Much of the Earth has been modified by humans, which has a flow-on effect on natural ecosystems, including the insects that carry disease. For the first time, researchers have examined when and how environmental change affects the transmission of insect-borne diseases, offering insights into future disease management.
Vector-borne diseases (VBDs) are spread by living organisms called vectors, which carry disease-causing pathogens from human to human or from animal to human. VBDs are higher in tropical and subtropical areas and disproportionately affect poor populations. Globally there are more than 700,000 deaths yearly from VBDs, according to the World Health Organization (WHO).
Mosquitoes are common vectors that can cause diseases such as malaria and the viruses chikungunya, dengue and Zika. Another common vector is the sandfly, which transmits leishmaniasis, a parasitic disease that can cause skin sores (cutaneous leishmaniasis) or affect internal organs (visceral leishmaniasis), usually the spleen, liver and bone marrow.
For the first time, researchers from Griffith University, Stanford University and the University of California have used cumulative pressure mapping and machine learning to map how environmental modification affects VBD transmission.
Cumulative pressures are environmental pressures that, combined with other past, present and future pressures, can produce additive, collaborative, or antagonistic effects. Environmental pressures include climate change, urbanization, land clearing, pollution, tourism, and industry.
VBDs are highly responsive to environmental change, and natural ecosystems are impacted by both large- and small-scale environmental modification. According to the researchers, we’ve just started to understand the impact of human pressure caused by this kind of modification.
“People are really good at modifying the Earth – as much as 95% of the Earth’s surface has been modified in some way by humans,” said Dr Eloise Skinner, the study’s lead author. “We know that when we modify the Earth’s surface we also change the species community, which includes plants, animals and insects.”
The effects of environmental change on VBDs are difficult to monitor, given the complex social and ecological factors affecting vector-human disease transmission.
For the current study, the researchers used a “human footprint index” as a single metric to capture the multidimensional influence of human interaction with the land. The human footprint relies on cumulative pressure mapping to calculate a scale of human pressure, ranging from zero to 50.
Specific ranges of human footprint are associated with variations in ecological integrity and functioning. Previous studies have identified that a human footprint equal to or greater than three is the tipping point at which species extinction occurs. For comparison, areas with a human footprint of less than four are considered intact ecosystems containing mostly natural habitat. Whereas a human footprint of greater than 12 indicates intense human pressure.
The researchers used Brazil to represent global patterns of human pressure, given its ecological and socioeconomic diversity and varied land uses. They studied the six most common VBDs endemic to the country: dengue, chikungunya, malaria, Zika, cutaneous leishmaniasis and visceral leishmaniasis.
“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,” Skinner said. “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?”
Using machine learning to compare human footprint with VBD transmission, the researchers showed that there were distinct thresholds of human footprint that impacted the prevalence of specific VBDs.
When the human footprint went from moderate (four to seven) to high (seven to 12) to intense (greater than 12), the occurrence of VBDs shifted from malaria, cutaneous leishmaniasis and visceral leishmaniasis to dengue, chikungunya, and Zika. Dengue, chikungunya and Zika are viruses that require a particular response to be treated effectively.
“These are diseases that require distinct responses in vector control, diagnostics, and environmental management,” Skinner said.
The study shows that human pressure on the environment – the human footprint – is an important predictor of VBD transmission. This greater understanding can inform our ability to predict future disease outbreaks.
“Because biomedical and chemical approaches alone have failed to sustainably eliminate these VBDs, managing the socio-ecological settings that promote pathogen transmission is a crucial frontier for planetary health,” the researchers said.
The study was published in Nature Sustainability.
Source: Griffith University