Invasion of exotic mosquitoes – a new threat in Europe

With summer it’s like with creamsicles – everyone likes them. Unfortunately, this highly anticipated season in our climate zone, in addition to its many advantages, also has some disadvantages. One of the biggest nuisances for anyone spending time on the water is certainly mosquitoes. Climate change is exacerbating the problem. Increasingly longer and warmer summers, as well as the heat waves and floods haunting our continent, are creating favorable conditions for the growth of these insects, not only native but also foreign species. An invasion of exotic mosquitoes is a potential threat to human health and life. These insects can transmit many infectious diseases, making the problem no longer just a harmless nuisance. Polish scientists in the journal Viruses report the confirmation of the Asian mosquito Aebes japonicus in Poland. It is a vector for many dangerous pathogens.

Three mosquitoes, one problem

For residents of temperate zones, mosquito-borne diseases are mostly known from literature, movies and news reports, possibly from travel to exotic countries. Malaria, yellow fever or Dengue hemorrhagic fever are problems not of our world. Except that our world is no longer as it was in Sienkiewicz’s time, and yellow fever may not be as exotic as it once was.

Climate change and the transport of goods between continents are two major forces driving the migration of species, including mosquitoes. For the past few years, there have been regular reports in the media about the presence of insects from Asia and Africa outside their natural range. Currently, they are focusing on three invasive mosquito species: the tiger mosquito(Aebes albopictus), the Egyptian (Ae. aegypti) and Ae. japonicus. All three are vectors of dangerous pathogens and have been recorded in Europe, but only the latter has been 2023. confirmed in Poland.

The European Center for the Study of the European Food Chain. The European Centre for Disease Prevention and Control (ECDC) is a member of the European Centre for Disease Prevention and Control. European Center for Disease Prevention and Control , ECDC) conducts regular monitoring of them. The constantly updated materials and extensive literature cited there [1] provide comprehensive information on their current geographic distribution, biology, ecology and threats.

Tiger and Egyptian not yet with us, but right on our doorstep – invasion of exotic mosquitoes

The most media coverage in Europe at the moment is of the tiger and Egyptian mosquitoes. The former, Ae. albopictus, is native to tropical rainforests in Asia, but has spread to Europe, North and South America and Asia over the past 30 years. His march through Europe began in 1979. in Albania, where it ended up as a result of transporting goods from China, to appear in more European countries in the following years.

Monitoring by the ECDC has shown its presence in more than 20 European countries. Of particular concern for Poland, since 2007. The species is recorded in Germany, and in recent years two independent observations have also been found in Slovakia: the first in 2012. near Košice and a second in 2023. In the densely populated municipality of Ružinov in Bratislava. During this second occurrence, the species likely established itself, increasing the risk to public health and safety. Contrary to repeated media reports, entomologists do not confirm its presence in Poland (as of 2023 [1]). So far!

The tiger mosquito is easily recognized by its contrasting coloration. It is distinguished by white or silver spots or stripes, clearly standing out against the black coloration of the body, especially visible on the legs. Studies indicate that it is a vector for 22 viruses, the most dangerous of which are dengue, yellow fever and West Nile viruses. This mosquito can adapt to moderate climatic conditions thanks to the diapause of its eggs, which enables it to passively colonize new areas. Climate warming, i.e. an increase in average annual temperature, leads to optimal conditions for the survival and spread of the species.

Egyptian mosquito Ae. aegypti is similar to the tiger mosquito (and often morphologically confused with it). It is native to Africa, but has spread to all tropical and subtropical regions of the world as a result of human activity. It was most likely brought to America and Mediterranean countries other than Egypt by ship. In the past, the species was sporadically recorded in Europe, from the Portuguese Atlantic coast to the Black Sea, but showed much larger ranges than today.

In the mid-20th century. The Egyptian mosquito disappeared from Europe (it was recorded in Spain until 1953 and in Portugal until 1956). The reduction in its range was likely due to eradication programs, including the use of DDT. Only at the beginning of this century were there reports of its re-colonization, such as in 2004. in Madeira or an incidental listing in the Netherlands in 2010. At a landfill for tires imported from the US. Fortunately, the outbreak was eliminated thanks to quick and effective action.

Unlike Ae. albopictus, the ability to settle in Ae. aegypti in regions with more temperate climatic conditions is limited due to low tolerance to lower winter temperatures and, in particular, high mortality of eggs exposed to frost. However, it looks like with the increase in average annual temperatures and warmer winter periods, it may have a chance to spread again. The most suitable for Ae. aegypti habitats are currently recognized coastal regions of the Mediterranean, Black Sea and Caspian Sea, as well as areas along large lowland rivers (Ebro, Garonna, Rhone and Po). In the future, with global climate change favoring further northward expansion of the species, this may change.

The third twin is already here!

The third invasive mosquito species recorded in Europe is Aedes japonicus, known as the Asian bush mosquito or rock pool mosquito (both translations seem severely unfortunate, but I haven’t found anything more fitting in the Polish-language literature so far). According to the group of specialists in the field. ISSG ( Invasive Species Specialist Group), the species is now on the list of the top 100 invasive species.

