2023-10-12 17:30:27
Tick-borne encephalitis is a potentially serious viral infection that occurs in temperate zones of Eurasia, in a geographical band extending broadly from the south-east of Great Britain to Japan, including part of Scandinavia, the Russia and a large portion of China.
As its name suggests, the virus responsible for this disease is transmitted by ticks. It belongs to the Flaviviruses, a viral genus which also includes the dengue and Zika viruses (which are transmitted by the tiger mosquito).
Tick-borne encephalitis is not a recent pathology, since it was formally described (and the virus isolated) in 1937, during an expedition to the Russian Far East. However, it is increasing throughout Europe, including in France.
Although the number of cases of annual infections in our country currently remains much lower than the number of cases of infection with the bacteria responsible for Lyme disease, health authorities are nevertheless taking the situation seriously. Here’s what you need to know.
Contaminations on the rise throughout Europe
Since the tick-borne encephalitis virus (TBEV) was isolated at the beginning of the 20the century, three major subtypes have been described. These are, in descending order, the severity of the symptoms of the Far Eastern virus, the Siberian virus and the European virus.
In Europe, the TBE virus is spread by ticks belonging to the species Ixodes ricinus. In the Far East (as well as in Central Europe), the vector of the disease is another species, Ixodes moved. It should be noted, however, that this distribution is no longer as strict as before. However, the tick Ixodes moved has also been detected in Europe, in the Baltic countries, where it overlaps with I. ricinus. Migratory birds might be responsible for this importation.
These are the same species of ticks that also transmit the bacteria responsible for Lyme disease (or borreliosis), Borrelia burgdorferi sensu lato. It is therefore interesting to draw the parallel between these two diseases transmitted by ticks.
A disease much rarer than Lyme disease, but which progresses
First observation: the epidemiology of tick-borne encephalitis and that of Lyme disease are slightly different. Ticks infected with TBE virus generally constitute microfoci where small rodents and insectivores play an important role, while ticks infected with the bacteria responsible for Lyme disease are uniformly distributed in the environment.
Second observation: the infection rates of ticks are also very different: 10 to 20% for the bacteria Borreliacompared to less than 1% for the TBE virus.
Finally, the number of contaminated people also differs very greatly, since it is estimated that 50,000 to 60,000 cases of Lyme borreliosis occur each year (according to the Sentinel network), while until 2016, less than ten cases per year were recorded for tick-borne encephalitis.
Nathalie Boulanger/DR, Provided by the author
But things are changing and cases are increasing across Europe. In France, 30 cases were detected in 2021, and 36 in 2022, according to the latest study by Public Health France. Part of Alsace, a region where the first case was diagnosed in 1968 and which was for several years the only endemic area, the disease is now spreading to Auvergne-Rhône-Alpes and Franche-Comté.
Read more: Beyond Lyme: Other Tick-Borne Diseases
Faced with this increase, the health authorities decided in 2021 to classify tick-borne encephalitis among the notifiable diseases.
How serious is it for human beings?
Infection with the TBE virus can lead in certain patients to damage to the central nervous system with meningitis (inflammation of the meninges, the membranes that surround the brain) and meningoencephalitis.
The European virus is the least dangerous of the three viruses, since the fatal risk associated with it is around 1%, compared to a mortality rate of around 30% for the Far Eastern virus.
Treatment involves treating the symptoms. For people particularly at risk of contracting the disease, such as foresters, there is an effective vaccination, which protects once morest all three viral subtypes.
How is tick-borne encephalitis transmitted?
Transmission of the tick-borne encephalitis virus can occur not only through tick bites, but also through consumption of dairy products made with raw milk from infected animals.
Remember that ticks Ixodes live on vegetation, in dead leaves and in humus, especially in forest areas. They evolve in three stages from the egg: the larva, the nymph and the male and female adults. If all stases are involved in the transmission of the virus, the nymph plays a particularly important role, because it is present in the environment in a uniform manner. In addition, its small size makes it easy to go unnoticed.
