ZIKA AND CLIMATE CHANGE
To me, it is one of the scariest scenarios I could imagine. Dangerous, disease spreading mosquito's conquering the world, making summer nights not only annoying but even deadly. Yet, Zika has shown that a world like this may become a reality.
Interestingly, I had never heard of Zika until recently, although it was already identified in humans in 1952. Perhaps that is because until 2007, no outbreaks of Zika were reported. In addition, Zika was perceived to be a relatively mild disease. Symptoms typically last for 2-7 days, and include headaches, fever, and skin rashes. Nothing to particularly worry about.
However, in 2013 and 2015 respectively, during large outbreaks in French Polynesia and Brazil, national health authorities reported potential neurological and auto-immune complications of Zika virus disease. Real concern about the potential threat Zika poses only started recently, when local health authorities in Brazil observed an increase in Zika virus infections in the general public as well as an increase in babies born with microcephaly. Although a relationship between these observations has yet to be confirmed, this is alarming.
CLIMATE CHANGE AND MOSQUITO-BORNE DISEASES
The Zika virus is transmitted to people through the bite of an infected mosquito from the Aedes genus, mainly Aedes aegypti in tropical regions. This is the same mosquito that transmits dengue, chikungunya and yellow fever. The thing about these mosquitoes is that they tend to thrive best in warm, wet climates, giving rise to concerns that future climate change may help them multiply and even spread into new areas, like new parts of the North American continent.
This 2006 photo provided by the Centers for Disease Control and Prevention shows a female Aedes aegypti mosquito in the process of acquiring a blood meal from a human host. (James Gathany/Centers for Disease Control and Prevention via AP)
The link between the spread of mosquito-borne diseases and a changing climate is far from simple and linear, however. Increased rainfall in normally dry areas, for example, can create stagnant pools of water where mosquitoes breed, which would favor the spread of mosquito-borne diseases. However, the same rainfall increase in wet regions could reduce malaria by washing immature mosquitoes away. A similar reasoning holds for temperature rise. Warmer weather speeds up transmission of malaria and enables the parasite to complete its life cycle more quickly, increasing replication. Increased temperatures in already hot regions, on the other hand, could reduce the spread of malaria mosquitoes by pushing temperatures higher than the mosquitoes can survive.
A changing climate is also likely to have effects on human behavior, in a way that may or may not make it easier for mosquitoes to get to us. In a warmer climate, we might go out earlier in the morning, and we might go out at different times of the day. People working outdoors, people not working outdoors, whether we are running air conditioning and staying indoors, the type of clothing we choose to wear or whether we apply insect repellent when we go out: they all play a role in whether mosquitoes are able to get to us.
HUMAN INFLUENCE
But there is more the mix than climate change alone. Human activities are also crucial to transmission of diseases. Forest clearance, for example, eliminates species that breed in water in tree holes (e.g., the forest Aedes species that transmit yellow fever) but provides favorable conditions for those that prefer temporary ground pools exposed to full sunlight (e.g., many of the Anopheles species that transmit malaria).
Other ecological and societal factors that may affect different types of mosquitos in different ways are water storage and disposal systems, agricultural practices, population density, living conditions, control programmes and health infrastructure. Many of these factors are expected to change dramatically in the near future as well.
Finally, an increase in the population of disease-bearing mosquitos does not automatically translate into an increase in disease. Human population's immunity to malaria and the parasite's levels of drug resistance, for example, also influence the transmission of the disease.
It becomes clear that all these factors together make it hard to predict how, when and to which extent mosquito-borne diseases will spread in the future. It is likely however that the habitat of mosquitos will change and even expand under a warmer, wetter climate. This means that especially the temperate world as we know it (for instance large parts of Europe and North America) is changing, and that we need to adapt to a life that involves disease-bearing mosquitos.