Dengue fever is now the most significant disease [Paywall] worldwide which is transmitted by arthropods, such as insects, ticks or arachnids. According to WHO data the number of infections rose dramatically in the last decades, with an estimated 390 million dengue cases per year. The dengue virus’ principal vector is the yellow fever or dengue mosquito Aedes aegypti, while the Asian tiger mosquito Aedes albopictus is a second, less effective vector.
Studies have already shown that rising temperatures in Europe may favour the spread of tropical diseases such as dengue and Chikungunya fever. The facts speak for themselves: In Greece in 2010 malaria cases appeared for the first time since 1974; in 2007 in northern Italy an outbreak of Chikungunya fever occurred – and in 2012 the first major dengue outbreak in Europe was seen since the 1920s; on the Portuguese island of Madeira about 2,000 people were affected.
Temperature fluctuations play a role
A research team led by Jing Liu-Helmersson of Sweden’s Umeå-University has now, using simulation calculations based on the increase in average temperatures and daily temperature variations, made the first predictions of dengue fever events in 10 European cities. The team looked at the so-called vector capacity: that is, the ability of mosquitoes to transmit the virus from person to person. In their analysis, the researchers considered actual and predicted temperatures from the years 1901 to 2099 and set various assumptions based on climate change – from a moderate to a strong increase in average temperatures.
“Both a warmer climate and greater temperature fluctuations affect the ability of Aedes mosquitoes to transmit dengue fever”, explains Jing Liu-Helmersson and her team. Higher temperatures promote the propagation and transmission of dengue viruses and cause female mosquitoes to bite more frequently. Simultaneously, the time window during each year when dengue fever is able to be transmitted will expand.
Dengue epidemics likely in southern Europe
The scientists’ calculations showed that dengue epidemics in Southern Europe in the summer would become likely as soon as Aedes-mosquitoes occur there. Accordingly, the probability is high that, in cities like Athens, Rome, Malaga or Nice, dengue outbreaks would occur. By way of increasing global warming, the risk areas could over time shift to the north, and extend the time window for the virus transmission. “Towards the end of the 21st century there could be seasonal dengue outbreaks throughout the whole of Europe in those regions where Aedes mosquitoes occur”, the researchers write.
Aedes albopictus mosquitoes are spread throughout southern Europe and occur all the way to the Netherlands. In contrast, Aedes aegypti mosquitoes, the principal vector of dengue virus, have only been recorded in Russia and Georgia. However, up to the 1950s the mosquitoes were widespread in European countries such as France, Spain or Portugal. Studies also predict that Aedes aegypti mosquitoes will spread by 2080 into the coastal regions of Europe. “Therefore, it is likely that Aedes mosquitoes will become an integrated part of the fauna in Europe”, says Liu-Helmersson.
In addition, dengue virus could increasingly be brought to Europe by international travellers, migrants and transported goods and introduced there into new regions. What’s more increasing urbanisation and a changing use of land might help [Paywall] tropical diseases spread in Europe.
Strategies to combat climate change could stop dengue
Nevertheless, the spread of dengue fever in Liu-Helmersson’s study greatly leans on one specific influence factor: emission rates and the associated increase in temperature. “This means that measures to reduce greenhouse gases are very important. They could greatly influence the spatial and temporal spread of dengue fever”, the scientists write.
The research team’s results also make it clear that effective measures to control the vector mosquitoes need to be developed. “The dengue outbreak in Madeira was a kind of wake-up call that something needs to be done in Europe,” says Liu-Helmersson.
Developing early warning systems
In order to detect regional outbreaks in time, the spread of tropical viruses and the vector mosquitoes should be continuously monitored, stresses a research team led by Francis Schaffner of the University of Zurich. There are in fact a number of institutions monitoring infectious diseases in Europe. However, they are currently not sufficiently prepared for the arrival and spread of tropical pathogens in Europe, says Jan C. Semenza from the European Centre for Disease Prevention and Control (ECDC) in a review article.
Several organisations, such as the European Environment and Epidemiology (E3) Network, which belongs to the ECDC, are already investigating which factors influence the spread of tropical diseases in Europe. For example, climate and weather, statistics on travellers and data from health registries are included here. Thus, the researchers calculated in one case study the probability of dengue fever being brought into Europe by international travellers. Such analyses can help in the development of early warning systems to predict the risk of dengue outbreaks. “This could serve as a basis to ensure the undertaking of swift preventive or mitigating measures”, says Semenza.
International cooperation important
One possible counter measure would be, for example, to eradicate the vector mosquitoes at an early stage after their introduction, explain Schaffner and his team. “When the mosquitoes have already spread, similar strategies to those in tropical countries could be successful”, say the scientists. “On the one hand the fight against mosquitoes, on the other hand the education of the public, for instance with regard to mosquito protection measures and the symptoms of the disease”.
The decisive factor in all of this is that European countries coordinate their activities. Thus, several countries in the Mediterranean area have already initiated measures, so as to prepare for the increasing risk of insect-borne diseases. Nevertheless, the measures are often still at a very elementary level, writes a research team led by Maya Negev at Israel’s Haifa University. “Here, the various countries should increasingly work together, to share information and coordinate their actions – at best under the scope of a politically neutral parent organisation”, says Negev and her team. “This is because the carriers of infectious diseases recognise no political boundaries”.
Climate Change and Aedes Vectors: 21st Century Projections for Dengue Transmission in Europe
Jing Liu-Helmersson et al.; EBioMedicine, doi: 10.1016/j.ebiom.2016.03.046; 2016