Not all mosquitoes are created equal. Different types of the flying pest thrive in different temperature ranges and transmit different diseases. From this starting point, a Stanford-led paper predicts for the first time how, when and where in sub-Saharan Africa malaria will abate and other mosquito-borne diseases such as dengue will rise dramatically. The article, published September 9 in Lancet Planetary Health, warns of a public health disaster if the region does not complement its focus on malaria with strategies tailored to other mosquito-borne diseases.
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“Climate change will rearrange the infectious disease landscape,” said Stanford biologist and study director Erin Mordecai. “Chikungunya and dengue outbreaks, like the ones we saw recently in East Africa, are becoming more and more likely in much of the continent. We must be prepared for this emerging threat. “
A story of two mosquitos
The nocturnal Anopheles gambiae mosquito transmits malaria, a disease that affects more than 200 million people in sub-Saharan Africa, killing more than 400,000 people there in 2018. For years, public health efforts in the area have targeted the Scourge, including using insecticide-treated bed nets and indoor spraying.
However, these malaria-focused control strategies do little to control the daytime biting mosquito, Aedes aegypti, which can transmit a number of devastating diseases such as rift valley fever, yellow fever, zika, chikungunya and dengue fever. Increasing urbanization has expanded the Aedes aegypti range by expanding the preferred breeding grounds – man-made containers such as discarded tires, cans, buckets and water tanks. In contrast, malaria-transmitting mosquitoes breed in naturally occurring pools of water, which are more common in rural areas. Expanding urban areas also form heat islands or microclimates that are several degrees warmer than the surrounding vegetation areas – another point of attraction for the warm weather-loving Aedes aegypti.
Previous research by Mordecai and others has found that warmer temperatures increase the transmission of vector-borne diseases to an optimal temperature, or “turnover point,” above which transmission slows down. Just as they transmit different diseases, different mosquitoes are adapted to different temperatures. Malaria is most likely to spread at 25 degrees Celsius (78 degrees Fahrenheit), while the risk of dengue is highest at 29 degrees Celsius (84 degrees Fahrenheit).
As a result, a warming world has meant more dengue fever – 2019 was the worst year on record for the disease, with cases in every region, including some countries, that had never seen dengue fever, according to the World Health Organization.
In sub-Saharan Africa, Mordecai and her colleagues said the shift towards mosquitos, which transmit diseases other than malaria, is likely to accelerate with climate change. While warming could increase the number of mosquitoes with malaria in relatively cool places like the highlands, it is likely to decrease in warmer lowland areas like Accra, Ghana, while the number of mosquitoes that transmit dengue and other diseases, increases.
Some places will take a double blow, according to the study. For both malaria and dengue fever, areas around Lake Victoria such as the cities of Kisumu, Kenya and Entebbe, Uganda will be at high risk by 2050, with that risk spreading west to areas such as the cities of Mbarara, Uganda and Mwanza (Tanzania).
A growing storm and a possible solution
In sub-Saharan Africa there is already evidence of a possible explosion of diseases transmitted by Aedes aegypti. In recent years, dengue infections and major chikungunya epidemics have increased in Kenya and Sudan. The researchers suggest that a lack of public awareness of mosquito ecology and the risk of exposure to Aedes aegypti increases the likelihood of developing mosquito-borne diseases other than malaria, while health systems in these regions miss many such cases due to a lack of testing and misdiagnosis than malaria.
“It is important to focus on fighting mosquitoes that spread diseases like dengue fever as there are no medical treatments for these diseases,” said the study’s lead author Desiree LaBeaud, professor of pediatrics at Stanford Medical School, which has documented the increase in arbovirus exposure to disease in Kenya for 15 years. “In addition, a shift from malaria to dengue can overwhelm health systems, as diseases introduced into new populations often lead to large outbreaks.”
As in America, new public health efforts in sub-Saharan Africa to combat Aedes aegypti and dengue, chikungunya and other viruses need to be added to existing malaria control measures, the researchers argue. In particular, the development of precise point-of-care diagnostics for dengue and chikungunya viruses as well as community-based mosquito control such as removing rubbish and covering standing water will become increasingly important for targeted care and prevention.