Climate change: exotic infectious diseases in Germany?

There are many ticks and mosquitoes native to Germany. Everyone has been bitten by mosquitoes now and then. Although bites from native mosquitoes are irritating they are mostly harmless. However, non-native species of mosquito such as the Asian tiger mosquito have been encountered in Germany from time to time for some years now. These species can pass dangerous pathogens that are relatively unusual in Germany on to animals and human beings. The pathogens and infectious diseases involved are referred to as “exotic” or “tropical”.

Ticks can also pass on pathogens. Well-known examples are native species of ticks such as the castor bean tick (Ixodes ricinus). They mainly transmit TBE viruses (tick-borne encephalitis) and Borrelia burgdorferi bacteria (Lyme disease or borreliosis). Non-native ticks from warmer regions of the world including the meadow tick (Dermacentor reticulatus) from the Mediterranean region are now increasingly being spotted in Germany. Like tropical mosquitoes they can pass on a wide range of exotic pathogens to people and animals.

Climate change is a key factor in the spread of exotic infectious diseases within Germany. Higher average temperatures, longer summers and milder winters create good conditions for the pathogens and their carriers exotic mosquitoes and ticks.

Ticks and mosquitoes are often carriers of pathogens. These include viruses, bacteria and single-celled parasites. The reason these pathogens can be passed on to human beings is that ticks and mosquitoes need blood to develop and reproduce. They are constantly on the lookout for a new “host” to provide that blood. However pathogens are only passed on by a tick bite or mosquito bite under certain conditions:

• The tick or mosquito must previously have fed on blood from an infected living creature and absorbed pathogens with it.
• The pathogens must have had enough time and the right conditions to reproduce in the tick or mosquito.
• For some pathogens transmitted by ticks, the feeding process must also have persisted for some time. For example, Lyme disease (borreliosis) bacteria are not transmitted for 12 hours at least. 

While a tick is sucking blood the pathogens can be transmitted to the person in the tick’s saliva. Most people know that a tick bite can be dangerous. That’s mainly because even native species can pass on diseases, particularly Lyme disease (borreliosis) and tick-borne encephalitis.

Little is known as yet about the capacity of native mosquito species to pass on diseases. However we do know that some native species can also pass on pathogens such as the West Nile virus that can cause disease. However the risk of passing this type of pathogen on is greater with tropical mosquitoes. So the risk is relatively low in Germany at present.

The main reasons for the increase in the spread of exotic pathogens is global trade in goods and travel, as well as climate change. The increase in mobility and international travel mean that exotic pathogens and their carriers are increasingly being introduced into the country. For example, many types of mosquitoes can lay their eggs in even the tiniest of pools of stagnant water. This allows mosquito eggs to be transported long distances.

Some pathogens such as the West Nile virus can also be introduced by migratory birds. They can then be transmitted from bird to human being by mosquitoes. But many pathogens can also be transmitted to animals such as horses, sheep and dogs.

Tropical pathogens would not be able to survive and spread in moderate climate zones like Germany if it weren’t for climate change and global warming. The increase in average temperatures means that exotic pathogens and their carriers can reproduce in Germany too. Increasingly mild winters mean that tropical mosquitoes and ticks can now establish themselves permanently in Germany. For example, the meadow tick, which comes from the south, has spread increasingly in Germany in recent years.

Meadow tick

The meadow tick (Dermacentor reticulatus) settled in Germany several decades ago and can now be found throughout the country. It originally came from warmer regions such as the Mediterranean and North Africa. Meadow ticks can transmit a range of pathogens including Babesia and Ehrlichia. However people are rarely bitten by meadow ticks. Horses and dogs are infected more frequently.

Babesia are parasitic pathogens similar to the malaria pathogen. They frequently infect dogs with babesiosis, also known as “canine malaria”. Human beings are far less likely to get babesiosis, and even if they do they only have mild symptoms. Typical symptoms of the severe form of the disease in human beings are high temperature, anemia and exhaustion.

Ehrlichia are bacteria that reproduce in human and animal cells. Three quarters of people with an Ehrlichia infection (ehrlichiosis) are asymptomatic. However older people and people with immune deficiency can become severely ill from it. Typical symptoms include, for example, high temperature, shivering and other flu-like symptoms.

Brown dog tick

The brown dog tick (Rhipicephalus sanguineus) also comes from North Africa and southern Europe. It prefers warm, dry regions. It is mostly introduced by people (or their dogs) returning from holiday to Germany. But it has also been found in dogs that have never left Germany. This suggests that it may already have become established in Germany. 

Although the brown dog tick mainly infects dogs there are also rare cases of it biting human beings. It transmits a range of pathogens to dogs and human beings, again including Babesia and Ehrlichia. It can also transmit other species of bacteria. However reports of transmission of these pathogens to people continue to be rare in Germany.

Hyalomma ticks

Hyalomma ticks are mainly found in Africa, southern Asia and southern Europe. There have been isolated cases in Germany in recent years and it is assumed that they were introduced by migratory birds. The majority of sightings of Hyalomma ticks in Germany to date were in the hot summer of 2018. Conditions during the unusual months-long heatwave were very favorable for the tick. Hyalomma ticks can probably spend the winter in Germany. It’s unclear whether the few that survive the winter are enough to allow the tick to become established in Germany in large numbers.

