A comprehensive study by EPFL and the University Hospital of Lausanne (CHUV) has enabled researchers to map the geographic distribution of ticks in Switzerland for the first time, as well as to determine whether they are carriers of chlamydia. Little is known about these bacteria, but fleas have the potential to transmit them to humans. The team found that the zone conducive to tick proliferation had grown by 10% over the past decade.
Pedestrians departing on one of the many walking trails in Switzerland often bring back beautiful photos, the occasional cramp, and – accidentally – fleas. These tiny akarids, which are in dense bush and on the edges of forests, are very active in hot weather and cling to human and animal hosts passing nearby. Despite their small size, they can transmit potentially serious diseases such as Lyme disease and tick-borne encephalitis. Recent research by the Institute of Microbiology at CHUV has shown that fleas are often carriers of large amounts of chlamydia, a still poorly understood bacteria that can be transmitted to humans and cause secondary disease.
Scientists recognize that tick breeding and activity are influenced by a variety of environmental factors, including temperature and humidity. However, data on its regional distribution over time in Switzerland are lacking, which has been classified as a risk area. At EPFL’s Geographical Information Systems (LASIG) Laboratory, Estelle Rochat’s thesis project aims to address this void and identify areas where ticks carry chlamydia. His extensive mapping work has been published at prestigious events Applied and Environmental Microbiology.
Three data sources
After identifying the environmental conditions conducive to the presence of castor bean infestation (Ixodes ricinus), the most common species of tick in Switzerland, Rochat created a map of their geographic distribution between 2008 and 2018.He drew on three databases: a 2009 field campaign carried out by the Swiss Army , where more than 60,000 ticks were collected and analyzed; thousands of entries to smartphone application which allows the user to indicate where they are observing ticks; and a Rochat-led flea collection project in 2018.
He then used machine learning (a form of artificial intelligence) to predict the possible presence of ticks and bacteria based on values taken from a set of environmental data (rainfall, temperature, humidity, etc.) around the collection point.
The machine learning program allowed Rochat to estimate the location of the ticks, and revealed that the surface area of the zone favorable for tick propagation grew from 16% of mainland Switzerland in 2008 to 25% in 2018.
Statistically, the model works well. Using the rule of probability, we can find out whether a particular location is tick friendly, or if, conversely, ticks are unlikely. Using a distribution model, we estimated the prevalence of bacteria within these sites. This in turn allowed us to identify subzones where favorable areas for ticks also support chlamydia.
“A landmark project”
The scope, originality and novel approach of Rochat’s research won praise from Gilbert Greub, a world-renowned chlamydia and lice expert and director of the CHUV Institute of Microbiology. “This is an important project, and contains enough detail at the national level to allow us to draw conclusions. We can clearly see that between 2008 and 2018, there was an increase in the area of high risk tick exposure, which in my opinion is a reflection of global warming. This indicates that the ticks have migrated 300-400 meters higher in the subalpine zone. “
For Greub, the study is a valuable tool for preventive purposes, as well as for awareness raising. In addition, it will be useful at the Institute, which conducts clinical studies of the impact of tick-borne chlamydia on humans.
The Rochat model is now available in open access and could be used in future research on tick-borne pathogens. “It’s interesting to see how the ecological niches overlap. We use chlamydia in this case because we are working with Gilbert Greub, a global expert, but our approach can also be applied to tick-borne encephalitis and Lyme disease. The algorithms needed to process environmental data are available free of charge and can be applied to other data sets, ”said Stéphane Joost, who supervised Rochat’s thesis at LASIG.
Joost sees an opportunity for the Swiss Federal Public Health Office to refine its risk map for head lice – which will be increasingly present in Switzerland as a result of global warming.
More fleas? Keep calm.