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.
“Our 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.