Cases in Italy: How dengue fever comes to Europe

None of the infected people are said to have traveled before; the disease was “locally acquired”. This means the infections took place in Italy. It is also said that there is no connection between the cases in the different regions.

The pathogen that causes dengue fever is transmitted by certain mosquitoes:

As the ECDC reports, increased attention is now being paid to new infections in the affected Italian regions, as they cannot be ruled out. The Italian authorities have also taken further measures, such as “vector control” of affected areas or checking blood and organ donations. As Italian media report, insecticides are also being sprayed in certain areas because the pathogen is transmitted by mosquitoes.

In fact, according to the World Health Organization (WHO), there have already been local infections with the tropical dengue virus in around two dozen European countries, including Spain, Croatia and France. People have also been infected with dengue fever on the Portuguese island of Madeira.

There have also been dengue cases in Germany – but among returning travelers. According to the Robert Koch Institute, no autochthonous transmissions are known yet – i.e. infections that were transmitted locally and did not occur through travel.

This is also confirmed by the doctor and virologist Jonas Schmidt-Chanasit from the Bernhard Nocht Institute for Tropical Medicine: “Most dengue cases are still caused by people returning from travel.”

However, it is not always possible to determine with certainty whether the dengue virus was newly introduced by a returning traveler or whether it had already been circulating in the local tiger mosquito population for a longer period of time and then caused an autochthonous infection.

“The tiger mosquito spreads through travel and goods traffic, through globalization,” explains Schmidt-Chanasit. “The mosquito can travel to Europe in the interior of an airplane, for example.” It has also been spreading in Germany for some time.

There are currently areas with populations in the southwest in Baden-Württemberg, Hesse and Rhineland-Palatinate, but also in places, for example in Fürth, Jena or Berlin.

And populations were also discovered on the Upper Rhine, as the municipal action group to combat the mosquito plague (Kabs) in Speyer reported last November.

The EU disease control agency ECDC warns of an increasing risk of mosquito-borne diseases such as dengue fever in view of climate change. It is said that heat waves and floods as well as longer summers provide good living conditions for invasive mosquito species such as the Asian tiger mosquito and the yellow fever mosquito.

In addition, the animals threaten to breed all year round in several regions of southern Europe – which would increase the potential for disease to spread. In central Italy, according to a team of researchers in the journal “Royal Society Open Science”, the tiger mosquito is said to have laid eggs in the winter.

Since the tiger mosquito was once again detected in a Berlin allotment this year, the Senate Department of Health also comes to the conclusion: “Successful overwintering is therefore proven and permanent settlement is to be feared.”

Already in June, the ECDC reported: Ten years ago only 114 regions in Europe were affected by the tiger mosquito, this year there are already 337 regions. After the dengue fever cases in Italy, the current report now says:

Since the Asian tiger mosquito is widespread in most parts of Europe, further virus introductions may occur in southern EU countries.

In order for dengue fever to spread, more Asian tiger mosquitoes would first have to become infected with the virus, explains Christina Frank, epidemiologist at the Robert Koch Institute. But she also says: “Due to the boom in long-distance travel, this isn’t that rare.”

For example, if someone travels back to Germany with dengue fever in July from a vacation in the Caribbean or Southeast Asia, Asian tiger mosquitoes can ingest the virus by eating a blood meal from that person.

“Then this mosquito can infect a person in Germany with the next blood meal – that would then be an autochthonous transmission.”

Virologist Jonas Schmidt-Chanasit is not worried by the cases of dengue fever on Lake Garda: “It’s nothing new, it’s nothing surprising.”

In the past, there have always been autochthonous cases – i.e. cases in which people became infected on site and not on a trip – for example in Italy, France or Croatia. But these are and were a few confirmed cases or smaller clusters.

Despite the spread of the tiger mosquito, the number of infected people in Germany will initially remain low, so there is no need to worry, he says.

It is to be expected that there will continue to be a few cases of dengue fever in Europe in the future due to returning travelers or infected mosquitoes. “In order for the virus to multiply well in the mosquito and then be transmitted, there must be higher temperatures over a longer period of time – and so far we only have these in the summer,” explains Schmidt-Chanasit.

This means that there is only a certain time window in late summer in which a few individual cases can occur.