Climate change is already suspected of causing legions of deaths every year as a result of extreme weather events: heat waves and floods; degradation of water supplies and sanitation with negative impacts on agriculture; and air pollution, which, in 2012, was responsible for 7 million deaths—one in eight deaths worldwide.
But climate change is also spurring an altered pattern of devastating infectious diseases on just about every continent today. Vector-borne diseases in particular—which are primarily spread by insect hosts and which cause more than one million deaths annually—are highly sensitive to alterations in the weather. The reason for this is that the behavior of the vectors themselves is sensitive to changes in the environment.
Indeed, vectors—living organisms, such as bloodsucking insects that can transmit infectious diseases between humans or from animals to humans—respond to a variety of climate variables, including temperature, precipitation, humidity, sea level, wind, and daylight duration. As our planet warms, the behavior of vectors is changing, and diseases like Chagas disease, dengue fever, leishmaniasis, malaria, chikungunya virus, and West Nile virus are migrating to northern climates, which have seen a pattern of temperature increases in recent years.
A good example is chikungunya virus, which is transmitted to humans by mosquitoes. Initially endemic in Asia and Africa, the disease has spread to the Caribbean, then gradually crawled north, and has now gained a toehold in Florida.
It’s also worth looking at Southern Asia, where malaria is already one of the most important vector-borne diseases. As changes in temperature and precipitation patterns take hold, malaria’s geographical range is expected to expand into more temperate and arid parts of the region. In India, for example, malaria distribution could well extend to higher latitudes and altitudes in the north, which have previously been unaffected by the disease. Due to the lack of tools for diagnosis, treatment, and prophylaxis for many of these diseases we are not prepared for this expansion pattern.
Right now, for instance, dengue fever, a viral infection spread by mosquitoes, mostly afflicts people below the equator—in South America, Africa, and South Asia. But researchers at the University of East Anglia say that the disease could move northward and break out in the highly populated coastal areas of the Mediterranean and Adriatic seas as well as the northeastern part of Italy, particularly the Po Valley.
A global research team led by the University of Pittsburgh Graduate School of Public Health has also recently shown that epidemics of dengue fever across Southeast Asia seem to be linked to the unusually high temperatures that accompany El Niño weather.
Moving from geography to medicine, the future is also worrisome. We’re making progress on a vaccine for dengue fever, but the first licensed vaccine is unlikely to protect completely against the disease. There are drugs to treat Chagas disease, but they are toxic, there is no approved vaccine, and diagnostics are crude to non-existent for different forms of this disease. We need more funding to combat chikungunya virus. And we’re working hard on a vaccine for West Nile virus.
Most of the global-disease-related headlines of the past decade have been devoted to the Ebola virus, SAARS, and MERS, which have been transmitted from human to human, thanks to a different effect of modern technology: rapid spread of isolated incidences through urbanization and global air transportation.
Now we should add the ways that climate change is changing the infectious disease landscape to our awareness and begin to ask how to address this significant global challenge.
We need to enhance vector surveillance and correlate it to climate information. Modeling and prediction of weather-related trends in vector movement and disease emergence could allow mapping in anticipation of countermeasure deployment.
Better science will also help us to understand the relationship between climate change and health outcomes. We need to study the linkages between climate and infectious disease transmission on a local or regional basis so that we can develop predictive models that are both relevant and actionable. Then we can develop and implement realistic preparedness and response plans for these health threats.
At the same time, we need to keep funding—and developing—cutting-edge, next-generation vaccines and diagnostics that will help us combat illness that is being accelerated by powerful and palpable global warming trends.
The changing burden of infectious diseases as a result of climate change could be quite onerous for the world. With the correct application of modern science, research anticipating these dynamic disease trends and the right application of available data we can help avert a burden of morbidity and mortality that could befall millions of people everywhere.
