Tick-borne encephalitis

What are the causes?

Tick-borne encephalitis is a viral infectious disease caused by a flavivirus known as tick-borne encephalitis virus (TBEV). 

It is a zoonosis (an infection communicable from animals to humans) endemic in large areas of Central and Eastern Europe, Scandinavia, Russia and North Asia (northern Japan, China, etc.). The disease is closely linked to environmental, climatic and behavioral factors (outdoor activities, exposed occupations). Incidence has risen in several European countries due to the expansion of tick habitats.

Once a subject has been bitten, the virus replicates locally and spreads in their body. Tick-borne encephalitis can either remain asymptomatic or progress to a central nervous system infection. 

What are the symptoms?

In most cases, the infection is asymptomatic. However, in symptomatic cases progression occurs in two phases following an incubation period lasting two weeks on average. 

The first phase manifests as an influenza-like illness, with symptoms including fever, shivering, intense fatigue, muscle pain (myalgia) and headaches (cephalalgia). 

Some patients then progress to a second phase with neurological conditions including meningitis, encephalitis and meningoencephalitis, sometimes combined with impaired consciousness, focal neurological deficits, or paralysis.

Clinical signs of central nervous system infection are exhaustion or restlessness, tremors, behavioral disorders, impaired alertness or consciousness, and occasionally seizures or coma. Although cases reported in France are rarely fatal (< 1% mortality rate), up to 40% of cases result in complications, mainly in the form of paralysis and behavioral disorders. 

How is the virus transmitted?

The virus responsible for tick-borne encephalitis is mainly transmitted through bites from infected ticks (Ixodes ricinus in Western Europe). It is also more rarely transmitted through the consumption of raw milk or unpasteurized dairy products from livestock carrying the virus, particularly goats and ewes.

No human-to-human transmission has been reported to date. 

How is the disease diagnosed?

Cases are diagnosed on the basis of exposure (subjects active in an at-risk area or bitten by a tick) and their neurological clinical profile. A serological assay is performed to test for specific antibodies (IgM and IgG) in the bloodstream and/or cerebrospinal fluid. Brain imaging may also be used to assess severity or rule out any other diagnoses. 

What treatments are available?

There are no specific antiviral treatments for the tick-borne encephalitis virus.

Treatment is exclusively symptomatic based on the treatment of fever and pain, hospitalization and neurological surveillance. Intensive care may be required for severe cases. Persistent neurological complications are possible.   

How can tick-borne encephalitis be prevented?

Vaccination and bite prevention are the most effective methods of protecting against the disease.

Vaccination reduces individuals' risk of contracting the virus and prevents the most severe clinical forms. It is particularly recommended for individuals in endemic areas as well as those at risk of exposure due to their occupations (farmers, foresters, etc.) or leisure pursuits.

Personal protective measures against tick bites are essential, e.g. wearing long clothes, applying repellents, checking the skin after at-risk situations (e.g. a walk in the forest), and quickly removing ticks before they bite.

Unpasteurized raw milk should not be consumed in at-risk areas. 

How many people are affected?

Between 5,000 and 13,000 tick-borne encephalitis cases are reported every year worldwide, with incidence variable by region and season. The disease is under permanent surveillance by Santé publique France.

Around thirty cases are diagnosed in France every year.