Forests, cities and gardens, ticks are now everywhere. For several decades, certain ticks, mainly forest ticks, have been expanding in France, spreading with them a procession of more or less serious diseases. These mites thus become a real public health problem.
Lyme disease has long been the only health risk mentioned in relation to the bites of these animals. But health authorities are reporting the emergence of other pathologies, the symptoms and number of cases of which are increasingly worrying in France and other European countries. At the beginning of July, the French public health agency published an initial assessment of one of these new pathologies: tick-borne encephalitis. This disease caused by a virus mainly transmitted by the Ixodes ricinus tick can affect the cerebral system with sometimes lifelong sequelae.
Cases are still rare, but the increase in their incidence in homes far from areas already known to circulate the virus, mainly the Grand Est, has drawn the attention of the authorities. Another great first, it is an infection transmitted both by ticks but also by food from infected animals. “Encephalitis transmitted by ticks to animals can then pass to humans through the consumption of products made from contaminated raw milk, such as certain goat cheeses”, indicates Professor Hansmann, head of the infectious and tropical diseases department and internal medicine at the University Hospital of Strasbourg.
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Anaplasmosis, which causes fevers and muscle aches, babesiosis, whose symptoms resemble those of malaria, or Crimean-Congo hemorrhagic fever are other diseases transmissible to humans about which little is known. incidence due to their rarity, but which could gradually settle in the territory. Crimean-Congo fever is not yet present in France, but its vector, the Hyalomma tick, circulates around the Mediterranean. “It is suspected that migratory birds are bringing specimens back to France, which are currently safe because the seasons are not conducive to their development, but this could change in the years to come with global warming”, explains Nathalie Boulanger, entomologist specializing in infectious diseases and emerging diseases and professor of parasitology at the University of Strasbourg.
Of the 900 species of ticks existing in the world, two of them predominate in France. The most widespread is the “sheep tick” (Ixodes ricinus), followed closely by the tick of the genus Dermacentor. Difficult to distinguish with the naked eye, or even to feel on the skin, these mites feed on the blood of various mammals, birds and reptiles. It is this particular way of feeding that makes them very effective vectors of disease. Adult female ticks can ingest up to 100 times their weight in blood over a period of about 10 days.
“The injection of anesthetic substances contained in the saliva of ticks is a decisive factor, because, by this means, any bite is painless, which allows them to go unnoticed”, explains Professor Yves Hansmann. Fortunately, not all ticks carry pathogens and their bites are mostly harmless. “Only 10% to 15% of Lyme disease ticks are actually infected. In addition, we know that the transmission of the pathogenic bacteria by the tick generally takes place 24 hours after the start of the meal, so if we manage to remove the animal early enough, there is no risk of transmission of the illness,” says Professor Hansmann.
Urbanization and landscape modification have helped create favorable conditions for ticks, and have expanded their habitat areas. Changes in logging practices in particular have had a significant impact. Until the 1970s, every part of the cut tree found a use. Today, a considerable amount of wood is abandoned on the ground, constituting new moist refuges for ticks, but also for their hosts.
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The prohibition of certain insecticides, such as DTT, or herbicides used for land clearing in the 1970s and 1980s also favored ticks. “Even if their ban is justified, this has indirectly contributed to the reappearance of ticks in the places where these products were once used”, underlines Nathalie Boulanger. Another favorable factor: the artificial feeding of game or “grazing”, which consists of depositing food at a distance from the fields in order to keep wild boars away. “The problem with these practices, which are in principle highly regulated, is that they bring together hosts of interest, including rodents, birds, deer, wild boars, in forest areas where ticks are also found”, explains the ‘entomologist. Added to this is a change in hunting practices, resulting in a significant increase in deer and other large wild ungulates, which are very favorable to tick populations.
On a much larger scale, the impacts of climate change are also held to be responsible for an increase in the presence of ticks, due to warmer temperatures and longer seasons. Generally, they are present from March to June, with a significant peak of activity in May. However, they could come to persist during milder winters or even colonize environments located at higher altitudes which are experiencing significant changes in vegetation.
