• Tick ​​populations are on the rise in Europe, according to our partner The Conversation.

  • Since ticks mainly live in the woods, all the practices that lead to an increase in forest cover encourage their proliferation.

    But these have changed somewhat in recent years ...

  • The analysis of this phenomenon was carried out by Nathalie Boulanger, teacher-researcher in parasitology at the University of Strasbourg (UR7290).

For more than fifty years, there has been an increase in tick populations in Europe, which has been accompanied by a greater spread of the diseases they carry.

Climate change, human activities, ecosystem modifications… Several factors have been put forward to explain the reasons for this situation.

More precisely, what are the causes and what is the role of the human being in this phenomenon?

Mainly forest ticks

In Europe, the ticks most affected by this population increase are

Ixodes ricinus

and, to a lesser extent, the tick

Dermacentor reticulatus

.

Ixodes ricinus

is the most abundant tick in the northern hemisphere. It feeds exclusively on blood from a wide variety of hosts: small and large mammals, birds and even reptiles ... More than 300 animal species are susceptible to bites! The human being, on the other hand, is an accidental host. After hatching, the development of this tick follows several stages (called “stasis”): larva, nymph, then adult. Small nymphs are particularly involved in the transmission of bacteria (such as that responsible for Lyme disease), viruses, and even parasites to humans.

Ixodes ricinus

needs a high humidity and a temperature of at least 5 ° C to be active.

It grows in vegetation, mainly in forests (although its presence in cities is increasingly reported, often in heavily vegetated areas).

She lives in humus and leaf litter, spending her time climbing on the grasses to hunt on the lookout and then descending to the ground to rehydrate.

In winter, she goes into diapause, ceasing all activity.

The Ixodes ricinus tick is the most frequent in our environment (female: orange color-black patch) but Dermacentor reticulatus is also widespread (female: marbled patch) © Nathalie Boulanger (via The Conversation)

As this tick mainly evolves in the woods, all the practices that lead to an increase in forest cover encourage its proliferation.

However, these have changed somewhat in recent years.

Evolution of logging practices

In the past, after logging, all parts of the trees were harvested: logs were used for furniture, the crown provided firewood or material for the production of wood chips or pulp. However, at present, a significant amount of wood is left on the ground. Rodents and birds can take shelter and nest there, while ticks find suitable protection (and enough to feed on…). Critical from an ecological point of view, ecobuage (clearing by burning) and clear cutting, which made the environment not very conducive to ticks, have been largely abandoned.

The presence of bark beetles on certain tree species led to the use of insecticides by dusting affected trees in the 1950s (and more recently by spraying pyrethrins), which probably also indirectly impacted ticks in the forest environment.

Changes in forestry practices lead to an accumulation of dead wood © Nathalie Boulanger (via The Conversation)

Not elsewhere, in our country, the proliferation of ticks could be favored by the modification of the landscapes: the forest today occupies 31% of the territory (67% of the stands are made up of deciduous trees).

However, the development of road infrastructure, intensive agriculture (new dimensions of farms) and urbanization have led to a fragmentation of the landscape.

In some regions, the forest stands are now isolated plots, in which proliferates a fauna favorable to tick populations (and associated pathogens), such as rodents and deer.

Role of wildlife

Ticks are strictly blood-sucking: they feed on blood only.

Lots of blood: female ticks need 100 times their blood weight!

To obtain it, they must have a varied fauna available.

Their privileged hosts are deer, especially deer, and suidae such as wild boar.

However, over the past 40 years, the populations of these two species have increased overall by a factor of 10, even if there are regional disparities.

Result: deer and wild boar are more and more often found in urban and peri-urban areas, where they feed in gardens.

Artisanal agrainoir for wild boars (May 2018) © Sebleouf / Wikimedia CC BY-SA 4.0

If the hunting of deer and suidae can participate in the regulation of their populations, certain practices such as agrainage (artificial feeding of game), on the contrary, promote their proliferation.

Indeed, if the deterrent agrainage aims to keep swine in the heart of forests, grazing sites attract not only wild boars, but also a whole fauna carrying ticks: rodents, birds and their predators, badgers, even deer and deer.

By concentrating more animals on a small area, fixed agrainages create spaces that not only promote the development of ticks, but also that of other diseases carried by wildlife.

The role of socio-economic changes

A number of socio-economic transformations may have favored the increase in tick populations in France.

In many regions, rural desertification and the abandonment of plots dedicated to agriculture have favored the planting of trees (or afforestation), most often spruces.

Brownfields have also developed, habitats of choice for small and large mammals, which are the privileged hosts of

Ixodes

ticks

.

Dead leaves, dead wood… Environments that can favor the proliferation of ticks by providing them and their prey with shelter © Analogicus / Pixabay

Added to this is the fact that certain human practices which altered the tick environment and probably kept their populations quietly have also disappeared.

Today we consume less game, we no longer collect the residual crown wood after logging, dead leaves are no longer used for manuring gardens ...

Climate change

The impact of climate change is often mentioned, in particular in the possible decrease in populations of

Ixodes ricinus

ticks

.

This tick indeed needs a high ambient humidity (at least 80% humidity).

If the frequency and intensity of heat waves increase, ticks could disappear from certain areas.

However, this impact on the populations of

Ixodes spp.

is currently limited.

Conversely, climate change could also create favorable conditions for the colonization of other places by ticks, especially at altitude if changes in vegetation occur.

They could also become active all year round in the event of a less marked winter season.

A tick in the middle of a “meal” © francelyme.fr

Many of these phenomena are well documented in the United States.

If the European context is not quite identical, we can expect the same observations on our continent.

Carry out coordinated actions

Given their wide distribution in the environment, it is unrealistic to hope to succeed in eradicating ticks.

As the reasons for the increase in their populations are multifactorial, reducing their size and controlling their spread requires coordinated actions involving many stakeholders: foresters, hunters, farmers, competent authorities, etc.

At the same time, we must also think about the definition of a biodiversity favorable to the cohabitation of human beings and ticks.

In a few years, we have gone from one extreme to the other when it comes to “tick environment”.

We must now ask ourselves how to properly revegetate our environment without making it conducive to the proliferation of these ectoparasites.

Installing barriers around your garden keeps wildlife at bay, and therefore ticks it can carry © Randy Fath / Unsplash

A few simple measures can already be applied by everyone to achieve this: installation of barriers to prevent suidae and deer from approaching homes, mowing the grasses around them (ticks do not like the drought), elimination of areas of humidity, deworming of domestic animals, etc.

Our file "LYME DISEASE"

The fight against ticks is more and more publicized, but still struggles to be part of a real public health problem in France.

While waiting for the situation to improve, it will therefore be necessary to continue to carry out a meticulous examination of the body after each walk in "tick zones" ...

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This analysis was written by Nathalie Boulanger, teacher-researcher in parasitology and head of the Borrelia group at the University of Strasbourg (UR7290), in collaboration with Catherine Bertaux, president of the Artémi55 hunting association.


The original article was published on The Conversation website.

Declaration of interests

Nathalie Boulanger has received funding from ANR and CNRS.

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