GE-I-5: Pathogen vectors – case study

Warmer climatic conditions can favour the establishment and spread of the Asian Tiger Mosquito Aeoles albopictus in Germany. This increases the risk that the bite from a mosquito can lead to the transmission and subsequent spreading of dangerous viral diseases by pathogens introduced to Germany by infected persons arriving from abroad. The finds of eggs and mosquitoes in traps in the area of the Upper Rhine area have increased distinctly over the past 15 years.

Dangerous Tiger Mosquitoes are spreading

Worldwide we are confronted with new and recurring fomites (infection agents) which in many cases can be transmitted between animals and humans; owing to their increasing global mobility they are able to spread rapidly. Both long-term climatic changes (temperature, precipitation) and the increase in extreme weather conditions play important roles in this process. In vector-transmitted infectious diseases such as West Nile Fever, malaria, dengue, leishmaniosis, zika, chikungunya or tick-borne encephalitis (FSME) there is the risk that in Germany the changed climatic conditions will enhance favourable conditions for animal vectors such as mosquitoes or ticks as well as pathogens, thus increasing the risk of infection in respect of humans and animals. For example, in recent years there have been cases in central eastern Germany where humans were infected by the autochthonous West Nile Virus (WNV) which is transmitted by indigenous mosquitoes (Culex).

The mechanisms of absorption, development and reproduction of pathogens in vectors, and the transmission to animals and humans remain to be clarified in the majority of cases. Changed climatic conditions can influence this interaction of pathogens and vectors in various situations. Changed climatic conditions can lead to changes, for instance, in the reproduction rate of animal vector organisms, their lifespan, their behaviour or their population density²⁵. Besides, short winters may result in the animals being active for longer in the course of the year, thus reproducing faster and producing more generations. Their efficiency in transmitting pathogens can also be affected. This can cause vector species previously not indigenous to Germany – introduced from warmer countries – to become established here and disperse widely.

The exploration of relationships between climate change on one hand, and the spreading of vectors / pathogens on the other, is still in its infancy. The recording of most infectious diseases associated with vectors is already being carried out systematically – in most respects on a nationwide basis – and covered by regulations laid down in the German Infection Protection Act (for instance. compulsory registration). However, there is nonetheless still a shortage of data collected systematically and continuously on the occurrence and distribution of vector species and their infection with pathogens. The illustration of the indicator is therefore limited to the example of one specific vector, namely the Asian Tiger Mosquito (Aedes albopictus), a mosquito species that was originally introduced from Southeast Asia. This species is considered a highly efficient vector which can transmit more than 20 different viruses.

The Tiger Mosquito which emanates from a species variant successfully adapted to non-tropical conditions in the USA, has meanwhile achieved wide distribution in southern Europe and also in parts of Central Europe. In recent years there have been frequent finds of eggs, larvae and adult individuals of this species in Germany. According to the current state of knowledge, the introduction takes place by means of vehicular traffic from the south (for instance from Italy). In areas where the Tiger Mosquito comes upon favourable conditions, it is able to establish colonies, and those populations can then also become the source of further passive (anthropogenic) introductions elsewhere. Of particular benefit to the establishment of the Tiger Mosquito is its invasion of areas where the immediate environment offers adequate breeding sites, blood hosts and safe havens such as allotment garden environments and housing zones with a high proportion of garden space.

In respect of the chikungunya virus it has been possible to show already that transmission by Aedes albopictus is, also in Germany, less limited by external temperatures than (especially) by an inadequate occurrence of mosquitoes²⁶. For the zika virus, laboratory tests have shown that the vector competence of Aedes albopictus is distinctly boosted by temperatures of 27 °C as against lower temperatures of 18 °C²⁷. In fact, the establishment of these mosquitoes has created the basic prerequisites for this pathogen to spread more widely in Germany too, provided it is introduced by infected individuals.

The region of the Upper Rhine plain in Germany is favoured by warm climatic conditions. This region also plays a major role as an important entrance point to Germany for thermophilic species via vehicular traffic from neighbouring countries, including Switzerland and Italy. Since 2005, there have been ongoing records of the occurrence of Tiger Mosquitoes in the Upper Rhine area, starting with the first record in 2007. This finding resulted from examining 105 traps where evidence was found in terms of five Tiger Mosquito eggs in one of more than one thousand samples. After a break in monitoring in 2010 and 2011, followed by the installation of new types of traps in 2012, findings were again positive: a total of eight individuals were trapped which means that one percent of all trap samplings was positive. From 2012 onwards, the number of samplings was expanded, leading to annually approximately 1,500 samplings in the Upper Rhine area from 2014 onwards. As early as 2013, 13 % of all traps and more than 2 % of all samplings resulted in evidence of eggs or adult mosquitoes. Subsequent years produced further increases in the number of positive finds. As early as 2014 approximately in 18 % of traps, and in 2017 approximately in 34 % of traps along the A5 and A6 motorways, evidence was found for Aedes albopictus. By the end of 2021, there were already well-established populations in 14 rural districts and in district-free towns along the river Rhine and in Fürth (Bavaria) as well as Jena (Thuringia)²⁸. In fact, the establishment of these mosquitoes has created the basic prerequisites for this pathogen to spread more widely in Germany too, provided it is introduced by infected individuals.

Since 2021, there have been no further resources available for monitoring work in the Upper Rhine area. It is true that the ZALF and the FLI continue to collect data on the distribution of mosquitoes for the ‘Mückenatlas’ (mosquito atlas) (cf. Indicator GE-R-4) while also conducting geographical campaigns for data collection by means of placing traps alternately in different locations. However, these surveys are not yet sufficiently standardised for establishing a time series.

^{25 - Beermann S, Dobler G, Faber M, Frank C, Habedank B, Hagedorn P, Kampen H, Kuhn C, Nygren T, Schmidt-Chanasit J, Schmolz E, Stark K, Ulrich RG, Weiss S, Wilking H: Auswirkungen von Klimaveränderungen auf Vektor- und Nagetier-assoziierte Infektionskrankheiten. – J Health Monit 8 (S3): 36-66. doi: 10.25646/11392.}

^{26 - Heitmann A., Jansen S., Lühken R., Helms M., Pluskota B., Becker N., Kuhn C., Schmidt-Chanasit J., Tannich E. 2018: Experimental risk assessment for chikungunya virus transmission based on vector competence, distribution and temperature suitability in Europe. Euro Surveill., 23(29): 1800033.
doi: 10.2807/1560-7917.ES.2018.23.29.1800033}

^{27 - Heitmann A., Jansen S., Lühken R., Leggewie M., Badusche M., Pluskota B., Becker N., Vapalahti O., Schmidt-Chanasit J., Tannich E. 2017: Experimental transmission of Zika virus by mosquitoes from central Europe. Euro Surveill. 22(2): 30437. doi: 10.2807/1560-7917.ES.2017.22.2.30437}

^{28 - Informationen des FLI – Friedrich-Loeffler-Institut zur Nationalen Expertenkommission „Stechmücken als Überträger von Krankheitserregern“: https://www.fli.de/de/kommissionen/nationale-expertenkommission-stechmuecken-als-uebertraeger-von-krankheitserregern/}

GE-I-5: Pathogen vectors – case study | Umweltbundesamt