Warming temperatures from climate change, coupled with urbanisation, could lower rates of malaria and increase outbreaks of mosquito-borne diseases such as dengue fever in Sub-Saharan Africa, new research reveals.
"Climate change, in conjunction with urbanisation, could drive a shift in most sub-Saharan African countries from climates most suitable for malaria transmission (by rural Anopheles mosquitoes) to climates more suitable for transmission of dengue and other arboviruses (by Aedes aegypti mosquitoes), with major consequences for public health and disease control strategies," says the research paper, led by Stanford University and published in Lancet Planetary Health this week.
Climate change is going to rearrange the landscape of infectious disease, said Stanford biologist and study lead author Erin Mordecai, in a statement.
“Chikungunya and dengue outbreaks like we’ve recently seen in East Africa are only becoming more likely across much of the continent. We need to be ready for this emerging threat," Mordecai said.
In regions where temperatures are regularly between 25°C and 29°C, including much of sub-Saharan Africa, a warming climate will become less suitable for malaria but more suitable for dengue, chikungunya, and other arboviruses transmitted by Ae aegypti, the research finds.
"Specifically, the highest density of people exposed to optimal temperatures for disease transmission (the so-called risk hotspot) for malaria is projected to shift towards higher elevations such as the Albertine Rift region in central Africa and higher latitudes in southern Africa.
"The risk hotspot for dengue, chikungunya, and other Ae aegypti-transmitted arboviruses is predicted to expand from west Africa throughout sub-Saharan Africa."
Together with climate change, urbanisation is driving widespread changes in habitat, microclimate, and human populations, and is "occurring more rapidly in sub-Saharan Africa than anywhere else in the world (although these transitions are complex and diverse)".
Urbanisation affects vector-borne disease transmission by altering the availability of vector breeding habitat and contact with humans.
Ae aegypti mosquitoes breed in human-made container habitats such as discarded tyres, cans, buckets, and water storage containers, all of which increase in density in urban areas but are also present in villages.
"By contrast, An gambiae and some other African malaria vectors breed in naturally occurring pools of water, which are more common in rural areas, although malaria transmission can occur in cities."
Urban areas, says the research, also form so-called heat islands with micro-climates that are several degrees warmer than surrounding vegetated areas, which can affect vector development and survival and could benefit warmer-adapted Ae aegypti over An gambiae mosquitoes.
"Therefore, urbanisation could act synergistically with warming climate to promote the shift from Anopheles-transmitted malaria to Aedes-transmitted arboviruses in sub-Saharan Africa."
Dengue, chikungunya, and their Ae aegypti mosquito vector are already widespread but under-recognised in Africa, based on studies of vector abundance, human serology, and acute infections from across the continent.
As climate suitability increases for arboviruses, these diseases could expand and overtake the public health burden of malaria.
Although malaria control efforts remain essential, arbovirus control using surveillance and vector control of container-breeding, day-biting Ae aegypti is a crucial emerging public health need in Africa, the research says.
Testing and diagnostic capacity for arboviruses, and awareness of vector ecology and exposure risk, lag behind that of malaria in most of sub-Saharan Africa, where climate change is expected to increase the incidence of dengue and other arboviruses.
The researchers note how disease control strategies that are effective against malaria, including long-lasting insecticide-treated bed nets, indoor residual spraying, and artemisinin combination therapy, are ineffective against dengue, which has the day-biting and container-breeding Ae aegypti mosquito as its primary vector and has no specific drug therapy or broadly effective vaccine available.
"A shift from malaria to dengue in sub-Saharan Africa would therefore require public health efforts to refocus to control an ecologically different vector and pathogen, a shift that has already taken place throughout much of Latin America and the Caribbean."
Malaria eradication efforts remain crucial. "However, given the year-round circulation of dengue and chikungunya and abundance of Aedes mosquitoes in Africa, public health efforts should also prepare for a potentially emerging threat of arboviral disease in Africa."
Prof Tiaan de Jager, the director of the Institute for Sustainable Malaria Control, at the University of Pretoria, said malaria has been around for millennia and it is still a challenging disease to eliminate.
According to the WHO, between 2030 and 2050 it is expected that climate change will cause around 250 000 additional deaths annually, because of malnutrition, malaria, diarrhoea and heat stress alone.
"Numerous strategies and tools have been developed over time to control and eliminate malaria. Surveillance is important, early diagnosis and treatment, work is being done to develop vaccines, advocacy etc," he said.
"In spite of the fact that malaria and other mosquito-borne diseases have similar challenges and risks, there are things to consider when approaching the diseases and the potential future impact of climate change on especially the arboviral disease.
The different mosquitoes display different ecologies and habits – for example Aedes aegypti are day-biters, Anopheles night-time biters. Malaria is caused by a parasite and other vector-borne diseases by viruses.
"They respond differently to types of treatment and methods of treatment. Malaria has been researched and control tools and public health strategies have been developed and used over many years.
"As temperature starts increasing these other mosquito-borne diseases may start becoming more of a problem. Due to lack of knowledge, it might become a challenge to actually control the diseases – health systems may not be geared to handle the ‘new’ challenge due to lack in infrastructure or the lack of diagnostic tools and treatment methods – possibly resulting in major outbreaks?"
Malaria is both treatable and preventable while dengue and other mosquito-borne diseases have no treatment (drugs) yet and require more research.
"It is imperative to understand the differences between malaria and these other diseases but also utilise the tools available to control and eliminate malaria, to guide efforts towards establishing strategies and developing the tools necessary to diagnose and control other mosquito-borne diseases."
Efforts, he says, must align controlling other diseases with that of existing malaria control strategies.
"The diseases may overlap in areas and some control mechanisms may be effective on both – prevention of mosquito biting, clearing of potential habitat spaces (pick up trash)."