Tick-borne encephalitis, a rare disease in France

Tick-borne encephalitis is caused by a virus (Flavivirus) transmitted to humans through the bite of an infected tick, primarily from spring to fall (the tick activity season), and more rarely through the consumption of raw milk dairy products. There are three types of this virus: European, Far Eastern, and Siberian.

The European subtype, the only one present in France, causes less severe illness than the other two.
Between 5,000 and 13,000 cases of tick-borne encephalitis are reported worldwide each year.

Very rare in France, tick-borne encephalitis is currently prevalent from Europe to northern Japan and China, between the 40th and 60th parallels.

The epidemiological situation in France is evolving. Historically, about twenty cases were diagnosed per year, mainly in Alsace and Haute-Savoie, or contracted abroad. Since the disease was added to the list of notifiable diseases in May 2021, the number of reported cases has reached approximately 35, but these data need to be consolidated over time.

In Europe, the countries most affected are the Czech Republic, Germany, and the Baltic states. The disease is spreading toward Northern and Eastern Europe.

Mode of transmission

The tick-borne encephalitis virus is transmitted through the bite of an infected tick (usually Ixodes ricinus), regardless of its stage of development. Infected vertebrate animals or tick carriers, such as cattle, do not directly transmit the disease to humans. More rarely, infection can occur through the consumption of raw milk or raw milk cheese, primarily from goats or sheep.

This type of virus transmission was observed in France for the first time in the spring of 2020, during a cluster of cases among people who had consumed the same goat cheese.

Certain occupational activities carry a higher risk due to exposure to tick bites: these are primarily professions practiced in forests or rural areas, in regions where the virus is present—such as loggers, farmers, foresters, and gamekeepers. However, the risk exists for anyone visiting forests for recreational activities.

Prevention Methods

The main preventive measure is to wear protection against tick bites (long clothing, checking the skin upon returning from the forest).

Two vaccines against tick-borne encephalitis are available in France. Vaccination against tick-borne encephalitis is recommended for adult and child travelers at risk in countries with high incidence rates. It reduces the individual risk of contracting the disease and prevents the most severe clinical forms.

Symptoms and Treatment

After an incubation period of one to two weeks, the disease begins abruptly, with fever, headaches, and muscle and joint pain.

Subsequently, in 20 to 30% of patients, symptoms appear due to involvement of the central nervous system (encephalitis, myelitis) or the peripheral nervous system (paresis or paralysis of a limb). Clinical signs of central nervous system involvement include prostration or agitation, tremors, behavioral disturbances, impaired alertness or consciousness, and occasionally seizures or coma. Death is rare with the European viral subtype (<1% mortality), but sequelae (primarily paralysis and behavioral disturbances) can occur in up to 40% of cases.

There is no specific antiviral medication for this disease. Treatment is solely symptomatic. Recovery from the disease is prolonged, with neurological or psychiatric sequelae that may persist for several years.