A study has found that nearly 44 per cent of the world’s population, approximately 3.5 billion people, could be exposed to zoonotic diseases — which are caused by pathogens transmitted from animals to humans.
The study titled Zoonotic Host Richness in the Global Wildland–Urban Interface conducted by Yale School of the Environment, also revealed that around 20 per cent of humans living in close proximity to wildlife with high zoonotic potential share their habitat with over 20 different host species.
Published recently, the findings highlight the risks of zoonotic disease transmission in areas where human populations intersect with wildlife. The study suggests that approximately 3.5 billion people, living within five per cent of the wildland-urban interface (WUI), could be at significant risk of exposure to zoonotic diseases originating from wildlife.
The WUI is defined as the zone where human settlements and wildlands meet, creating areas where human-environmental conflicts are concentrated. The study states that these regions are particularly vulnerable to zoonotic disease spillovers. The authors further explained that activities such as hunting, fireweed collection, and changes in land use can expose humans to pathogens from wildlife, directly increasing the likelihood of disease transmission.
In addition, the presence of peri-urban agriculture can exacerbate these risks, as livestock often act as intermediary hosts for zoonotic diseases, transferring pathogens between wildlife and humans. This study is the first to examine how rapid urbanisation in the WUI could increase the risk of zoonotic spillover, a phenomenon that remains poorly understood.
The researchers emphasised the significance of understanding the epidemiology of zoonotic diseases, especially given the global impacts of pandemics like COVID-19. "Given the ramifications that zoonotic diseases can have for humanity, there is a critical need to understand the risk factors involved in the epidemiology of urban zoonotic diseases in the WUI," the study asserts.
To quantify global zoonotic risk, the researchers analysed the distribution of 686 terrestrial mammal species linked to 144 zoonotic diseases.
This information was then mapped to identify potential areas of concern within the WUI. They found that certain generalist host species, such as the natal multimammate mouse (Mastomys natalensis), the tanezumi rat (Rattus tanezumi), and the red fox (Vulpes vulpes), are widely distributed across the WUI, making them ideal candidates for zoonotic disease transmission.
The host species richness mirrored global trends in mammalian biodiversity, with the highest concentrations of host species found in South America and equatorial regions, including parts of Central America, equatorial and southern Africa, and Southeast Asia.
The study also observed that regions in southern India and southwest China showed pockets of high host richness, although the WUI in these areas had a lower overall host concentration compared to other parts of Asia. When considering the mammal species found near human populations, bats (Chiroptera) ranked highest, followed by rodents (Rodentia), apes, monkeys, and lemurs (Primates). Other notable groups included ungulates (Artiodactyla), carnivores (Carnivora), and hedgehogs and moles (Eulipotyphla).
The study also identified the most prevalent zoonotic diseases in these areas, including rabies, leptospirosis, toxoplasmosis, plague, and leishmaniasis (both visceral and cutaneous forms).
The research found that poverty significantly amplifies the risk of exposure to zoonotic diseases. Most of the populations living in WUIs with high zoonotic potential are from Low- and Middle-Income Countries (LMICs), where healthcare access is limited, and living conditions are often substandard.
Specifically, 73 per cent of people living in WUIs with high host richness – around 520 million people – come from LMICs, primarily in Africa, including countries such as Kenya, Uganda, Ethiopia, Tanzania, and Zimbabwe. Other nations in West Africa, such as Nigeria, Ghana, Burkina Faso, and Côte d'Ivoire, also face high zoonotic risks.
The study explains that informal settlements in LMICs, marked by poverty, inadequate housing, poor healthcare access, and insecure land tenure, make people in these regions particularly vulnerable to zoonotic disease transmission.
Southeast Asia also saw a high population exposure, with 175 million people living in WUIs with high zoonotic potential, representing 25 per cent of the region's population. India and China were categorised as having moderate zoonotic potential, but with high human population densities, 10 per cent of the combined population of these two countries (around 275 million people) live in regions with high zoonotic risk.
In South America, over 47 million people (11 per cent of the population) reside in WUIs with high zoonotic potential, particularly around urban centres such as Rio de Janeiro and São Paulo in Brazil. The study also highlighted areas in Central America and the Caribbean, where 31 million people live in high-risk zones.
Interestingly, the researchers identified highly urbanised regions such as northeastern North America and west-central Europe as having high zoonotic potential, despite not being known for their species richness. This finding is attributed to the large WUI areas in these regions and the potential biases in geographic sampling. In these regions, 53 per cent of the population – around 393 million people in west-central Europe – live in WUIs with over 20 zoonotic host species. In contrast, most of North America's population resides in WUIs with low to moderate host richness.
In conclusion, the scientists underscored the importance of understanding human-animal interactions in WUIs.
By doing so, they could improve our knowledge of how diseases like Ebola and other zoonotic diseases, which are linked to urban poverty and forest fragmentation, spread through rapidly urbanising areas.