A rise in temperatures, and the increased survival of some mosquito species in particular regions, could lead to the spread of fatal viruses in the Global North.
[Dr. Duncan R Smith | Policy Forum]
The deadliest animal in the world – at least to humans – is the humble mosquito, which is believed to kill more than a million people a year. There are more than 3,500 different mosquito species that thrive in a wide range of habitats, but only a small proportion of them are involved in human disease transmission.
Diseases are passed to humans when an infected female mosquito takes a blood meal to obtain nutrients required for egg development. The mosquitoes of most pressing concern in terms of human health are Anopheles mosquitoes – transmitters of the malaria pathogen – and Aedes mosquitoes – spreaders of a number of viruses including the dengue, yellow fever, and Zika viruses.
Like everything else living on this earth, mosquitoes have a preferred temperature range. Below approximately 10 degrees Celsius, mosquitoes tend to shut down; at around 26 to 29 degrees Celsius, they are most active.
Female mosquitoes target food sources that have an ambient temperature of up to approximately 40 degrees Celsius, while all avoid targets above this temperature. However, the exact optimum conditions for all aspects of mosquito biology depend on the specific species of mosquito.
Given that global warming will change the temperature profile of the earth, it can be predicted that global warming will have a significant impact on the transmission of mosquito-transmitted diseases, but exactly how the distribution of diseases will change is far from clear.
The main problem is that mosquito-vectored disease transmission is not solely mediated by temperature, but is also mediated by factors like precipitation, water availability, sanitation levels, vector density, and population immunity.
Malaria, for example, used to be a significant cause of mortality in parts of the UK. This was the case until the early part of the 19th century, when its eradication was largely and unintentionally achieved through the drainage of marshes and fens, improved housing, and the availability of quinine as an anti-fever agent.
The decline of malaria in the UK preceded knowledge of the role of mosquitoes in the malaria life cycle, which started to formulate at the end of the 19th century.
Even today, however, Anophelese mosquitoes are present in the country and could act as vectors for transmission. It must be noted, however, that even with multiple cases of malaria being imported via infected travellers, indigenous malaria transmission has not been detected.
Similarly, much of Europe eradicated malaria with the help of national sanitation programs that included insecticide spraying, removing breeding sites, and prompt treatment for infected people. In doing so, countries successfully eliminated transmission.
With its prior experiences to draw on, countries of northern Europe and North America are unlikely to suffer from high levels of transmission of malaria as a result of global warming.
The same cannot be said, however, of the viral diseases spread by Aedesspecies mosquitoes. Two species of Aedes mosquitoes – Aedes aegypti and Aedes albopictus – are already highly invasive species, both with footholds in lower Europe and North America. Global warming is likely to increase the spread of both species northward.
These species have already introduced ‘tropical’ viruses into more northern latitudes, as seen with outbreaks of chikungunya fever in Italy and France and Zika fever in the US.
One intriguing aspect of global warming is that it is likely to reduce the transmission of some mosquito-spread diseases in Asia and parts of Africa. Higher expected temperatures in these regions will most likely be unfavourable for Aedes albopictus mosquitoes – a main vector of viruses such as chikungunya and dengue.
This leaves us with an interesting scenario in which not remediating global warming will shift the burden of these diseases away from the poorer tropics to the richer northern countries. On the other hand, returning global warming to a pre-industrial baseline will largely confine transmission to tropical and subtropical countries.
But the worst scenario of all will come if the world only partially addresses global warming. Transmission will be maintained in the tropics and subtropics, while increasing the distribution of the disease in what are now more temperate climes.
While completely remediating global warming will, in the richer north, keep an estimated one billion people free from potential mosquito-transmitted viruses, it comes at the cost of worsening the burden of disease on some of the poorer communities in the world. This, of course, should be compensated for in some way.
The pessimist in me says that global warming won’t be fixed in time, and that the conditions in the tropics will become such that a major south-north migration occurs. It is likely that those migrants will find that many of the diseases they are escaping will already be entrenched in their destination countries.
We need to stop thinking of mosquito-transmitted diseases as a tropical and subtropical country problem, and address them as a global concern.
Western countries must invest heavily in research on mosquito ecology and in understanding disease transmission, as well as fast-tracking the development of protection through vaccines and drugs. They have the choice to pay for it today when the problem is still at arm’s length, or to pay much, much more tomorrow when they find the deathly threat in their own backyards.
Dr. Duncan R Smith is a Professor at the Institute of Molecular Biosciences, Mahidol University, Thailand.