For us, it is important because its presence has just been confirmed in Poland. In late April of this year, in the journal Viruses, Polish scientists confirmed its listing in October 2023. in Mikolow, Lower Silesia [2]. Identification of the species was made on the basis of morphological features and then confirmed by genetic studies. The second observation, not genetically confirmed, is from the same period (October 2023) from Kielce. Thus, our native culicidofauna, numbering about 50 species (47 or 49 according to various sources), has expanded with a new exotic resident.

Aedes japonicus is larger than the previous two invasive mosquitoes, measuring as much as 6 mm in length, and is brighter and less contrastingly colored. It occurs naturally in East Asia (Taiwan, Hong Kong and Japan, as well as parts of Russia, China and the Korean Peninsula). After the first detection in France and subsequent reports from Belgium and Switzerland, it expanded its reach in Europe, taking in more areas. According to ECDC’s 2023 data. Its occurrence in Europe includes the Netherlands, Germany, the Czech Republic, Slovakia, Hungary, Austria, Switzerland, Liechtenstein, Luxembourg, Italy, Slovenia, Croatia, Spain and Romania. Presence Ae. japonicus has also been confirmed in the United States, Canada and Hawaii.

Its geographic expansion is facilitated by human activity, primarily international trade. Interestingly, the importation of used tires is often cited as a source of infestation (for other mosquito species as well) – water lingering in their crevices promotes the survival of these organisms. Aedes japonicus has shown the ability to colonize urban environments, and diurnal activity patterns increase the risk of human contact and potential transmission of diseases such as Dengue hemorrhagic fever and chikungunya.

According to the authors of the paper in Viruses after numerous publications, under controlled laboratory conditions Ae. japonicus has shown competence as a vector for a very large number of arboviruses (arthropod-borne viruses), including chikungunya virus (CHIKV), dengue (DENV), eastern equine encephalitis (EEEV), Japanese encephalitis (JEV), Cache Valley virus (CVV), La Crosse virus (LACV), Rift Valley fever (RVFV), St. Louis encephalitis (St. John’s encephalitis), and other viruses. The disease is also known as St. Louis (SLEV), West Nile fever (WNV) or Zika (ZIKV). Of course, it is important to remember that the study was conducted under controlled experimental conditions, which could have affected the results. In order to determine the actual role of the mosquito in the transmission of specific pathogens in a given area, further research and evidence collected in the field are needed.

Features conducive to invasion

The invasion success of this species can be attributed to its ability to spread over long distances and its significant tolerance to winter temperatures in temperate regions. Unlike the more thermally demanding tiger mosquito, the species can survive winter conditions by entering diapause at the egg stage. The eggs are resistant to frost and desiccation, allowing the species to survive adverse conditions and facilitating long-distance transportation. Also larvae Ae. japonicus tolerate a wider range of temperatures than the larvae of other exotic mosquitoes. On the other hand, some data suggest potential reductions in life functions in habitats with water temperatures above 30°C. This could potentially be a limiting factor in its future expansion into Southern Europe.

Model studies on the ecological niches of the tiger mosquito and the bush mosquito indicate that the habitat requirements of the two species are different [3]. Modeled temperature niches Ae. japonicus were found to be narrower with a maximum at lower temperatures compared to niches Ae. albopictus. On this basis, it is predicted that while the tiger mosquito will be promoted by climate change in Europe, the range of conditions inherent in the Ae. japonicus will decrease. This prediction is based on the assumption that the latter is unable to adapt to warmer climatic conditions.

On the one hand, this is good news, because it means that if one gets better, the other will get worse. On the other hand, the temperature niches of the two species are not fully separable, so it is possible that they will co-occur in the same areas.

Vaccination calendar to be revised?

Since the bush mosquito has been reported in our country, and its brethren, the tiger mosquito and the Egyptian mosquito, are already regularly reported in neighboring countries, should we be afraid? The whole argument is not about scare-mongering, but spreading awareness that the emergence of mosquitoes carrying pathogens hitherto unknown in our country is a fact of life, and we probably have little influence in stopping this phenomenon. Believing in the magical power of chemical de-smoking is rather illusory, as it may reduce the problem, but certainly will not eliminate it.

As the climate warms, exotic species will colonize areas that were once inaccessible to them due to excessively low temperatures during winter periods. This phenomenon is observed quite commonly with not only mosquitoes, but also many other groups of animals and plants. Our world is becoming tropicalized. It is highly likely that the currently recommended vaccination calendar in Poland will have to be expanded to include diseases that we have so far only vaccinated against before going on exotic vacations.

Photo. main: Erik Karits/Unsplash

In the article, I used, among others. From the works:

[1] European Centre for Disease Prevention and Control (ECDC). Factsheet for experts – Mosquito factsheets: https://www.ecdc.europa.eu/en/disease-vectors/facts/mosquito-factsheets (updated October 2023);
[2] Gierek M., Ochała-Gierek G., Woznica A.J. et al. (2024). Winged threat on the offensive: a literature review due to the first identification of Aedes japonicas in Poland. Viruses 16(5):703. https://doi.org/10.3390/v16050703
[3] Cunze S., Koch L.K., Kochmann J. et al., (2016). Aedes albopictus and Aedes japonicas – two invasive mosquito species with different temperature niches in Europe. Parasites Vectors 9, 573. https://doi.org/10.1186/s13071-016-1853-2