In a world characterized by rising temperatures, deforestation and other human influences on the environment, the spread of infectious disease is a hot topic. Many recent studies suggest that environmental changes can affect the transmission of everything from malaria to the Zika virus — and it’s increasingly important to understand these links, scientists say.
This week, a new study has provided new evidence that environmental changes can increase the threat of disease. It concludes that unusually warm temperatures caused by 2015’s severe El Niño event — probably compounded by ongoing climate change — may have aided in the rapid spread of the Zika virus in South America that year. And while there are many complex factors at play in the spread of mosquito-borne diseases, the study may help scientists better prepare for the kinds of future effects we might see in our warming world.
“The start of the mission was simple — trying to address where the risk will be, where is it going to move next, where could Zika happen on the planet on a global scale,” said Cyril Caminade, a research fellow at the University of Liverpool and the new study’s lead author. To that end, the authors designed a study that would help them determine how climatic changes have impacted the mosquito-borne transmission of Zika.
There are two main species of mosquito known to carry the Zika virus — Aedes aegypti, or the yellow fever mosquito, which is widespread in the tropics; and Aedes albopictus, or the Asian tiger mosquito, which lives in both tropical and temperate regions of the world. Scientists also believe Zika can be sexually transmitted, but the new study focused only on mosquito transmission.
For the study, the researchers collected published information on the distribution of these two mosquito species and how temperature variations can affect them. Studies suggest, for instance, that up to a certain point, rising temperatures can cause mosquitoes to bite more frequently. The researchers also collected global historical climate data from the past few decades and used all the information to build a model of Zika transmission worldwide.
The model produced an unusually high disease transmission potential in the tropics for the year 2015, including in Colombia and Brazil, the countries hit hardest by Zika. Similar results occurred between 1997 and 1998, one of the only other times on record to experience such a brutal El Niño event.
“[O]ur model indicates that the 2015 El Niño event, superimposed on the long-term global warming trend, has had an important amplification effect,” the researchers note in the paper.
The model also helped the researchers identify the ideal seasonal climate conditions for Zika transmission around the world. In South America, for instance, the model suggests that the potential for transmission should peak in the winter and spring.
In the southeastern U.S., on the other hand, summer is ideal. In fact, the model suggests this region has a high potential for disease transmission during this time, due partly to the high temperatures and partly to the fact that both mosquito species are found there.
That said, reports of Zika have been limited in the U.S. so far — and this speaks to the complexity of vector-borne disease transmission, Caminade said. Climate can certainly play a significant role in setting up the right conditions for an outbreak, but epidemics also depend on many other factors, including population density, access to healthcare and the use of pesticides and other anti-mosquito interventions in any given location. Some of these factors — which were not accounted for in the new study — can probably explain why there hasn’t been much Zika transmission in the U.S. so far.
Caminade also pointed out that after a population has been exposed to a mosquito-borne disease like Zika, a phenomenon called “herd immunity” often occurs — this happens when so many people have already been exposed, and developed an immunity, that there aren’t enough new people left to infect to continue the epidemic. This is the probably part of the reason we didn’t see Zika epidemics in other tropical parts of the world in 2015, despite the new study’s results. And some experts have suggested that herd immunity will likely cause the current situation in South America to burn itself out within a few more years.
But Caminade cautions that there’s still the potential for Zika outbreaks in other parts of the world where the conditions are right, including the United States and even southern Europe. The result would likely be milder than what’s been experienced in South America in the past year, but there’s “still risk,” he said.
According to Caminade, one of the study’s major takeaway points is that extreme climate conditions can lead to all kinds of unusual events — droughts, floods and wildfires are only a few examples — and disease outbreak is just one more potential disaster scientists should be looking out for when these conditions occur.
And such events may only be exacerbated by future climate change. Some studies have suggested that the kinds of “monster” El Niño events seen in 1997 and 2015 may be more likely in a warmer world, Caminade pointed out. But he added that scientists wishing to make more precise predictions about the future spread of disease must take a wide variety of factors into account — climate is just one of them.
“I won’t overplay the role of climate for the future,” he said. “It’s still a disease, and there are still parameters which are going to be very important.”