Vaccination programs and public awareness campaigns could reduce the number of people affected by tick-borne encephalitis virus, which is sometimes foodborne, according to researchers.

Tick-borne encephalitis (TBE) is a viral infection of the central nervous system. Humans mainly acquire TBE through tick bites, but it is occasionally contracted through consuming unpasteurized, raw milk products from affected animals.

Researchers analyzed foodborne TBE cases, mainly from Central and Eastern Europe. Most infections were reported during the warmer months in April to August and were associated with unpasteurized, raw dairy products from goats. The median incubation period was short at 3.5 days and neuroinvasive disease was common, according to the study in Emerging Infectious Diseases.

Overall, 19 studies were included, describing 410 patients across Europe. Countries reporting cases from 1980 to 2021 included Slovakia, Czech Republic, Poland, Hungary, Estonia, Germany, Croatia, Austria, Russia, and Slovenia.

Patient details
Of 273 patients with data on season of infection, 243 were infected from April to August and 30 from September to November. Patients ranged in age from 1 to 85 years old. Most cases were in months that match tick season in Europe.  

Of the 120 patients wher vaccination status was recorded, only one was vaccinated. This person had their last TBEV vaccination booster more than 15 years before infection.

Among 232 patients, epidemiologic investigation revealed consumption of raw goat milk or cheese; raw sheep milk or cheese was reported in 88 cases, drinking unpasteurized cow milk in 23 cases, and consumption of a mixture of unpasteurized dairy products in seven cases.

For 124 of 138 patients for whom the incubation period was reported, it was less than two weeks. For 14 patients who reported the exact infection timeline, the median incubation period was 3.5 days.

Although TBE is a mandatory reportable disease in Europe, nearly all cases occurred in specific regions. This might be explained by habits for unpasteurized dairy products in different regions, but data on the frequency of such consumption in various parts of Europe is lacking, said researchers.

TBE: A public health concern
A recent outbreak occurred in the Ain department of France, wher TBEV had not previously been detected. Investigations revealed that all but one of 43 patients had consumed unpasteurized goat cheese from a local producer.

Researchers said there could be underdiagnosis, underreporting, variations because of the low number of patients involved in some outbreak reports, and incomplete epidemiologic investigations. 

Another explanation might be variability in the viral load of infected dairy products because the exact TBEV dose required for human infection via the oral route is unknown and might be different from the viral load for clinical infection through tick bites. 

Foodborne transmission of TBE is uncommon but has the potential to cause outbreaks affecting many people, making it a major public health concern. Such transmission could be eliminated by education campaigns that encourage people to consume only pasteurized dairy products and through vaccination, said researchers.

Another study, in the International Journal of Molecular Sciences, has reported on a family outbreak of disease, initially suggestive of human-to-human infection, during early summer in Austria.

The index patient was a 39-year-old man, who was hospitalized with a four to five-day history of headache and high fever, with preceding malaise and dizziness for one to two days. His 14-year-old son and 41-year-old wife were also admitted three and eight days later, respectively. They were later discharged with resolving headaches and no fever.

After questioning, it was found that the family had unpasteurized goat’s milk from a farm in Braunau in Upper Austria, two weeks before onset of symptoms. Tick-borne encephalitis was diagnosed following consumption of the goat’s milk and the virus was detected in frozen milk samples. 

The man and his wife were unvaccinated. The 14-year-old son had received three doses of the vaccine, but not according to the recommended immunization schedule. Another 7-year-old boy was not infected despite drinking the milk and not being vaccinated.