Due to climate change, two species of tropical mosquitoes are spreading into Europe at increasing rates. Their presence is linked to a range of tropical viruses, including dengue, yellow fever, Chikungunya and Zika. With an increase in disease-ridden insects, we’re likely to start seeing these infections at higher rates in European countries, in Belgium and beyond.
Deadly disease vectors
A group of international scientists, including researchers from the Institute of Tropical Medicine (ITM) in Antwerp, have been studying the spread of Aedes aegypti (the yellow fever mosquito) and Aedes albopictus (the tiger mosquito).
Aedes sp. get their scientific Genus name from the Ancient Greek word ἀηδής, meaning "unpleasant". The name is an apt description: both species are considered arbovirus vectors, transmitting deadly diseases, including dengue, yellow fever, Chikungunya and the dreaded Zika virus.
Contracting any of these diseases is no laughing matter: an infection in humans typically results in symptoms including fever, headaches, achiness and dizziness. Some of the viral strains, such as Chikungunya and Zika, can also result in a severe skin rash. Zika also results in birth defects, even when the infected mother shows no symptoms herself. Most arbovirus infections can also lead to death, most commonly as a result of meningitis and organ failure.
For some of these diseases, like yellow fever and dengue, there are vaccines to be had. However, the vaccines often cause problems of their own (dengue vaccine can trigger deadly reactions) and for others, like Zika and Chikungunya, there is still no vaccine and no available treatment at all.
Over the last decades, the tiger mosquito’s estimated rate of expansion in Europe is about 100 km per year. - Wim Van Bortel, ITM
An article published earlier this year estimated that shifts in Aedes distribution caused by climate change may result in up to half a billion more people being exposed to these viruses by as early as 2050. The disease burden on European and global healthcare systems would be enormous.
A warmer world means more tropical mosquitos
Originally, Ae. aegypti mosquitoes were restricted to their native Africa, but they are now found in tropical, subtropical and even temperate regions all around the world. Similarly, although Ae. albopictus originated in Southeast Asia, their range has recently increased to include Europe, the Americas, Africa and the Middle East.
Image: Aedes albopictus (James Gathany, Centers for Disease Control and Prevention's Public Health Image Library (PHIL)).
The reason for this spread is clear, according to researchers: it is largely driven by both global mobility by humans and the presence of suitable climate. In other words: as global temperatures rise, the warm regions, where tropical mosquitoes like Ae. aegypti and Ae. albopictus thrive, are expanding. Their ranges are becoming more northerly and people who were once protected from tropical diseases will become susceptible.
By creating a series of predictive maps, researchers have established a range of areas where these insects are likely to become increasingly common in the near future. The maps, published in Nature Microbiology last month, were based on projections as to how the mosquitoes adapt to alterations in the environment caused by climate change. They also used data about intercontinental passenger and freight transport to create models simulating the distribution of mosquitoes.
The researchers were able to predict the most likely spread of Aedes aegypti and Aedes albopictus for the years 2020, 2050 and 2080, based on three different greenhouse gas emission scenarios. ITM researcher Dr. Wim Van Bortel commented:
“The study shows, among other things, that the tiger mosquito could become widely established in our region. This certainly demonstrates the importance of monitoring exotic mosquitoes in Belgium."
Of course, a few tiger mosquitoes in Belgium does not necessarily mean that the country is on the verge of a viral outbreak. But, if the mosquito population increases, so does the likelihood of local infections. Tropical viruses such as dengue and Chikungunya have, in the past, usually entered European countries only via infected travelers arriving from abroad. However, with more tropical mosquitoes in our midst, the diseases may now start establishing themselves in the local European population. France and Italy have both experienced outbreaks in recent years. Dr. Van Bortel agreed; the risks are severe:
"Over the last decades, the tiger mosquito’s estimated rate of expansion in Europe is about 100 km per year. Our recent findings in parking lots in the south of Belgium are probably a first indication that the mosquito has reached our country from regions where it is already firmly established."
Killing a killer bug
It’s not all bad news, however: using the predictive maps created by the ITM team, we should be able to improve successful vector control activities and hopefully stay a few steps ahead of the bugs.
The tiger mosquito could become widely established in our region. This (study) certainly demonstrates the importance of monitoring exotic mosquitoes in Belgium. - - Wim Van Bortel, ITM
Because arbovirus diseases are carried and transmitted by mosquitoes, controlling the insect populations is currently thought to be the best method for preventing infection. Traditional methods for pest control have included source reduction (getting rid of stagnant waterbodies where the mosquitoes breed), pesticide spraying (both to kill larvae and adult mosquitoes) and the use of mosquito traps.
There are also other approaches under development: a bacterium called Wolbachia is being used as a method of biocontrol (the bacterium renders the mosquitoes immune to arboviruses); another method uses genetic modification to suppress mosquito populations altogether. Genetically modified mosquito strains are being engineered to express a self-limiting gene that prevents female offspring from surviving. Male modified mosquitoes, which do not bite or spread disease, are released to mate with the wild females. Their offspring inherit the self-limiting gene and females die before reaching adulthood. This genetic technique, called a gene-drive eventually leads to a complete population crash.
Image: How gene drives spread through a population, with genetically modified males (red) mating with wild-type females (blue) until there are no females left and the population crashes (Mariuswalter, Wikimedia commons).
Both the Wolbachia and gene-drive methods have widespread support, ranging from smaller companies like Oxitec to large institutions and conglomerates like the Gates Foundation and Alphabet Inc. (Google’s parent company). Field trials have already been conducted in South America, but the programs have yet to be rolled out on a large scale, as there are still concerns about the future implications of the methods.
In the meanwhile, here’s hoping forewarned is indeed forearmed: that these maps and predictions published by ITM lead to the development of strategic control programs that help us eliminate the serious arbovirus threat to global public health.
Cover Image: Aedes aegypti (Muhammad Mahdi Karim, Wikimedia Commons).