Part 3: The role of mosquitoes in the transmission of West Nile virus in Germany
The West Nile virus is a flavivirus that was isolated first in Africa in the 1930s. It is transmitted by mosquitoes, which serve as so-called vectors. In recent decades, the virus has spread to many regions of the world. By now, it has also reached Germany. Since 2018 there have been the first confirmed cases of West Nile virus in Germany in animals and in 2019 the first infection was confirmed in a human in Germany.
Part 1 and part 2 over our interview series on the West Nile virus in Germany are dedicated to the significance and danger of the virus for animals and humans. In part 3 we are taking a closer look at the role of the mosquito which is a key element in the transmission of the disease. For this purpose the German Research Platform for Zoonoses spoke to the entomologist Dr. Helge Kampen at the Friedrich-Loeffler-Institute (FLI). As office manager of the National Expert Commission "Mosquitoes as vectors of pathogens" and co-founder of the “Mosquito Atlas” of Germany, he is one of the national experts in the field of vector ecology and epidemiology.
Note: the interview has been translated from German. See the original German version
ZOOP: Unlike in other regions of the world, until recently a mosquito bite in Germany was considered harmless. But now there is an infection risk with the West Nile virus by infected mosquitoes. Can we be certain that the West Nile virus is indeed transmitted by mosquitoes in Germany?
Kampen: Yes, we know from literature and experimental work that some species of mosquitoes native to Germany can act as potential vectors, including the common house mosquito (Culex pipiens), which is widespread in Germany, and other representatives of the genus Culex. The species Culex modestus and Culex torrentium are to be mentioned here. Within the framework of mosquito monitoring in Germany, the first evidence of the virus in a mosquito population in Germany was also obtained at the Tierpark Berlin.
ZOOP: Recently, a lot could be read about invasive vector species in Europe. Has the spread of exotic mosquito species favoured the spread of West Nile virus in Germany or can the virus also be transmitted by native species?
Kampen: Among the invasive species, there are also some that can potentially transmit the virus, including the Asian tiger mosquito (Aedes albopictus) or the Asian bush mosquito (Aedes japonicus). However, it can be assumed that these species play a negligible role in the transmission of the West Nile virus in Germany, as they are by far not as common as the common house mosquito. In addition, they probably have a lower vector competence. In view of their distribution, the contribution of invasive species in comparison to the common house mosquito is likely to be rather small.
ZOOP: In 2018 and 2019 we had rather warm and dry summers. Do the climatic conditions have an impact on the West Nile virus spreading by mosquitoes?
Kampen: The temperature has a great impact both on the mosquito as well as on the virus development in the mosquito. The mosquito can digest a blood meal faster at higher temperatures. Consequently, the mosquito will bite more frequently, the egg production takes place faster and the reproduction cycle is accelerated. Additionally, the seasonal activity period of the mosquito is longer. The virus benefits from higher temperatures as well, because it can develop faster inside the mosquito and becomes infectious more quickly. Therefore, at warmer temperature we have mosquito populations that bite more frequently and multiply faster in combination with a faster viral development cycle.
Dryness on the other hand can become a problem for the mosquito. Since mosquitoes carry out their larval development stage in water, they are dependent on the presence of water. Some species require major water accumulations or flooded areas. For other species, small accumulations of water, which are often found in gardens, are sufficient. For example, the tiger mosquito or Culex pipiens are synanthrope species that breed predominantly in the immediate vicinity of humans in artificial water accumulations. Watering one's own garden in dry summer months can thus contribute to the reproduction of these mosquitoes.
ZOOP: Are mosquitoes somehow affected by a West Nile virus infection?
Kampen: It has not yet been observed that West Nile virus causes any symptoms of disease in mosquitoes and no changes in behaviour have been observed in infected animals. The situation is partly different for other arthropod-borne diseases. Malaria pathogens or filariae can also be pathogenic for the vector. It is also known that an infection of mosquitoes with malaria pathogens can lead to the insects biting more frequently. However, such effects on the mosquito are not known for the West Nile virus.
ZOOP: Mosquitoes are often referred to as “bridge vectors” in disease spreading events. What exactly is meant by that and why is it relevant?
Kampen: Bridge vectors are transmitters that do not feed host-specifically, but suck blood from different species, so-called generalists. This allows them to transmit pathogens between taxonomic groups. In the case of West Nile virus, we are looking at species of mosquitoes that bite both birds and mammals such as humans or horses. This is how they spread the virus from one species to another.
There are two different biotypes of the common house mosquito. One bites preferentially birds and the other preferentially mammals. Hybrids of the two biotypes, which occur mainly in the late summer, are less host-specific and feed on both avian and mammalian blood. Therefore, they can function as a bridge between the reservoir host (bird) and the dead end host (human, horse) for the virus.
