Science

Ticks and Lyme disease: reasons for the increase in incidence

Close-up on a black background with the word apocalypse… On the cover of its latest August 8 issue, the American weekly The Nation is sounding the alarm: Lyme disease, spread by the famous parasitic dust mites, is progressing uncontrollably. The United States alone now accounts for 476,000 new cases… every year. Lyme disease diagnoses increased 357% in rural areas and 65% in cities between 2007 and 2021, according to the nonprofit FAIR Health, which regularly analyzes private health insurance data in the US, with an acceleration in the latter period.

In Europe, the pressure is also growing. “Now we see ticks in places where they did not develop before,” confirms Professor Jeanne Bruger-Picout, veterinarian and member of the National Academy of Medicine. One example among others: twenty or thirty years ago we didn’t see them in God. region, between Annecy and Aix-les-Bains. Today they are part of the landscape.” Doctors know this and treat the resulting lesions, from erythema migrans to the most troublesome Lyme arthritis.

Nathalie Boulanger, a medical entomologist, leads the research group on vector-borne diseases transmitted by ticks at the University of Strasbourg. This confirms the trend. “In the United States, on the east coast, we are seeing a large spread of cases of Lyme disease that are spreading to the Great Lakes and increasingly entering Canadian lands. In France, even if some regions such as Grand Est or Limousin have suffered more than others, in recent years the number of cases has generally increased,” she explains at a recent symposium organized on the premises of the French Academy of Agriculture.

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Is this a “real” increase or a reflection of better informed patients and doctors? Probably both, says the expert. As with Covid, the more tests we run, the more positive patients we find. However, other reasons undoubtedly contribute to the reproduction of ticks, starting with an increase in the animal reservoir. Deer (highly valued by female ticks, which are 100 times their own weight in blood) have increased their population significantly in recent years. As are rodents. “In the Cenard forest in the Paris region, you will find, for example, many squirrels from Korea, former domestic animals infested with ticks,” notes Jeanne Bruger-Picoux.

Great houses for ticks

Thus, by changing his environment, a person bears a share of responsibility in the current situation. He acts without even knowing it, on many levels. Land fragmentation, for example, seems to lead to a concentration of animals (deer, rodents and ticks) in areas of the forest where physical contact is easier to establish. In another register, the just cessation of the spread of some insecticides in our forests has undoubtedly benefited the mites. As well as the evolution of some methods of forestry management. “Less burning, more branches left on the ground after pruning … This makes for great homes,” Jeanne Bruger-Picoux laments.

Even urban greening is sometimes problematic. Because evidence of tick-infected citizens is accumulating near green spaces. “Just like bats and mosquitoes, these animals need to be closely monitored,” warns Jeanne Bruger-Picoux. Especially since they can transmit other diseases besides Lyme disease. This applies, for example, to tick-borne encephalitis. “We are currently seeing an increase in cases in several European countries and in particular in Switzerland reaching Spain, making surveillance necessary in the south of France.”

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“Making Lyme disease a major public health issue is insulting,” Professor Eric Kom recently reminded L’Express. However, the increase in the incidence makes it necessary to adapt: ​​a new order of clearing in forests, a reduction in the population of some animals, research on possible tick-borne predators (mushrooms, nematodes, etc.). There are many options. But they are not yet the subject of a real plan of attack.

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