- Flooding boosts Aedes aegypti mosquito populations, a new study from Kenya has found.
- With a changing climate and extreme weather like floods expected to become more frequent and intense, this could mean more outbreaks of mosquito-borne disease such as dengue in the coming years.
- Dengue already afflicts millions of people every year, almost all of them in tropical countries, where the A. aegypti mosquito thrives.
- Though the new paper did not find a greater abundance of mosquitos leading to greater infection risk, the link between larger mosquito populations and disease outbreaks warrants further investigation, the authors said.
The Aedes aegypti mosquito, with its distinctive white-spotted spindly legs, is one of our species’ most irksome foes, sucking blood and capable of spreading dangerous diseases like dengue.
Attempts to control their populations have largely been unsuccessful, and now scientists are increasingly worried about yet another complication: climate change.
The jury is still out on one particular aspect of this problem: how does frequent and intense flooding impact their populations? All that additional water could be promoting mosquito breeding, a new study from Kenya in the journal PLOS Neglected Tropical Diseases says.
“We find that floods lead to increase in abundance, both of eggs and adult mosquitoes,” said Cameron Nosrat, first author of the paper published on March 18.
During unusually wet times, pools of freshwater where these mosquitos lay their eggs multiply, which could explain the population boom. However, some other studies have also found that too much water can lead to a “flushing out” effect because the mosquito larvae develop only in standing water.
Dengue infections are caused by a flavivirus transmitted to humans by female A. aegypti mosquitoes. In the past two decades alone, the number of dengue cases has grown 15 times, with millions of people contracting the disease every year.
A. aegypti thrives in rainy, tropical climates, and is unable to survive in colder climes. However, a 2019 study in Nature Microbiology found that with the planet warming, the mosquitoes’ range will expand in Asia, Europe, North America and Australia. By 2080, the paper predicted, 60% of the world’s population, or 6 billion people, would be at risk from dengue.
“Few studies have retrospectively looked at the effects of extreme weather events on the abundance of disease-transmitting mosquitoes as well as dengue transmission,” said Jane Messina, first author of the 2019 paper, whose work at the University of Oxford centers on geography and epidemiology.
The new analysis relied on NASA temperature data and NOAA rainfall data for Kenya between 2013 and the start of 2019.
While above-average temperatures are also linked to the proliferation of mosquitoes and disease outbreaks, the new study did not find a link between the two. It might be explained, Nosrat said, by the fact that mosquito populations are more responsive to microclimates, which their data did not capture.
“We looked at land surface temperature based on satellite data, which is definitely a limitation,” he said.
Another surprise was the absence of a link between an increase in mosquito population and the risk of infection.
The researchers gathered medical data at four sites where the study was focused: Chulaimbo, Ukunda, Kisumu and Msambweni. The first two are urban settings in Kenya while the other two are located in rural areas. The analysis did not show a significant increase in dengue infections even in places where the mosquitoes experienced a population boom following extreme rainfall episodes.
“Although the authors do not find actual changes in dengue transmission due to the extreme events, they rightly point out that other factors related to human behaviors are important in mitigating these effects,” Messina said.
The authors did not analyze which actions by the communities might be protecting them against infection, she noted. “That said, this is an interesting study which opens up several questions for future research, especially if settings with a wider range of socio-economic characteristics could be studied,” she said.
The researchers argue that altered behaviors, like staying indoors during inclement weather, could explain why infection rates did not rise when mosquito populations swelled.
Climate models forecast that the average temperature in Kenya will rise by 2.5º Celsius (4.5° Fahrenheit) between 2000 and 2050. A changing climate is expected to lead to more frequent and intense flooding as rainfall becomes more unpredictable.
There is sound evidence that as average temperatures rise and environments in which communities live change, public health will be affected. The modalities of how it happens are still being uncovered.
Nosrat said he hopes their work will make it easier to predict where these health impacts might occur. It would allow authorities to issue early warnings and prepare their constituents better for disease outbreaks. Even simple actions like proper outdoor garbage disposal and clearing of potential breeding sites during floods could help save lives.
Nosrat, C., Altamirano, J., Anyamba, A., Caldwell, J. M., Damoah, R., Mutuku, F., … LaBeaud, A. D. (2021). Impact of recent climate extremes on mosquito-borne disease transmission in Kenya. PLOS Neglected Tropical Diseases, 15(3), e0009182. doi:10.1371/journal.pntd.0009182
Messina, J. P., Brady, O. J., Golding, N., Kraemer, M. U., Wint, G. R., Ray, S. E., … Hay, S. I. (2019). The current and future global distribution and population at risk of dengue. Nature Microbiology, 4(9), 1508-1515. doi:10.1038/s41564-019-0476-8
(Banner Image: An Aedes aegypti mosquito. Image courtesy: mika mami via Pixabay.)