ZOOP: Are there already control strategies in place to contain the spread of West Nile virus by mosquitoes in Germany?
Kampen: There are control strategies against mosquitoes in general but none that are dedicated specifically to control West Nile virus spreading. At that point it is important to mention that Culex pipiens, as a vector for West Nile virus, is very common in Germany and can be found nearly everywhere in Germany where people live. In the context of our mosquito monitoring in Germany with the help of the mosquito atlas, Culex pipiens made up the largest part of the entries. A widespread control of this mosquito species is therefore not possible.
The situation is different with invasive species such as the tiger mosquito. In Germany, this species is specifically fought against. Furthermore, mosquito control measures have been in place in the Upper Rhine region for over 30 years. However, these are directed against species that lay their eggs in floodplains and floodplain landscapes. During flood events, the larvae of these mosquitoes hatch and cause an explosive increase in the mosquito populations in these areas. For this reason, a biocide against mosquitoes has been used in this region for years.
ZOOP: What could control measures against the spreading of West Nile virus relevant mosquito species look like?
Kampen: In the fight against the common house mosquito, the most important transmitter of West Nile virus in Germany, the application of biocides is not an option. In the end, it is up to each and every citizen to take care in his or her own garden, for example, not to great places, like water accumulations, where the mosquitoes can lay their eggs. Attaching insect nets can also help to keep your own living space free of mosquitoes. In the long term, we need a good education policy so that everyone in Germany is informed about the life cycles and distribution of mosquitoes and is aware of the potential risks posed by mosquitoes. This will enable each individual to adapt his or her behaviour.
ZOOP: Do you think control measure are sensible and necessary?
Kampen: Yes, even though the eradication of specific mosquito species might not be applicable, it is important to keep mosquito populations small nevertheless, especially when it comes to invasive species. Because the mosquito population density together with the number of infection sources determines the infection risk for mosquito-transmitted diseases. This applies to West Nile virus as well as to Zika, Dengue, Chikungunya or Rift Valley Fever virus in other parts of the world. We should therefore attempt to keep the population densities of vector-competent mosquitoes at a low level in all cases.
ZOOP: To what extent could other species be affected by the control measures? Is there a potential danger for ecosystems?
Kampen: The biozide used in the Upper Rhine region is a bacterial protein derived from Bacillus thuringiensis israelensis (BTI). The BTI toxin is ingested by the larva through their food and destroys the intestinal wall of the mosquito larva, which consequently will die. The toxin works relatively specific on mosquitoes and close relatives. However, it is controversial to what extent the toxin does also harm other insects larva.
On a long-term perspective, one needs to be aware of the fact that every control strategy against mosquitoes will affect the ecosystem because mosquitoes as well as their larvae are an important component of the food chain. In the end, it will be a political question if you do prioritise the protection of nature, e.g. the protection of fish, amphibians and birds which feed on mosquitoes, or if you prioritise the health of humans and livestock. In both cases collateral damages are unavoidable.
ZOOP: Is it possible that other vector species, like ticks or biting midges, might contribute to the transmission of West Nile virus?
Kampen: The virus could already be detected in other blood-sucking arthropods by means of RNA detection. However, these findings do not tell us whether these species are also transmitters of the virus. It is not sufficient to take up the virus via a blood meal alone because the virus doesn´t necessarily further develop or multiply and spread in the haematophagous arthropods. In general we can not exclude that other vector species exist. Nevertheless, I doubt that the contribution of other species play an epidemiologically relevant role.
ZOOP: What is your prediction for the future? Do you think the West Nile virus can establish itself permanently in the mosquito population in Germany?
Kampen: I share the opinion stated by my two collegues in part one and part two of the interview series. The evidence that the West Nile virus has managed to hibernate in Germany from 2018 to 2019 suggests that the virus will continue to establish itself in our domestic mosquito populations. Taking into account climate warming we need to assume that we will have to deal with “mosquitoes as vector for diseases” on a regular basis in Germany. In this context, the spreading of invasive species like the tiger mosquito will also play a crucial role.
ZOOP: Dr. Kampen, thank you very much for the interview.
Interview: Dr. Dana Thal for the German Research Platform for Zoonoses
The interview is part of a series of discussions with experts on the subject of West Nile virus in Germany. If you would like to learn more about the West Nile virus in humans, please read the answers given by Prof. Jonas Schmidt-Chanasit from the Bernhard Nocht Institute for Tropical Medicine in Hamburg in Part 1. Further information on the potential dangers of the West Nile virus for animals can be found in Part 2 in an interview with Dr. Ute Ziegler from the Friedrich-Loeffler-Institute, the German Federal Research Institute for Animal Health in Greifswald.