Spring is tick season, and, with that, it is also TBE season. What is TBE and why is it so dangerous?

The TBE virus belongs in the same family as the viruses that cause Yellow Fever, West Nile Fever, Dengue, and Zika. Like these diseases, TBE is a disease of the central nervous system (the brain and spinal cord). Because these are very sensitive tissues, infection here can be really severe and damage can have long-lasting consequences.

TBE is a caused by a virus that is carried by ticks. This means that if we are bitten by an infected tick, the virus can then be transmitted to us. Since the TBE virus lives in the ticks’ salivary glands, the transmission happens right away after the bite. As a comparison: the bacteria that cause Borreliosis live in the gut of the tick. For transmission, the bacteria need to detach from the gut and migrate to the salivary glands and then into our skin. Because of this, the transmission can take a day or two and can sometimes even be prevented by removing the tick quickly.

Could you get both diseases at the same time?

The ticks here that carry TBE can also carry Borrelia bacteria, so technically, yes, you could contract both at the same time. While there are a couple of different tick species around here, the Ixodes ricinus tick - commonly known as the sheep tick - is the most common and they can carry many diseases.

Where do the ticks get it from?

From small animals. There is a greater range of reservoir hosts for Borrelia than for the TBE virus, but it’s generally small mammals.

We hear a lot about “one health” approaches these days. In that line of thinking: would there be a way to vaccinate the hosts, meaning these animals, and try to contain the disease like that? Because my cat could also carry a tick, right?

Well, the main reservoirs are mice.

Ah, that makes vaccination a bit hard.

Indeed. For us, it is important to have antibodies. However, we currently believe that between mice a lot of the transmission may be non-viremic. This means that these mice actually have circulating antibodies and the virus is not necessarily transmitted from the blood of the animal to the tick. It seems that ticks who are feeding next to one another transmit virus to each other by coming in contact with infected saliva.

There have been several studies on this, and it seems to be an important mode of transmission. However, it is not super well understood and people do question it. Still, it is the prevailing wisdom at the moment and there is really no good way yet to interrupt this type of transmission.

There is another - relatively uncommon - mode of transmission where people can get the virus by consuming unpasteurized milk and eating products made from it, like soft cheese. This is because when an animal is bitten there is a period of viremia where the virus can be transmitted into the milk. If that milk is not pasteurized the virus can be transmitted. This is really uncommon though, so I don’t think it’s necessary yet to vaccinate all the cows in Switzerland :)

In some areas, they try to kill ticks with pesticides, but this has drawbacks. Pesticides can stay in the environment and end up back in our drinking water and food when not used carefully. They also tend to kill more than just the insect you target which interrupts the food chain, since insects are an essential food source for many animals. So in short: controlling the virus in the environment is really tricky.

What do our readers need to know about TBE in Switzerland?

TBE in Switzerland is a much bigger deal than most people are aware of. While the disease was first reported in the 1970s in just the northeastern part of the country, it has spread widely since. This is partly because, as mentioned, it’s very difficult to control the spread of ticks and the animals that carry them in the environment.

Switzerland has become one of the most highly TBE endemic countries in Europe – the disease incidence is similar to what’s seen in Latvia, Czechia, Estonia, Slovenia and Sweden. It is on the higher side of what is observed in the world. Now most areas of the country are considered risk areas. So, one needs to be careful here!

However, because it’s still a relatively new issue in Switzerland, awareness is pretty low – A recent study found that, among TBE endemic countries in Europe, TBE awareness was lowest in Switzerland.

A couple years back the news reported an unusual amount of TBE cases in hospitals and it seemed that from one year to the next the whole country was a red zone. Why did TBE suddenly spread so fast?

There is a combination of factors and it’s also thought to be driven, in part, by climate change. The winters are not quite as cold, and there is a longer warm season where people are outside. Since there is not much we can easily do about the spread of infected ticks in the environment, vaccination becomes an issue. If we can’t control the virus in the environment, then we really need to protect ourselves.

Which regions have the highest incidence?

Some of the regions with the highest incidence are Thurgau and St. Gallen. Also Appenzell Ausserrhoden and Appenzell Innerrhoden. The BAG has a really good map of this. Vaccination coverage is highest in Zurich, followed by eastern Switzerland.

So, the highest incidence is in the northeast, where TBE started to spread from.

Exactly! On the other hand, we see the fewest cases and the lowest vaccination coverage in Geneva and Ticino which is why these cantons are currently not labeled as risk areas and there isn’t a vaccination recommendation.

However, I would also say that Switzerland is a small country and people move around a lot. If you live in Ticino and you go hiking in the northern parts of Switzerland, you are at risk. It is not that far away.

Ah, the ticks have not yet crossed the mountains.

Actually, this begins to happen. There are game trails where large animals go through the alps and they are bringing ticks with them. So they track diseases along these pathways.

BAG card on TBE vaccination recommendation.

How well does the vaccination really protect and why is TBE also a risk for travelers? Learn more about this in the next part.