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Public health authorities are preparing for future threats and are projecting the likely impact of climate change on public health risks. In doing so, it is easy to get overwhelmed by the volume of heterogeneous information in scientific articles and public opinions. During the last fourteen years, working as an entomologist in the Public Health domain, I frequently participated in discussions among various stakeholders. In each instance, participants were passionate, but not necessarily talking about the same thing. The reason is that both vector-borne diseases and climate change are broad concepts. Therefore, important questions to ask ourselves are i. What are we talking about? ii. What do the numbers tell us? and iii. What are probable causes of the changing numbers?

What are we talking about?

Vector-borne diseases are caused by pathogens that are transmitted by blood sucking arthropods. Globally, the burden in humans is dominated by diseases transmitted by mosquitoes, especially malaria and dengue, followed by those transmitted by ticks, sandflies and to a lesser degree by triatomine bugs, lice, fleas and mites. In livestock, productivity loss is mainly caused by pathogens transmitted by biting midges. In total, vector-borne diseases comprise well over a hundred diseases that are caused by viruses, bacteria, protozoa, and helminths; many are zoonoses. The intricate biological processes underlying the interactions between pathogens, vectors and their vertebrate hosts make the predictability of disease risk a risky business and very difficult. [1]

Climate change can be defined as any significant long-term change in the expected patterns of average weather of a region or the entire world over a significant period of time. In current debate, climate change is synonymous with global warming, whereas climate change also involves rising sea levels, increasing UV radiation, changes in rainfall and wind patterns and consequently relative humidity. Climate change also impacts land use and land cover, crop suitability and agricultural patterns and human behaviour.

Since an important part of the life cycle of pathogens takes place outdoor in cold-blooded vectors, global changes including climate change can lead to changes in the transmission dynamics of the pathogens with the ultimate consequence of a change in disease risk. [2] However, without explicitly specifying ‘which disease, where, when and how’, making generalized statements that go further than ‘climate change affects vector-borne diseases’ can be misleading. A contextual approach is needed to understand climate and human health and to develop public health strategies. [2] To set a good example, in the following, I choose to discuss the case of dengue in Brazil.

What do the numbers tell us?

In their “Dengue – Epidemiological Update of 13 September 2019”, the Pan American Health Organization reported that in Brazil, between epidemiological week 1 and 34 of 2019, a total of 1,439,471 probable cases of dengue were reported, of which 1,015,124 cases were laboratory-confirmed. Among the confirmed cases, there were 591 deaths, and 486 deaths remain under investigation. The incidence rate is about 6 times higher than in the same period of 2018.[3]

Since the large year-to year variation in dengue incidence is characteristic for endemic countries, long-term surveillance data are necessary to see whether 2019 was an exceptional epidemic year. Nunes et al. showed that over the last 30 years Brazil (1986-2015) experienced extensive epidemics, characterized by emergences and re-emergences of different dengue serotypes and a strong increase in the number of severe and fatal cases. [4] The latter is illustrated by the case numbers (fatalities) of the main epidemic year in each of the three decades: 104,399 (0) in 1991, 696,472 (159) in 2002 and 1,649,008 (986) in 2015. While the numbers for 2019 are comparable to those of 2015, the long-term data show a dramatic increase of cases of dengue in Brazil during the last three decades.

What are the probable causes of the changing numbers?

Dengue epidemiology is determined by the interaction between the virus, mosquito vector and humans, and we need to look for its drivers and for changes therein. The dengue virus has four related but antigenically distinct serotypes (DENV 1-4), each having several different genotypes. Viral genotype and serotype, and the sequence of infection with different serotypes, are known to affect disease severity, also known as antibody-dependent enhancement of infection. [5] The mosquito species responsible for vectoring the vast majority of dengue cases in the world, including Brazil, is the yellow fever mosquito Aedes aegypti. This mosquito species feeds almost exclusively on humans and thrives in urban settings. [1] Many factors have been identified as having contributed to the emergence of epidemic dengue. In 2011, Gubler recognized three principal drivers: 1) urbanization, 2) globalization, and 3) lack of effective mosquito control. [6]

Between 1986 and 2015, the percentage of Brazil’s population living in cities grew from 70.7% to 85.8%.[7] In the same period, the total Brazilian population grew from 138 to 206 million inhabitants. [7] As a result, the number of people living in cities nearly doubled (98 to 176 million) in 30 years. Scaling theories suggest that global urbanization is driven by a phenomenon known as superscaling, in which doubling the size of a city increases wealth and innovation by approximately 15% per capita. [8] However, besides positive aspects, there are negative influences as well, such as increases in the amount of garbage, AIDS and possibly dengue. [8]

