Tick-borne encephalitis : tracing the origin of cases of transmission via cheese

In spring 2020, 43 inhabitants of the Ain département of France suffered from meningitis, meningoencephalitis and flu-like symptoms caused by infection with tick-borne encephalitis virus (TBEV). They had all eaten goat cheese made with raw milk.

These were the first known cases of foodborne infection with TBEV, which is usually transmitted through tick bites. "In April 2020, the first lockdown encouraged the consumption of local products” affirms Gaëlle Gonzalez, project manager in the Virology Unit of ANSES's Laboratory for Animal Health. “The fact that this was a cluster of cases made it easier to identify the source of infection”.

ANSES's Laboratory for Animal Health, Laboratory for Food Safety and Laboratory for Rabies and Wildlife took part in the investigations carried out on the farm that had produced the cheeses suspected of being responsible for the infections. The results of this scientific work, which also involved other French infectiology teams, were recently published in Frontiers in Microbiology.

Tracing the virus from cheeses to ticks

It was found that all the cheeses involved came from the same farm. Following the health alert, the cheeses were removed from the market to prevent other cases of infection. At the same time, the goats on the farm were kept indoors.

A quarter of the goats had antibodies against TBEV, suggesting that they had been exposed to the virus and that it was quite widespread in the area. The virus itself was detected in the milk of three of them. Considering that TBEV can be excreted in milk for up to 23 days following infection, this means that the goats had only recently become infected, probably by virus-carrying ticks that were found in the undergrowth in their pasture area.

These first cases of foodborne infection in France underlined the importance of having effective methods for the detection of TBEV in dairy products. ANSES's scientists therefore helped assess the method used, to ensure that it was sufficiently sensitive and specific to TBEV.

Infections outside of the virus's known geographical area

For the first time, these infections revealed the presence of TBEV in Ain, whereas its occurrence had previously been known primarily in Alsace, Lorraine, Savoie and Haute-Savoie.  However, this was not surprising for the scientists involved, who were aware that the virus's range has tended to spread from Eastern Europe and that the virus can go unnoticed. “Tick-borne encephalitis does not cause symptoms in animals. In humans, only 10 to 30% of cases present with meningitis or encephalitis. The rest of the time, the symptoms resemble those of the flu and therefore go unnoticed” explains Gaëlle Gonzalez, the first author of the article published in Frontiers in Microbiology.

Studying the factors that can influence the risk of infection

Since the first cases were observed in spring 2020, some other cases of foodborne infection have also been reported in France. Studies are currently being undertaken in ANSES's laboratories to identify the factors that may influence the risk of infection.

One of these studies is attempting to understand how the microbiota affects the risk of transmission. It will take into consideration all of the micro-organisms contained in the digestive systems of ticks and domestic animals and in milk.

Moreover, it is known that milk pasteurisation eliminates the virus and the virus does not survive in cheeses that have been ripened for several months. Thesis work was initiated in 2021 to determine how the stages of raw-milk cheese production impact the virus : do they reduce the amount of virus compared with that found in milk? Is the amount of virus homogeneous throughout the same cheese? This additional knowledge should enable appropriate monitoring and preventive measures to be taken.