Tick-borne diseases spread across Europe, boosted by global warming

Laure BOURNEZ, head of the TIQUoJARDIN project, in front of a microscope with a female tick, in a rabies and wildlife laboratory of the National Agency for Food, Environmental and Occupational Health Safety in Malzéville, June 3, 2022.

From north to south of France, the tick is everywhere. Four species have shared the metropolitan territory for several years, capable of transmitting various bacteria, viruses and other parasites harmful to humans. If Lyme borreliosis, caused by the bacteria Borrelia burgdorferiis the best known of these tick-borne diseases, new threats have been observed under scrutiny by health authorities in recent years, including tick-borne encephalitis and Crimean-Congo fever.

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In the northern hemisphere, ticks are the primary vectors of disease, both in human health and in veterinary health. The number of cases of Lyme borreliosis increases significantly according to Public Health France (SPF), the incidence rate fell from around 40 cases per 100,000 inhabitants in 2009 to 90 cases per 100,000 inhabitants in 2020, with fluctuations depending on the year. Since 2016, there has been an oscillation ranging from 47,000 to 70,000 new cases per year.

Bacterial contamination can be quickly reversed by antibiotic treatment, when the diagnosis is made soon after the bite, after spotting and removing the tick, or on the appearance of erythema migrans – an inflammatory red patch around the bite. But in case of delay in diagnosis, the disease can progress, with first symptoms such as headaches, fatigue and muscle or joint pain, then disseminations in multiple organs. “Lyme disease, anyone can catch it and there is no protective immunity, which means that you can contract it several times in your life”explains Nathalie Boulanger, a medical entomologist who leads a research team on vector-borne diseases transmitted by ticks at the University of Strasbourg.

The species responsible for the transmission of this disease is theIxodes ricinuspresent everywhere in Europe as far as the Arctic Circle, as shown in maps compiled by the European Center for Disease Prevention and Control. This same species transmits about twenty other pathogens, one of which is of particular concern to the authorities: the tick-borne encephalitis virus. Still rare, it has caused 71 cases notified in France since 2021, according to a recent report by SPF.

“A distribution in full evolution”

This disease, with low mortality but significant sequelae, first results in fever and pain, but can subsequently lead to serious infections of the nervous system, such as encephalitis or meningitis. In France, a country less affected than Austria or Germany, the two best-known outbreaks are in Alsace and Haute-Savoie. But in 2020, an outbreak was identified in Ain, linked to the consumption of raw milk cheeses, infected goats having secreted the virus in their milk. “The circulation of the virus is wider than expected”alerts Alexandra Mailles, epidemiologist in the infectious diseases department of SPF, citing cases in certain departments hitherto unidentified at risk such as Ardèche and Cantal. “And the majority of cases were not considered cured after hospitalization”, she adds. Closely monitored, the disease has been notifiable for two years.

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