A map with the potential evolution of fasciola hepatica in switzerland

Swiss Fluke Forcast


Table of Contents

Introduction
The parasite life cycle
Modelling the life cycle
Creating the map
Summary
Bibliography

Bovine Fasciolose (infection of cattles by common liver fluke) is a serious disease. A calculation of the true prevalence by Schweizer et al. (2006) showed that nearly every fifth cattle is infected by Fasciola hepatica. This leads to an immense finacial loss of about 60 Mio. EUR per year. As shown in human medicine we know that maps may help to protect from diseases. For this reason we developed a map with the potential risk of a liver fluke. The map had to be presented printed and on screen, a task that is easy to do with SVG.

The common liver fluke (fasciola hepatica) is a parasite from the phylum of flatworms. The eggs of the parasite can be located in the bile duct and gallbladder of infected animals. They leave the host via intestine and feces. Outside of the host the embryo grows in the egg. By temperatures between 23 and 26 degree Celsius a miracidium releases from the egg. The miracidia search buoyant their intermediate host. In switzerland this host is lymnea truncatula (aka gabla truncatula) - a little freshwater snail.

The miracidium drills in the foot or the mantle cavity of the gastropoda and undergos a methamorphosis. It losts its agility and degenerates to a inactive sporocyst. After a while the sporocysts convert to redia and this redia dissipate to dauther rediae or to cercariae. The cercaria leave the mollusc and swim to a adequat place to become a metacercaria.

Metacercariae are a rather resistent form of cerariae that encysted. The can be found near to water bodies with the intermediate host such as water wells and mud and swampy meadows. The metacercariae are absorbed oral by a potential primary host of the parasite such as cattles, sheeps, deers, hares, rabbits, pigs, boars, goats, horses and humans. After the digestion in the stomach the adult fluke drills trough the intestine to the liver. Six to ten weeks after the absorbtion the primary host starts to excrete new eggs of the liver fluke.

In literatur we found estimations about optimal conditions for the evolution for each part of the parasites life cycle. From this estimations we defined functions that describe the probability of a optimal evolution depending on temperatur, soil, rainfall and wood. Unfortunately the descriptions in literatur were neither well formed nor complete. Therefore we interpolated the probablities between the extreme values.

The whole cycle runs sequentiel and there are no negative reaction couplings. It reaches its optimum if each part of it runs optimal. The probability of how optimal the cycle runs is a multiplication of the probabilities of each part.

For a more detailed reading about the modelling of the parasite cycle we propose Rapsch et al. (2008).

According to the data we were able to calculate the relative hazard probablity with a grid size of 100 m. These information had to be represented as a map. The target was a printed map as well as a interactive map presented in the www.

The preferred grafic format was SVG. This because SVG is a web-standard. It is possible to combine raster and vector elements and the rasters and vectors can be displayed as top-quality graphic.

The map was created and finalized by a veterinarian, which had only few knowledge in programming and webgraphic. The diffent map layers and the Layout were merged in an XML-Editor. The map navigation tools were created with the GUI-Elements of Andreas Neumann available on carto.net. As reported by the map-author it was not a big problem to add and edit all this elements.

The map with the hazard of the evolution of fasciola hepatica is a tool for farmers and veterinarians. It was developed with informations from literature and observations made by the authors. The calculated hazard was compared with direct observations. We found that the model is not contradictory to this observations.

If a farmer doesn't know about the hazard of this parasite and the resulting finacial loss, the maps can give him some information, whether he lives in a area with a high or low risk. If necessary he can fight the parasite with medicaments.

Due to SVG it was possible to create a map that can be printed and presented in the internet with top-quality graphic. Concerning the graphic format, we had no serious difficulties to create the map. The use of SVG was profitable.