Strengthening the surveillance of tick-borne encephalitis to improve disease prevention

A rare disease that can have serious consequences

In France, around 30 cases of tick-borne encephalitis are recorded every year. Although infections are relatively rare, the health impacts of the disease can be significant. “Ten to thirty percent of infected people develop symptoms, which are generally flu-like. Among those with symptoms, 20% to 40% experience neurological signs resembling meningitis that can lead to long-term sequelae and loss of autonomy” explains Elsa Quillery, co-coordinator of the expert appraisal.

Beyond the health consequences, the disease also has a significant economic impact. As part of its expert appraisal, ANSES estimated that the current annual cost of the disease in France is €3 million. This estimate includes the cost of treating the disease, loss of income and time for unpaid activities, and the decrease in quality of life in the event of disabling sequelae.

A worrying geographical spread

Several findings are raising concerns about an increased risk of transmission of tick-borne encephalitis virus in France. In 2020, the first outbreak of infections associated with the consumption of raw-milk cheese was identified, in a département where the virus was not known to be circulating. This is a route of transmission that had not previously been reported in France, as the virus is mainly transmitted by bites of infected ticks of the genus Ixodes. In addition, the virus is expanding its area of circulation, with cases reported outside Alsace, the region that has historically been affected. Lastly, the number of cases is increasing in several European countries, including France.

The expert appraisal carried out by ANSES identified some categories of people, such as farmers and foresters, who are at higher risk of infection because they are more frequently exposed to tick bites. These groups are 13 times more likely to be infected than the general public.

When ticks carrying the virus bite dairy animals, this can contaminate their milk. Raw goat’s milk and dairy products made from raw goat’s milk appear to pose a higher risk of transmission than dairy products from other animals. They are responsible for the majority of cases of foodborne transmission in Europe. The Auvergne-Rhône-Alpes region is most affected by this risk, due to the confirmed circulation of the virus and the large number of goat farms with frequent access to the outdoors.

Stepping up the surveillance of tick-borne encephalitis to improve prevention

The first step in combating tick-borne encephalitis is to improve surveillance of the virus in order to better identify infected areas and livestock farms and deploy appropriate preventive measures in these areas. Currently, this surveillance mainly consists in identifying human cases of tick-borne encephalitis. However, these are late indicators of the virus being present in a given area. ANSES recommends including domestic and wild sentinel animal species, such as goats, cows and deer, in the tick-borne encephalitis surveillance strategy and monitoring dairy products and ticks themselves.

ANSES also recommends taking steps to prevent transmission in the areas affected, in consultation with the stakeholders involved, particularly farmers. To prevent milk from becoming contaminated, these steps could include limiting the exposure of ruminants, particularly milk-producing goats, by installing fences to prevent contact with the areas most favourable to the presence of ticks, such as wooded areas and hedges, or by implementing rotational grazing. These measures could be supplemented by milk pasteurisation in emergency situations, for example when human cases occur due to foodborne transmission or when the infectious virus is detected in collected milk.

To keep the virus from being transmitted to humans through tick bites, it is advisable to wear long clothing when outdoors, especially in forests. Anyone can be vaccinated against the virus if they wish; this may be particularly relevant for the most exposed groups of people, including forestry workers, livestock farmers and people who engage in leisure activities in forests. Lastly, the Agency recommends better informing the general public, exposed workers and healthcare professionals about the risks and symptoms of this still little-known disease.

Multiple causes that remain unclear

It is likely that multiple factors are behind the geographical spread of tick-borne encephalitis virus, although these remain poorly understood. Ticks of the genus Ixodes, which carry the virus, are already present throughout mainland France, except along the Mediterranean coast. Several hypotheses have been put forward, such as landscape fragmentation, which leads to domestic animals and humans being increasingly in contact with ticks, and increased visits to forests by the general public. On the other hand, climate change does not seem to play a major role.

“Many questions still remain about the mechanisms by which the virus circulates and spreads” concludes Elsa Quillery. “To better identify areas and situations at risk, research needs to be conducted focusing on various aspects, such as the strains circulating in France, the risk factors for transmission and the impact of milk components and milk processing methods on the persistence of infectious viruses in dairy products. In addition, tools for detecting tick-borne encephalitis virus need to be improved so that it can be more accurately detected in humans, animals and dairy products”.