Warmer and wetter conditions facilitate transmission of mosquito-borne diseases, which may have added to spread, says lead climate change scientist
The outbreak of Zika virus in Central and South America is of immediate concern to pregnant women in the region, but for some experts the situation is a glimpse of the sort of public health threats that will unfold due to climate change.
“Zika is the kind of thing we’ve been ranting about for 20 years,” said Daniel Brooks, a biologist at University of Nebraska-Lincoln. “We should’ve anticipated it. Whenever the planet has faced a major climate change event, man-made or not, species have moved around and their pathogens have come into contact with species with no resistance.”
It’s still not clear what role rising temperatures and altered rainfall patterns have had on the spread of Zika, which is mainly spread by mosquitos; the increased global movement of people is probably as great an influence as climate change for the spread of infectious diseases. But the World Health Organization, whichdeclared a public health emergency over the birth defects linked to Zika, is clear that changes in climate mean a redrawn landscape for vector and water-borne diseases.
According to WHO, a global temperature rise of 2-3C will increase the number of people at risk of malaria by around 3-5%, which equates to several hundred million. In areas where malaria is already endemic, the seasonal duration of malaria is likely to lengthen. Aedes aegypti, the mosquito that carries Zika and other diseases, is expected to thrive in warmer conditions.
As climate change reaches almost every corner of the Earth’s ecology, different diseases could be unleashed. Increased precipitation will create more pools of standing water for mosquitos, risking malaria and rift valley fever. Deforestation and agricultural intensification also heightens malaria risk while ocean warming, driven by the vast amounts of heat being sucked up by the oceans, can cause toxic algal blooms that can lead to infections in humans.
“We know that warmer and wetter conditions facilitate the transmission of mosquito-borne diseases so it’s plausible that climate conditions have added the spread of Zika,” said Dr Diarmid Campbell-Lendrum, a lead scientist on climate change at WHO.
“Infectious agents in water will proliferate with more flooding. It’s clear that we need to strengthen our surveillance and response to a range of diseases. Globalization, the movement of people, is an important factor too. In a world where we are disrupting the climate system we’ll have to pay the price for that.”
WHO estimates that an additional 250,000 people will die due to climate change impacts – ranging from heat stress to disease – by 2050, but Campbell-Lendrum said this is a “conservative estimate”.
“It is based on optimistic assumptions that the world will get richer and we’ll get better at treating these diseases,” he said. “We do need to get better at controlling diseases at their source and we do need to drive down greenhouse gases because there is a limit to our adaption. By moving to cleaner energy sources we will also help relieve one of the largest health burdens we have, which is the air pollution that kills seven million people a year.”
Until now, efforts to push back the threat of infectious diseases has been successful. Malaria, for example, used to be found in the New York area – and there is evidence to suggest it was once present in southern England; much earlier, the Romans used to retreat to the hills at certain times of the year to avoid mosquitos carrying the disease. Vaccines have been developed for a range of diseases including, belatedly, Ebola.
The eradication of threats like these makes wealthy western countries fret over outbreaks like Zika. As the world warms, there may be a lack of preparation for other diseases not currently considered threats.
“This is likely to become an equal opportunity crisis,” said Brooks. “The developing, poorer countries are impacted disproportionately but they deal with these diseases all the time, they are not surprised by them. But in Europe and North America, people have lived in a bubble where we think our wealth and technology can protect us from climate change. And that’s not true.
“The thing that worries me most is a death by a thousand cuts. I don’t think an Andromeda strain will wipe out all humans. But the amount of time, money and effort needed to combat these many different problems can overwhelm a healthcare system.”
So which climate-fueled diseases are likely to pop up next? Some experts believe that water-borne diseases could escalate, which would have significant consequences for countries such as Bangladesh – a low-lying nation with plenty of rivers that has a public health system already struggling to meet its population’s current needs.
“There’s not nearly enough attention paid to diseases that cause diarrhea, crypto spiridium, Hepatitis A,” said Aaron Bernstein, a pediatrician at Harvard Medical School.
“We’ve seen outbreaks of these diseases in the past due to extreme precipitation. The build environment we live in wasn’t designed for the climate we will soon be living in; when you consider half the world’s waterways have been engineered by man, they won’t be able to contain the extra water that will flood them.
“Flooding will certainly lead to mosquito-borne diseases but also cause water-borne diseases and also a lack of drinking water. People in Asia and Africa, particularly those living on the coast, will be very vulnerable, climate change could be the straw that breaks the camel’s back in terms of public health.”