One point is important to emphasize: in the case of tick-borne encephalitis, transmission of the virus occurs immediately, from the start of the bite, because it is present in the salivary glands of the tick. This differs from the transmission of bacteria responsible for Lyme disease, which must mature in the tick’s intestine before transmission in saliva (which requires a delay of approximately 24 hours, and implies that removing the tick before this limit generally prevents you from getting Lyme disease).
Read more: Ticks are back: what works to prevent bites
The TBE virus transmission cycle involves three main players: the virus, the tick (vector of infection) and a natural host. The natural hosts are generally small forest mammals (field mice Apodemus spp. or the voles Myodes spp.), but birds might also constitute reservoirs. Cervids and more particularly deer (Capreolus capreolus), for their part, actively participate in maintaining tick populations.
But the reservoir of the virus is firstly constituted by the tick, which transmits the virus from stasis to stasis (transtadial transmission), as well as from the female to the eggs (transovarial transmission). This allows the TBE virus to survive well in the environment. Ticks can also become infected by “co-feeding” when they are grouped together on an animal (which is often the case on wild animals, deer for example).
If one of them is infected, it will transmit the virus during the bite to those who feed around it. In fact, the tick carrying the virus inoculates the virus with its saliva into the skin of the host. Other ticks nearby then suck up the blood and the inoculated virus.
Nathalie Boulanger/DR, Provided by the author
It should be noted that recent studies in Germany and Europe confirm the role of another tick in the circulation of the virus in the environment: the tick of the genus Dermacentor. Like the tick Ixodes, its population is expanding following climatic, environmental and socio-economic changes. In the case of Dermacentorunlike Ixodesonly adults bite (they mainly attach to the scalp).
The cohabitation of these two species of ticks in the same ecosystems (we speak of “sympathy”) explains the particular circulation of the tick-borne encephalitis virus in Ixodes et Dermacentor.
How is the increase in the number of cases explained?
The overall increase in cases of tick-borne encephalitis in Europe in recent years can be explained not only by environmental changes, but also by socio-economic changes.
The Baltic countries, Slovenia or the Czechia for example, are particularly affected by the infection. The increase in cases in these regions occurred following the fall of the Soviet bloc, and its origin is multifactorial. In particular, to compensate for the loss of purchasing power linked to the economic crisis which followed the collapse, certain populations would have visited the forests more to collect wild berries and mushrooms. Added to this is a drop in vaccination coverage and the use of pesticides.
Currently, the increase in cases of tick-borne encephalitis is also explained by the geographic expansion of the tick. Ixodes, which is, once more, multifactorial. It results not only from the modification of its forest ecosystem, now more vegetated and more fragmented, but also from the increase in certain wild ungulates such as deer.
What prevention?
For this tick-borne disease, the best prevention is vaccination. A vaccine, developed in 1976, is available and has shown its effectiveness. Austria, a country particularly affected by the tick-borne encephalitis virus due not only to tick bites, but also to the consumption of contaminated raw milk cheese, carried out massive vaccination of its population, which made it possible to significantly reduce the incidence of this disease.
Vaccination is all the more important because, as mentioned previously, transmission of the tick-borne encephalitis virus occurs instantly, at the time of the bite.
Concerning the transmission of the virus through food, in Europe, 1% of human infections are associated with it. It is therefore advisable to be careful in the three regions of France where cases of tick-borne encephalitis have already occurred, when consuming products made from raw milk, particularly goat’s milk. Note that in the case of a foodborne infection, the incubation is shorter, the clinical symptoms are less, and the occurrence is characterized by cases grouped around the consumption of contaminated product.
To conclude, let us remember that the period of tick activity is maximum from April to June then less from September to November, when temperatures drop. In an oceanic climate, with current climatic changes (mild winters), ticks Ixodes are active all year round; in continental climate, they observe a winter diapause, and begin to be active once more when temperatures rise above 7°C.
During periods of tick activity, one way to protect yourself is to observe a few rules when going out to infested areas, including, in particular, wearing covering clothing, if possible in a light color: this helps limit bites. , and to better spot ticks.
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