GE-I-4: Pathogen carriers – case study
2019 Monitoring Report on the German Strategy for Adaptation to Climate Change
2019 Monitoring Report on the German Strategy for Adaptation to Climate Change
Warmer climatic conditions can favour the establishment and spread of the Asian Tiger Mosquito in Germany. Thishas created the basic prerequisites for this pathogen to extend its range here in Germany provided it is introduced by infected individuals. The evidence of eggs and mosquitoes caught in traps as well as positive samplings in the Upper Rhine area have shown a distinct increase.
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 very fast. Both long-term climate change (temperature, precipitation) and the increase in extreme weather conditions play important roles in this process. In vector-transmitted infectious diseases such as malaria, dengue, leishmaniosis, zika, chikungunya or early-summer meningoencephalitis (FSME) there is the risk that in Germany changed climatic conditions will enhance favourable conditions for animal vectors such as mosquitoes or ticks, thus increasing the risk of infection for humans and animals. This goes to show the close links between the health of humans, animals and their environments (One Health).
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 entail changes e.g. in the reproduction rate of animal vector organisms, their lifespan, their behaviour or their population density. Their efficiency in transmitting pathogens can also be affected. Short winters may entail that the animals are active for longer in the course of the year, reproduce faster and produce more generations. This can cause vector species previously not indigenous to Germany, introduced from warmer countries, to get established here and disperse widely.
The examination of relationships between climate change and vector or pathogen dispersal is still in its infancy. The recording of most infectious diseases associated with vectors is already carried out systematically and in most respects on a nationwide basis, but owing to the regulations laid down in the German Infection Protection Act (e.g. compulsory registration), there is nonetheless a shortage of data on occurrence and distribution of vector species and their infection with these pathogens. The illustration is therefore limited to the example of one vector, i.e. the Asian Tiger Mosquito (Aedes albopictus), a mosquito species that was originally introduced from Southeast Asia. It 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 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 (e.g. Italy). In areas where the Tiger Mosquito comes upon favourable conditions, it is able to establish and spread. Of particular benefit to the establishment of the Tiger Mosquito is its release in areas where the immediate environment offers adequate breeding sites, blood hosts and safe havens such as allotment garden areas 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 Ae. albopictus is, also in Germany, not much limited by external temperatures but especially by an adequate occurrence of mosquitoes7. For the zika virus laboratory tests have shown that the vector competence of Ae. albopictus is distinctly boosted by temperatures of 27°C as against lower temperatures of 18°C8 . The establishment of these mosquitoes has created the basic prerequisites for this pathogen to spread widely in Germany too, provided it is introduced by infected individuals.
The Rhine Plain in Germany is favoured by warm climatic conditions. It also plays a major role as an important entrance point to Germany of vehicular traffic for thermophilic species from neighbouring countries (e.g. Switzerland and Italy). There has been ongoing recording of the occurrence of Tiger Mosquitoes in the Upper Rhine area since 2005. Evidence was first found in 2007. This was done by examining 105 traps when evidence was found for five Tiger Mosquito eggs in 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 found which means that one percent of all trap samplings were positive. From 2012 on, the number of samplings was expanded, leading to annually c. 1,500 samplings in the Upper Rhine area from 2014 onwards. As early as 2013 13% of all traps and c. 2% of all samplings resulted in evidence of eggs or adult mosquitoes. Subsequent years produced further increases in the number of positive findings. Already in 2014 approximately in 18% of traps and in 2017 approximately in 34% of traps along motorways evidence was found for Ae. albopictus. Besides, in Baden-Württemberg there are reports of at least four established populations. In locations such as Heidelberg and Freiburg evidence has been found for three consecutive hibernations from 2015 to 2016, 2016 to 2017 and 2017 to 2018. Since 2015 efforts have been ongoing to develop and enhance a monitoring system for mosquitoes9 .
Nevertheless, the infestation with Ae. albopictus is lower in Germany than in southern Europe. Likewise, the warm-weather periods are still shorter in Germany. Even in southern Europe, only rare individual cases and small clusters of transmissions of dengue and chikungunya virus were identified apart from two chikungunya virus outbreaks, It is therefore considered that although the risk of individual transmissions in Germany cannot be dismissed, there seems to be limited danger of major outbreaks.
7 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, 2018. Euro Surveill. 2018;23(29):pii=1800033.
8 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. 2017;22(2):pii=30437.
9 Since 2015 Germany has been developing continuous, active monitoring of gnats/mosquitos occurring in Germany. This process involves setting up specific gnat traps and the separate collection of larval forms which covers the geographic and seasonal occurrence of gnat species and the associated transmission of animal and human pathogens in Germany. Also since 2015, the Citizen-Science research project ’Mückenatlas’ (gnat atlas) has been conducted (see also https://mueckenatlas.com). By spring 2019 more than 22,000 participants had captured more than 120,000 gnats for scientific purposes and sent these to experts working at ZALF (the Leibniz Centre for Agricultural Landscape Research) and at the Friedrich Loeffler Institute. This made it possible to produce evidence of numerous additional specimens of the Asian Tiger Mosquito occurring away from motorways and even to identify populations which hibernated in Thuringia, Heidelberg and Erding).
At both Federal and Länder level, additional data ought to be collected and analysed in order to recognise epidemiologic developments in Germany in time, to understand their causes and relationships, to improve risk assessments and to develop preventative and intervention strategies. (DAS, ch. 3.2.1).