Since F. hepatica transmission is linked to its intermediate host L. truncatula, information on suitable environmental conditions can help locate possible areas with enhanced infection risk by means of cartography.

The aim of this study was to create a map based on multimedia cartography illustrating regions with good environmental conditions for the development of L. truncatula and the free-living stages of F. hepatica as basis for the implementation of control strategies.

Section snippets

Risk model

The occurrence of L. truncatula and the transmission of F. hepatica mainly depend on temperature, moisture, soil conditions, and solar irradiation (Thomas, 1883, Kendall and McCullough, 1951, Ollerenshaw, 1959, Ross, 1970a, Armour, 1975, Christensen et al., 1976, Christensen et al., 1978, Petzold, 1989). Therefore, a risk model was developed based on temperature, rainfall, soil condition and forest cover as these data were readily available. The model's output is an environmental relative risk

Risk model

The risk density distributions for the factor temperature for eggs of F. hepatica, for L. truncatula and for metacercaria as well as the overall temperature risk curve are displayed in Fig. 3.

The x-axis displays the temperature in °C and the y-axis shows the corresponding density curve which when integrated over the whole temperature spectrum equals one. Hence the whole risk based on the factor temperature is set to one and each density of a temperature range refers relatively to the whole

Discussion

Maps predicting fasciolosis risk have been created for several endemic areas. In Cambodia Tum et al. (2004) created a map based on inundation, proximity to rivers, land use, slope, elevation, and the density of cattle and buffaloes. Other studies in east Africa were based on moisture and temperature (Malone et al., 1998, Yilma and Malone, 1998).

However, in none of these studies, temperature setting of the free-living parasite stages and of the intermediate host have been modelled this detailed

Acknowledgements

Temperature data were provided by the Federal Office of Meteorology and Climatology MeteoSwiss.

We thank the Atlas of Switzerland for providing the data on rainfall, soil condition and forest stand.

We thank Novartis for the financial support.

We thank Dipl. Geogr. Andreas Neumann and Juliane Cron for their assistance in creating the interactive maps.

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