Second, globalization has implications for the way in which countries interact with each other and for people’s behaviour. The Dengue virus has also globalized as people travel more than ever before. In Brazil, this has resulted in the co-circulation of the four dengue virus serotypes and the increasing occurrence of severe and fatal cases. [4,5]

Third, like elsewhere in the world, Brazil lacks effective mosquito control strategies. [6] Shortly after World War II, the use of synthetic insecticides against larval and adult mosquitoes grew in importance. [1] At the same time, it became apparent that mosquitoes quickly develop resistance to any newly developed insecticide, leaving the vector controllers nowadays largely empty handed. [9] This has led to a (re) appreciation of population reduction or replacement by sterile insect techniques, which are neither easy nor cheap and have their own challenges.

More recently, the possible influence of climate change on the emergence of dengue worldwide has entered the discussion. This may be due to the fact that the temperate world has been confronted with the incursion of invasive mosquitoes. [10] However, when focusing on Brazil, Aedes aegypti has been inhabiting Brazilian cities in vast numbers, ever since the investments in vector control programs were abandoned in the mid-1970s. In addition, the effect of the increase in temperature due to climate change has been most pronounced in temperate areas where temperature is a limiting factor. For the biggest part of Brazil, this is not the case. The impact of climate and climate change, including precipitation patterns, on the geographic distribution of dengue is still uncertain and needs to be investigated.[11]

What about the netherlands?

Higher summer temperatures in Netherlands will likely increase the risk of the emergence of mosquito-borne diseases. However, other factors, particularly anthropogenic ones, will undoubtedly play more important roles. For example, neither the climate nor the change therein plays a significant role in the introduction of Aedes albopictus or Asian tiger mosquito in the Netherlands. These mosquitoes are introduced via imported goods, and prevention is currently focused on this type of incidental introduction. Kraemer and colleagues state that the distribution of the main dengue vector, the Aedes aegypti and its relative Aedes albopictus, over the next 5-15 years is predicted to occur independently of extensive environmental changes, as both species continue to expand into their anthropogenic ecological niches through spatial dispersal. [10] The increase of imported dengue in Europe is predominantly due to the fact that global dengue incidence was high (see above) and therefore also the related risk (of the ever increasing number) of international travellers importing the virus into Europe. [12]

In conclusion, while climate change currently dominates the public debate, other global and local changes are more important when it comes to the spread of dengue.

References

  1. Braks M, Giglio G, Tomassone L, et al. Making vector-borne disease surveillance work: new opportunities from the SDG perspectives. Front Vet Sci. 2019; 6: 232.
  2. Braks M, van Ginkel R, Wint W, et al. Climate change and public health policy: translating the science. Int J Environ Res Public Health 2013; 11(1):13-29.
  3. PAHO. Epidemiological Update Dengue: 13 September 2019. https://www.paho.org/hq/index.php?option=com_docman&task=doc_download&gid=50321&Itemid=270&lang=en.
  4. Nunes PCG, Daumas RP, Sánchez-Archila JC, et al. 30 years of fatal dengue cases in Brazil: a review. BMC Public Health 2019;19(1):329.
  5. Katzelnick LC, Gresh L, Halloran ME, et al. Antibody-dependent enhancement of severe dengue disease in humans. Science 2017:358(6365):929-32.
  6. Gubler DJ. Dengue, urbanization and globalization: the unholy trinity of the 21st century. Trop Med Health 2011; 39(4 Suppl):3-11.
  7. Gapminder. [2 October 2019]. www.gapminder.org
  8. West G. Scale: the universal laws of life and death in organisms, cities and companies. 2019, London, UK.: Weidenfeld & Nicolson.
  9. Salgueiro P, Restrepo-Zabaleta J, Costa M, et al. Liaisons dangereuses: cross-border gene flow and dispersal of insecticide resistance-associated genes in the mosquito Aedes aegypti from Brazil and French Guiana. Mem Inst Oswaldo Cruz 2019;114: e190120.
  10. Kraemer MUG, Reiner RC Jr, Brady OJ, et al. Past and future spread of the arbovirus vectors Aedes aegypti and Aedes albopictus. Nat Microbiol. 2019;4(5):854-63.
  11. World Health Organization. Quantitative risk assessment of the effects of climate change on selected causes of death, 2030s and 2050s. WHO Geneva, 2014.
  12. European Centre for Disease Prevention and Control. Autochthonous cases of dengue in Spain and in France. Rapid Risk Assessment. 2019: Stockholm.