More dengue, less malaria. That may be the future in parts of Africa on a warming planet, depending on where you live.
New research says it’s all about which mosquitoes will thrive. And the methods to control one don’t necessarily work on the other.
The mosquito that spreads malaria prefers relatively cool temperatures of 25 degrees Celsius (77 degrees Fahrenheit). The dengue mosquito does best at 29 degrees Celsius (84.2 degrees Fahrenheit).
Because of this difference in optimal temperatures, “We would actually predict that climate change might have opposing effects [on disease transmission],” said Erin Mordecai, assistant professor of biology at Stanford University and lead author on the study. “Climate change might make it less suitable for malaria to be transmitted but more suitable for dengue to be transmitted.”
Using mosquito optimal temperature data and population density, the researchers predicted the risk of malaria and dengue in Africa under “worst-case, business-as-usual” climate projections.
The dengue mosquito — which also spreads a lot of viruses that cause diseases such as chikungunya, Zika and yellow fever — is expected to expand its range, increasing the risk of these diseases throughout sub-Saharan Africa by 2080.
In contrast, the areas of greatest risk for malaria are predicted to shrink, shifting further south and into high-elevation regions.
The researchers say that rising urbanization in Africa may further enhance the risk of dengue. Malaria is often more of a problem in rural areas because the mosquito breeds in natural bodies of water such as ponds and streams. But the dengue mosquito prefers to breed in tiny, human-made containers “as small as a bottle cap” that are common in cities, said Mordecai.
Urban areas also tend to be warmer than surrounding rural regions, providing more suitable habitat for the heat-loving dengue mosquito.
“We’re predicting that dengue is going to become a much bigger problem in Africa. And I think that, itself, is a very big deal, because Africa on the whole is probably not well-prepared, because they’ve been focused on another very important vector-borne disease — malaria,” said Desiree LaBeaud, professor of pediatric infectious diseases at Stanford University and co-author of the study.
Public health measures such as insecticide-treated bed nets have helped curb malaria because they protect against the nighttime-biting malaria mosquito. But nets are little help against dengue because the mosquito bites during the day.
Growing cases of dengue and other viral diseases may pose new challenges for Africa. According to Mordecai, diagnostic tools for dengue are not widely available in many parts of the continent, which can lead to misdiagnosis and inappropriate treatments.
“Generations of scientists and control people have been trained and have experience in controlling malaria vectors. But for dengue, you need to start retraining people essentially for a very different creature and a different enemy,” said Philip McCall, professor of medical entomology at Liverpool School of Tropical Medicine, who was not involved with the study.
According to McCall, other studies have also shown links with climate change and dengue.
“It’s more likely you would have an increase in dengue, or possibly chikungunya and Zika, becoming an emerging serious urban phenomenon,” he said. “But I can’t see malaria, which is so established in Africa, disappearing easily. So, it could be like double trouble.”
Experts say this study shows just one possible scenario for mosquito-borne diseases in Africa.
“This study is only looking at the very high emission, very fossil fuel-intensive future, which some people think is a little bit unlikely,” said Joacim Rocklöv, professor of epidemiology at Umeå University, who did not contribute to the research. “I think if you would look at another scenario which would perhaps be more plausible, or given we make changes in controlling emissions, then you might see quite different results, actually, in regards to malaria.”
The best way to control dengue is reducing breeding habitats for the mosquito by removing containers that hold standing water or making sure they’re fitted with a tight cover.
But there is new hope for a different type of dengue control.
A naturally occurring bacterium called Wolbachia blocks dengue virus replication in the mosquito and prevents transmission. A trial involving release of Wolbachia-infected mosquitoes in Indonesia was found to reduce dengue cases.
“They have reduced transmission in a huge area of Yogyakarta by 77%, which is incredible,” said McCall.