Deforestation can lead to the spread of pathogens as wild animals infected with them often end up being in closer proximity to people and their livestock. But there are other ways in which pathogens can spread, involving vectors such as mosquitoes.
The destruction of tropical rainforests harms the diversity of mosquito species, causing more resilient species of them to become more prevalent. The viruses they carry can then become more abundant.
This is according to researchers from Charite - Universittsmedizin Berlin, who teamed up with the Leibniz Institute for Zoo and Wildlife Research to investigate how clearing rainforests in West Africa to make way for plantations or settlements affects the prevalence of certain mosquitoes and the viruses they carry.
They examined mosquitoes around Ta National Park in Cte d'Ivoire where land use ranges from pristine rainforests to secondary forests to cacao and coffee plantations as well as villages. This provided the scientists with ample opportunity to compare various mosquito habitats.
"We identified the species of mosquitoes we had caught and tested them for viral infections," explains Kyra Hermanns of the Institute of Virology at Charite, the first author of a study on the findings. "Then we looked at how the composition of mosquito species differs across the different land use types, where certain viruses are present and how prevalent they are."
Healthy ecosystems such as a pristine rainforests support a wide range of viruses in animal species that act as hosts for them. A change in the ecosystem affects the viruses as well, however.
"We discovered 49 virus species, with the greatest diversity of hosts and viruses observed in untouched or minimally disturbed habitats," notes Prof. Sandra Junglen, head of the Ecology and Evolution of Arboviruses at the Institute of Virology at Charite, who led the research.
Most of the 49 different virus species were relatively rare in the areas under review, but nine of them were commonly found in multiple habitats. The prevalence of five virus species increases in disturbed habitats and it reached the highest numbers in human settlements, according to the scientists.
"This means that the clearing of tropical rainforests causes a decrease in biodiversity across mosquito species, which changes the composition of host types. Some resilient mosquito species have multiplied very successfully in the cleared areas, bringing their viruses with them," Junglen says.
The composition of species in an area also has a direct bearing on the prevalence of viruses they carry. "If one host species is very abundant, it is easier for viruses to spread," the virologist explains.
"All of the viruses we found to be more common were demonstrated to be present in a certain mosquito species. The viruses belong to different families and have different properties," she notes. "That means we were able to show for the first time that the spread of the viruses is attributable not to a close genetic relationship, but to the characteristics of their hosts, especially those mosquito species that adapt well to changing environmental conditions in habitats that have been disturbed."
The viruses the researchers studied only infect mosquitoes and cannot be transmitted to humans, but we may not be so lucky with other viruses.
"Our study makes clear just how important biodiversity is, and that decreasing biodiversity makes it easier for certain viruses to thrive because it causes their hosts to become more abundant," Junglen stresses.
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Source: Sustainability Times