Mosquitoes on the move: climate change and its effect on vector-borne diseases

a_albopictus

If you live in Florida, like I do, you come to truly fear and loathe these guys: the Aedes albopictus, better known as the tiger mosquito.  Or any other mosquito, for that matter.  In the US, we actually have it pretty easy – we are usually only concerned about the annoyance of bug bites and the subsequent itchiness.  Other parts of the world, usually in hot and humid places in Southeast Asia, Central America, and the Western Pacific, mosquitoes carry vector-borne diseases, such as yellow fever, West Nile, and dengue fever, thus carrying much more clear and present danger with them.

Now, a new study in BMC Public Heatlh suggests that countries in northern latitudes may need to further their public health preparations for a dengue invasion.  Bouzid et al have developed a model predicting dengue incidence in Europe taking into account the increased temperature and humidity due to climate change.  Mosquitoes love that climate, so these increased temperatures will likely make it easier for them to survive, especially near water.  The results show a significantly increased risk of dengue infection in Europe, but all this risk is localized on the coasts of the Mediterranean and Adriatic Seas as well as the northern part of Italy.  Let’s look a little deeper into these findings…

Two breeds of mosquito, A. albopictus and A. aegypti, carry dengue – the blue in the map shown above indicates areas infested with A. aegypti and the red indicates endemic dengue fever locations:

Figure courtesy of safety.worldnomads.com

Figure courtesy of safety.worldnomads.com

Mild dengue can lead only to a fever and muscle aches, but a more severe, hemorrhagic variant has led to about 12,000 deaths globally per year.  This is a major public health concern, and one that is increasing in northern latitudes and more cases are being seen in the southern US and Europe.

To investigate how climate change will affect this trend in Europe, Bouzid et al used a Generalized Additive Model (GAM), similar to a linear regression model that determines significant relationships between input predictor variables and outcomes, but also accounts for nonlinearity between variables.  But Europe is currently cool enough that dengue is not largely present, so the authors needed to figure out a way to determine significant relationships between temperature/humidity (predictors) and dengue incidence (outcomes) using data elsewhere.  Their solution was to use a proxy – Mexico.  Mexico has a high prevalence of dengue and a large amount of relevant data collection, so they used GAM to determine the relationships between these variables there, then extended these findings based on predicted temperatures/humidity in Europe due to climate change.  The COSMO-CLM European climate model was used to determine climate change data, which predicts about a 2.8 C surface temperature increase by 2090-2099.

There are definitely drawbacks to this design, as one cannot completely assume that temperature and humidity are the only factors determining dengue incidence.  For one, public health systems will have a major impact on spread, which are likely better in Europe.  But it at least gives a sense for how climate change will affect future trends.  So what exactly did they find…

1) The Mexico Data:

The model used -including climate and population data (socioeconomic status, population, urbanization, etc.) explained 44% of the variance in the data, which is quite good.  This means they are capturing almost half of the effects on dengue incidence.  The major climate factor leading to high dengue incidence is average monthly minimum temperature, followed by monthly humidity.  Population density and urbanization were also associated with higher dengue risk – more people crowded together, more chance for transmission!

2) The Europe Data:

The authors then took this data and extrapolated it for Europe based on predicted temperatures from the climate change modeling.  Incidence begins at about 2/100,000 people in 1990 – by 2071, significant incidence increases are seen in the coastal Mediterranean and Adriatic regions over 10/100,000 people.  See the nice maps below that show this trend over time, from 1960-1990 (top left), all the way to 2071-2100 (bottom right).  The scale of colors goes from dark blue, indicating less than 0.1/100000 incidence, to dark red, indicating over 10/100000 incidence.

Figure courtesy of [1]

Figure courtesy of [1]

One can clearly see that southern Spain and a large part of the east cost of Italy are at risk, as well as southeastern Europe.  Large, dense cities are at particular risk near the end of the century.  Dengue is not really on the radar right now for these European countries, but this data should be taken as a strong message to consider preventative public health measures.  Also, although this study only looked at dengue, similar results will also likely be true for any mosquito-born vector disease like yellow fever.

So this is an important warning, but also one taken with a bit of salt.  First, Mexican data is used to predict outcomes in Europe.  Well, dengue is already endemic in Mexico, meaning that it sustains without any external influences.  But it’s not in Europe – this difference will change the spread of transmission drastically.  Also, the type of mosquito in Mexico is A. aegypti, however in Europe, A. albopictus is the primary vector.  Apparently, the latter is much less competent in effectively transmitting disease, so the outcomes above likely overestimate the incidence.  Nonetheless, an important first look and a finding that should improve awareness about how climate change can affect vector diseases.

References

1)

ResearchBlogging.org

Bouzid, M., Colón-González, F., Lung, T., Lake, I., & Hunter, P. (2014). Climate change and the emergence of vector-borne diseases in Europe: case study of dengue fever BMC Public Health, 14 (1) DOI: 10.1186/1471-2458-14-781

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