Editor’s Choice 59:1: Taking the road less fragmented slows disease spread

Associate Editors, Bret D Elderd and Anibal Pauchard, introduce this month’s Editor’s Choice article by Prist et al., which demonstrates that the building of roads that crisscross pristine habitat can lead to an increase in vector dispersal and Yellow Fever Virus (YFV) cases.

The relationship between human and animal disease and environmental integrity has been highlighted by the recent COVID19 pandemic. However, quantitative studies on the role of landscape degradation and the spread of zoonotic diseases are limited. 

A fragmented landscape is composed of a variety of different habitat types that may or may not be connected. Both the structure and configuration of the landscape affect animal movement. If the animal happens to carry a disease, landscape structure will also affect the movement of their pathogens.  If a pathogen is transmitted through infected vectors (e.g., mosquitoes and ticks), the dispersal of those vectors across the landscape becomes important as well and therefore the effects of human activities on vector movement becomes critical.

In their article, Prist et al. use a series of novel techniques, network theory and electrical circuit theory, to identify the factors that contribute to the spread of YFV vectors and the disease itself in the South American Tropics.

Specifically, the authors modelled movement of primate cases of YFV using a network approach where cases were nodes and the link between cases (edges in network parlance) were connected events.  To estimate movements, the authors relied on electrical circuit theory to characterize the flow of YFV through the nodes of their system. To model the spread, the authors used data collected from records of dead non-human primates (i.e. epizootic events), in particular, an Alouatta species that tested positive for YFV.

In terms of virus dispersion, roads next to forested areas as well as roads near forest edges and shrub areas helped to spread the virus. While urban areas, agriculture, large water bodies (e.g., lakes), and core forest areas served as barriers to the dispersal of the vectors and therefore, the virus. 

As would be expected, time of year also matters and depends on the activity of the YFV mosquito vectors with lower dispersal occurring in the winter. The authors conclude that “highly fragmented landscapes with a wide road network and large densities of forest edges facilitate yellow fever virus propagation, and that the maintenance of large blocks of forest can help to inhibit this spread.”

EC blog fig 1
Landscapes considered to have low (−) and high (+) dispersal potential for yellow fever virus

This study demonstrates that building of roads that crisscross pristine habitat can lead to an increase in vector dispersal and YFV cases. Forest edges, which are 100-meters wide, next to agriculture or where water is present represent a “habitat” type that promotes vector and, thus, disease dispersal.  From an applied perspective, the above results should be taken into consideration when planning road and agricultural development that helps mitigate YFV risk.

But there is always a flip side to the problem, as the authors point out. Mitigating for YFV may end up having negative consequences in terms of ecosystem services. These services, such as pollinator services, are often promoted when there is more contact between forest and agricultural areas.

Caution should be used when trying to extend the results from YFV to other zoonotic diseases which are caused by other pathogens and where other vectors are involved. Trade-offs between having more forested areas and creating barriers from the movement of pathogen vectors should be pondered.  Although we would like to have our cake and eat it too, management choices often involve weighing the options and choosing what best fits the community at large, but as Prist et al. show us, we need real world data and innovative analysis to get there.

Read the full paper Roads and forest edges facilitate yellow fever virus dispersion in Journal of Applied Ecology.

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