Hyalomma ticks can be distinguished from other ticks by their large size and striped legs. This is what a Hyalomma tick looks like:

Hyalomma ticks can also pass on rickettsia, the pathogens that cause Rickettsial disease. These pathogens have already been detected in Hyalomma ticks found in Germany. Crimean-Congo hemorrhagic fever (CCHF) is an infection that can be fatal and is also transmitted by Hyalomma ticks. However the Crimean-Congo hemorrhagic fever virus has not yet been detected in ticks in Germany. 

You can send the ticks to the following address:
Robert Koch Institut
ZBS 1 – “Zecke”
Seestraße 10
13353 Berlin

Asian bush mosquito

The Asian bush mosquito (Aedes japonicus) has spread widely around the world as a result of Asia’s globalization. It has also been found in Germany since 2008, mainly in the south of the country. It can transmit the West Nile virus, dengue virus, chikungunya virus and Japanese encephalitis virus.

Transmission of the West Nile virus to human beings was first reported in Germany in 2019. Individual cases were also reported in 2020. It is suspected that there were other infections which were not detected. That’s because just 20% or so of the people infected actually become ill with West Nile fever and have symptoms that are similar to those of flu. Just under one in every 100 people infected have the severe and sometimes fatal form of the disease. Since the West Nile virus now appears to survive the winter in Germany, experts believe it could spread even further in Germany.

There are no known cases as yet of transmission of the dengue, chikungunya and Japanese encephalitis viruses in Germany. Since they can only be transmitted at relatively high temperatures, the risk is currently classed as low in Germany.

Asian tiger mosquito

The Asian tiger mosquito also comes, as the name suggests, from Asia. It was first discovered in Italy in the 1980s. It has now become fully native to the country. It has been found in parts of southern Germany since 2007, initially in small numbers but now in larger numbers during the summer. It is also known to survive the winter in southern Germany.

It is believed that this mosquito can act as carrier for over 20 different viruses. Like the Asian bush mosquito, it can transmit dengue virus, West Nile virus and chikungunya virus. Theoretically, it can also transmit the Zika virus. However this virus needs rather long periods of very high temperatures to reproduce: at least 14 days of temperatures of 27 degrees or more.

As far as we know, none of the pathogens listed has been transferred by the Asian tiger mosquito in Germany. However this cannot be ruled out given the increasingly higher temperatures. Given the sharp rise in the number of cases of dengue and chikungunya in travelers returning from affected countries the Federal Environmental Agency has already classed the Asian tiger mosquito as a potential health risk.

Although there have been a few cases of exotic infectious diseases in Germany in recent years they remain rare, isolated cases. The risk of becoming infected with exotic pathogens in Germany is currently low. If you still want to protect yourself against the pathogens or are spending time in an at-risk area abroad, you need to protect yourself against the carriers, ticks and mosquitoes. However it’s a good idea to apply tick repellent in Germany, too, to protect yourself against Lyme disease (borreliosis) and tick-borne encephalitis.

Protection against ticks and the pathogens spread by them

There is no vaccine or any effective treatment for many of the pathogens spread by ticks. There are no vaccination or effective treatment options for some pathogen such as the Crimean-Congo virus. So you should avoid mosquito bites as far as possible to protect against infection.

The following measures can help:

• Try not to spend time in grass or undergrowth. You’re particularly likely to encounter ticks in these places.
• Wear long clothing and closed shoes when you are out in the countryside, particularly woods and meadows. It can also be helpful to tuck your trousers into your socks. That makes it harder for the ticks to access your skin.
• Use tick repellents such as insect spray before spending time in the countryside. It’s best to apply the spray to your skin and your clothing to get the best effect.

It’s also a good idea to search for ticks after spending time in the countryside. Sometime ticks can be discovered before they attach themselves. Ticks do not usually attach themselves immediately, but look for a suitable site first.

Removing ticks immediately after they bite can protect against transmission of some pathogens such as borrelia. It generally takes 12 to 24 hours for the Lyme disease (borreliosis) bacteria to be transmitted.

However transmission of viruses is mostly quicker and often cannot be prevented by removing the tick. Many tropical species of tick, unlike native species, leave their hosts very quickly after biting them.

Protection against mosquitoes and the pathogens spread by them

There is a vaccine against dengue virus. However it is only approved for persons who live in regions with a high prevalence of dengue virus who have already been infected with the virus. As yet there are no vaccines against West Nile virus, chikungunya virus or Zika virus. So the best way to protect yourself against infection is to protect yourself from the mosquitoes. Anti-mosquito sprays are an option here. The most effective deterrent is DEET (diethyl toluamide).

If you want to protect yourself against mosquitoes indoors, mosquito nets in front of windows or over beds are a good option. To stop mosquitoes from reproducing you should avoid areas of stagnant water, which mosquitoes can use as breeding sites, or at least empty them completely once a week. These include, for example, bird baths or rainwater butts.