It’s easy to lump all mosquitos together: tiny, blood-sucking insects that hover around you, their rapid wingbeats producing that annoying, telltale whine. But when it comes to transmitting disease, not all mosquitoes are alike. Malaria, which kills more than 600,000 in a year, is passed through mosquitos from the Anopheles genus. Sicknesses including Zika and dengue, meanwhile, arrive with the bite of Aedes mosquitos.
That difference means a lot when it comes to where people are most likely to contract certain infections. Anopheles mosquitos – and the malaria they spread – are found chiefly in rural or wild places, because the insects prefer to breed in clean, unpolluted water. Aedes mosquitos, by contrast, are city slickers, happy to lay eggs in water pockets found in discarded car tires and other trash.
This split between rural and urban diseases has been recognized for a long time. What’s harder to know is at what level of development this split occurs.
Now, a group of scientists say they have found a tool that could help better predict where certain diseases are likely to dominate based on the human footprint. The development offers another way to forecast which diseases will rise or wane as people alter the world around them.
“With increasing human pressure, one would expect transitions in the occurrence of different diseases – for example, dengue is a highly urban pathogen while malaria occurs at the frontiers of deforestation,” said Eloise Skinner, a Stanford University scientist who led the new research. “But how urban does an area have to be for dengue to become a risk? How much forest has to be converted before we start to see increases in malaria?”
To help answer that question, Skinner and several other researchers turned their attention to Brazil. The country provides an ideal test case. It’s host to an array of insect-borne diseases and the full spectrum of landscapes, from remote jungles of the Amazon to the slums and skyscrapers of São Paulo.
Past studies have used satellite images to gauge how rural or urban a region is. But Skinner’s team opted for an approach offering more detailed and nuanced picture. They enlisted a measurement developed in the early 2000s that sums up the human footprint on a landscape. The result is a single score based on eight factors, including population density, transportation and electric infrastructure, farmlands and the amount of land covered by buildings, parking lots and other human-made structures. Anything less than a 4 is relatively untrammeled country. Agricultural areas earn between 4 and 7. Scores up to 12 rank as “high” levels of development, while beyond that is deemed “intense.”
In the new research, the scientists calculated the human footprint for 5,500 Brazilian municipalities. They then compared the scores with reported infections for six insect-borne diseases: malaria, dengue, chikungunya and Zika (all from mosquitos) and cutaneous and visceral leishmaniasis (from biting sandflies).
The results revealed a clear pattern of different diseases rising to prominence as the human footprint increased. Rather than a gradual shift, the researchers uncovered abrupt swings in which diseases reigned. Malaria proved the quickest to fade as people reshaped the land, falling to 50% of peak infection levels at a footprint score around 5, and fading to almost nothing by 10. The two leishmaniasis diseases also eased as buildings rose, dropping to 50% of their peaks around a footprint of 10 and bottoming out around 15, the researchers reported March 13 in Nature Sustainability.
The Aedes-related illnesses flipped the script. There was little sign of those diseases in natural settings. But dengue took off soon after an area crossed the threshold into farmland, surging to 50% of its peak just as the landscape became more urban. It plateaued around a peak as an area reached “intense” development. Chikungunya and Zika followed similar paths, though they both exploded once an area shifted from farm to city.
At a time when public health experts have sounded the alarm links between disease and human incursions into natural habitats, these new results offer a clear illustration of how a switch from one level of development to another comes with a dramatic shift in which insects are vying for your blood, and which diseases they spread.
Skinner, et. al. “Human footprint is associated with shifts in the assemblages of major vector-borne diseases.” Nature Sustainability. March 13, 2023.
Image: @Anthropocene Magazine