Tick-borne encephalitis (TBE) pathogens can be transmitted to humans by the consumption of raw milk and raw-milk products from infected goats. The risk is judged to be low, and is most likely to come from on-farm consumption of the (unpasteurised) products.
Cases of tick-borne encephalitis (TBE) have been increasing in Switzerland for several years now. Although this viral infectious disease is usually transmitted to humans by tickbites, in recent years cases in the German-speaking area have come to light which were caused by the consumption of unpasteurised goat’s milk and goat’s-milk products. Severe disease progression is usually biphasic, with the infected person exhibiting flu-like symptoms in the first phase. In the second phase, the infection may attack the central nervous system (brain, meninges, bone marrow), causing neurological damage (e.g. signs of paralysis) which can lead to death.
Experts from Agroscope, the Federal Food Safety and Veterinary Office (FSVO) and the Spiez Laboratory have compiled a risk assessment for TBE virus-contaminated goat’s milk in the alpine regions of the canton of Valais. The calculations here are based on current data from the literature and on a study conducted in Valais canton on the seroprevalence (i.e. the levels of specific antibodies in the blood serum) of TBE-specific antibodies in goats.
The probability of TBE virus-contaminated milk is very low
The calculation assumes that an infected goat only excretes the virus in her milk during a short viraemic phase lasting seven days on average. Furthermore, only household milk, which accounts for approx. 4% of the goat’s milk produced annually in Switzerland (as of 2018) was taken into account for the risk assessment. This milk is processed directly on the farm, and is intended for on-farm consumption. Since this starting product is not subject to any external inspection, and – unlike marketed milk – is also consumed or further processed without heat treatment, it constitutes the highest risk of virus transmission for the risk assessment. For the risk assessment, the experimentally proven seroprevalence within goat populations in the Valais region was taken into account, and the assumption was made that in an average-sized herd, all animals experience an infection with TBE viruses within the first three to four years of life. The probability of virus-contaminated household milk calculated from this lies between 0.0012% and 0.024% (i.e. around one in every 5000 milk samples).
The risk is low – but the problem is highly topical
The calculated probability of goat’s milk in the selected alpine area in Switzerland containing infectious TBE viruses is very low, and is consistent with the low TBE case numbers in the German-speaking area in which the disease was demonstrably acquired via the alimentary infection route. Despite this, in recent years a total of eight cases have been documented in Germany and Austria in which the consumption of fresh goat’s cheese made from raw milk led to illness.
The most recent case of alimentary transmission of TBE in Central Europe was documented last year (in 2020) in the Auverne-Rhône-Alpes region of France, not far from the Swiss border. A total of 37 people who exhibited symptoms characteristic of TBE infection after consuming raw-milk goat’s cheese were examined. The alimentary transmission of TBE was scientifically proven for the majority of the examined individuals.
Targeted measures can prevent illness from TBE viruses. On the one hand, the TBE vaccine offers effective protection against infection. On the other, the virus can be inactivated by thermal treatment. Thus, for raw-milk processing, an adequate thermal treatment by pasteurisation, UHT sterilisation or boiling is recommended.
- TBE viruses can be transmitted from infected goats to humans via the consumption of raw milk and raw-milk products.
- Although the probability of milk contaminated with TBE viruses is very low, a number of cases of alimentary transmission of TBE have been observed in Central Europe in recent years.
- TBE viruses can be inactivated by adequate thermal treatment of the raw milk.
- The TBE vaccine offers effective protection against the viral pathogen.