You may have never heard of it before but it will be talked about more and more. tick-borne encephalitis is a viral infection due to the TBE virus (Tick-Borne Encephalitis) responsible for neurotropic infections, that is to say which attack the central nervous system. This virus from the flavivirus family (dengue virus, Zika virus, etc.) is mainly transmitted to humans by ticks, these parasitic mites that like wooded and humid areas.
The incidence of tick-borne encephalitis is increasing in Europe with 4,000 cases recorded in 2020. Faced with the increase in the area of circulation of the virus and the extension of the period of the year during which it circulates (from March to October with a peak during the summer season) it was listed among notifiable diseases in France, in May 2021. Since that date, cases and conditions of infection have been recorded by Santé Publique France.
What are the symptoms and risks?
As with other flaviviruses, the vast majority of infected people, around 70%, do not show symptoms. And among the 30% who have symptoms, about fifteen days after contamination, half will simply have non-specific infectious syndromes (fever, joint pain, muscles) without other consequences. But for the other half of the cases, after a few days of apparent improvement in their condition, a second phase appears which reveals an infection of the central nervous system.
“There may be encephalitis with limited damage to the brain, myelitis when the spinal cord is affected (paralysis of the limbs) or meningitis, explains Alexandra Mailles, epidemiologist at the Department of Infectious Diseases Public Health France. The mortality is low but the burden is the sequelae after these central nervous system attacks which may take time to resolve or not at all”. There is no specific treatment based on symptoms. Patients who suffer from severe brain damage are sometimes hospitalized in intensive care, with non-specific treatment.
What does the enhanced surveillance of this virus since 2021 tell us?
After two full years, the national public health agency has counted 71 cases with a majority of men, 15% of whom were occupationally exposed to tick bites, seven of whom worked on a farm. In half of the cases, people have reported tick bites, but these can also very easily go unnoticed. The median age of people infected with the virus is 48, and of the 71 cases, four were children, two of them under ten. Fifteen cases, or a fifth of the total number, are aged over 65, although age is a possible risk factor.
It should be noted that since there is no follow-up during the first phase, where people are asymptomatic, there is an overrepresentation of the most serious cases. Among them, the agency found 38% encephalitis, 13% meningoencephalitis, 37% meningitis and 3% encephalomyelitis. “In subjects over 65, it is very mainly encephalitis, so there is a need to protect themselves for them”, underlines Alexandra Mailles. The majority of these serious cases have been hospitalized and there have been no deaths over the past two years. “The majority of cases (around 60%) do not leave the hospital cured and require neurological, cognitive and neuropsychological follow-up”, specifies the epidemiologist.
How is the virus transmitted?
TBE virus is transmitted to humans mainly by ticks. It’s the same vector as Lyme disease but it’s a very different disease. “Certain mammals including ruminants can be infected and excrete the virus in their milk, in particular goats, explains Alexandra Mailles. And so we can end up with raw milk products that contain the virus. It is the second source of contamination for humans. In 2020, after the end of the first confinement, a fairly large epidemic took place in Ain after the consumption of artisanal cheeses made from raw milk.
How to prevent tick-borne encephalitis?
When you frequent wooded areas, it is recommended to wear covering clothes, and to use a repellent to prevent a tick bite. It is also necessary to monitor the appearance of the bite and remove the tick as soon as possible if necessary, using a tick shot. All prevention tips is available on the Public Health France website.
Two vaccines are authorized on the French market for travelers going to countries where infections are very common. In Europe, the most affected countries are the Czech Republic and Germany and the Baltic countries. An extension of the circulation of the disease is observed in the direction of Northern and Eastern Europe.
What are the risk areas in France?
There are two moderate historic outbreaks in France: in Alsace and Haute-Savoie and since 2016, we know that cases have been identified in Auvergne, mainly in the Forez massif. In light of the questioning of people infected in 2021 and 2022, we know that the vast majority (86%) were infected on national territory, mainly in Haute-Savoie. The disease is developing in Ardèche and Cantal but also in Marne.
“We have a slightly wider circulation than expected, with affected departments when they are a little distant from historic areas”, concludes the epidemiologist.