We’ve all heard that climate change is affecting our environment in a myriad of ways. But what is the impact on mosquito-borne diseases that afflict tropical and subtropical countries such as dengue fever? In the first long-term study, a research group found that temperature, rainfall, and access to piped water affect dengue incidence in Mexico and they predict that with climate change cases will climb up to 40 percent by 2080.
Dengue fever – caused by a virus transmitted through mosquito bites – is ominously spreading to new areas: prior to 1970 only nine countries had reported epidemics but now over 100 countries are grappling with the disease. According to the WHO, dengue incidence rose 30-fold in the last 50 years and now over 40 percent of the world’s population is at risk.
Aedes aegypti, the mosquito responsible for transmitting dengue fever, having a human blood meal.
Image: Wikipedia
Unfortunately, no cure exists for dengue and the only way to treat it is through early detection. It is best to avoid getting infected in the first place by protecting yourself from getting bitten and through the control of dengue-transmitting mosquitoes.
Scientists conducted the first study covering the all the diverse climatic regions in Mexico using provincial dengue incidence data from 23 years until 2007. They studied the effect of temperature range, rainfall, GDP, urbanization, and access to piped water on dengue incidence in all provinces of Mexico.
Researchers used statistical models (using generalized additive modeling regression) to estimate how changing climate – such as an increasing rate of temperature rise and irregular rainfall – may affect dengue incidence in Mexico in 2030, 2050, and 2080 in three scenarios. The first scenario is characterized by rapid economic growth, a mid-century peak in global population, and introduction of more efficient technologies. The second scenario features regional economic development, continually rising global population, and slower technological growth. Finally, the third scenario envisions a more service-based economy, population growth similar to the first scenario, and more resource-efficient technologies.
Mexico has both tropical and subtropical areas with varying climates making predictions applicable to other regions with similar climates around the world.
They found that minimum temperature played an instrumental role in dengue incidence, with almost zero cases below 5° Celsius, and a substantial rise in cases when the minimum temperature rose to above 18°C. As the maximum temperature surpassed 20°C, cases continued to rise until 32°C – possibly their highest transmission efficiency – after which they began to fall.
Dengue incidence rose with increasing rainfall until 550mm, after which they declined. They suggest that with higher rainfall more outdoor water-filled areas are available as breeding sites for mosquitoes to lay their eggs. But when rainfall becomes too high, water washes out of the areas, so mosquitoes may not proliferate easily.
An interesting finding that was in contrast with other findings was that increased access to piped water was associated with an increase in dengue cases. The reasons for this are unclear and complex but may be due to increased storage of piped water possibly due to erratic delivery of the piped water, and this may facilitate breeding sites for dengue mosquitoes.
The relationship between average monthly dengue incidence (cases per 100,000) and (A) minimum temperature, (B) maximum temperature, (C) rainfall, and (D) the proportion of the population with access to piped water.
(Colón-González, Lake, and Hunter 2013)
The model revealed that climate change overall caused average annual dengue cases to surge by up to 18 percent by 2030, 31 percent by 2050, and as high as 40 percent by 2080, while holding other factors constant. The impact of the second scenario was most severe in 2080. Interestingly, they noticed that dengue incidence rose sharply in provinces where dengue is endemic or constantly transmitted whereas it was subdued in epidemic-prone provinces where transmission is sporadic. While overall incidence increases, provinces north of the Yucatán Peninsula and in the northwest may experience a drop in dengue incidence presumably due to lower rainfall.
A 40 percent rise in dengue incidence translates to around 7,000 more cases of dengue fever per year in Mexico. But the authors suggest that there will probably be far more cases – going into hundreds of thousands – because for each official case it is estimated that there are up to 27 cases unreported. Even worse, taking into account asymptomatic infections, the incidence may be even higher. The CDC estimates that half of all dengue-infected individuals are asymptomatic.
However, on the brighter side we may develop effective strategies to adapt to and alleviate the effects of climate change, so the projected rise may not be as steep as this model predicts.
Nevertheless, this study shows that dengue incidence in Mexico is greatly affected by increasing temperature, rainfall, and access to piped water. We can control dengue transmission by limiting breeding sites for mosquitoes through minimizing storage of water both indoors and outdoors. But it will take a concerted worldwide effort to combat increasing temperatures that may have potentially dire consequences on the spread of mosquito-borne diseases – and dengue fever is just one of them.
Reference
Colón-González FJ, Fezzi C, Lake IR, Hunter PR (2013) The Effects of Weather and Climate Change on Dengue. PLoS Negl Trop Dis 7(11): e2503. doi:10.1371/journal.pntd.0002503
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