The COVID-19 pandemic remains something of a mystery, as scientists struggle to find out where the virus came from and how it entered the human host. This line of study is not merely academic, as it alone can tell if future pandemics caused by unknown agents are likely to arise, and where, and why. After all, this is not the first outbreak of zoonotic disease in the past few decades, and definitely not the first caused by pathogenic coronaviruses known to infect animal species in the wild.
Why Zoonoses Emerge
A new study published on the preprint server medRxiv* in August 2020 discusses the root causes underlying the emergence of such infectious diseases following their crossover of the line separating animal viruses from human. This initial event is of enormous significance.
In the present pandemic, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has a 96% genomic sequence identity with a strain found in horseshoe bats. It is in these animals that we find most of the earlier ancestors of the SARSr-CoVs, making them the probable reservoir for these viruses. However, it is unclear whether this occurred directly or through an intermediate jump from bats to another species, such as the Malayan pangolin, and thence to humans.
Nonetheless, it is quite clear that a growing number of contacts between animals in the wild and humans are driving the entry of animal viruses into the human community. The researchers in the current study point out that this is the effect of several linked factors that decrease the distance and increase the rates of contact between humans and wild animals, as well as between humans themselves.
Intermediate Horseshoe Bat (Rhinolophus affinis). Image Credit: Binturong-tonoscarpe / Shutterstock
Factors that Promote Human-Wild Animal Contact
These factors include a burgeoning population restricted in various ways from spreading out into less crowded spaces, increased growth of cities; a rise in per capita income in mid-income countries; and changes in dietary patterns linked to these shifts. That is, the increasing demand for animal products that accompanies the greater availability of free cash causes more of the forest land to be claimed for farming and animal husbandry – also called forest encroachment.
This inevitably affects the ecology of the forest systems. The entry of humans into areas formerly inhabited by wild animals, the pursuit of hunting, the clearing of forests to create pastureland for livestock to graze or for animal farming, may all encourage animal viruses to enter the human community. And in addition, the loss of old forest and breaking up of a single forest into smaller, nonviable fragments causes niche animal communities to die out while favoring the survival of generalist species.
Finding the Link: Land Use and Pandemic Emergence
This research, therefore, focuses on finding a link, if such exists, between the change in the way land is used in modern times, and the COVID-19 pandemic, including as the chief drivers the fragmentation of forest and the encroachment of humans into the homeland of the wild. The horseshoe bats of genus Rhinolophus is used as an index system to demonstrate how future outbreaks of coronavirus infections can occur, given the great variety of such bats in China, as well as of bat coronaviruses (CoVs) resembling the SARS-CoV.
The researchers found that two of the four CoV genera are found in bats, namely, alpha and beta-CoVs. All four of the betaCoV subgenera infect bats, and this includes SARS-related CoV (SARSr-CoV). SARS itself began first in southeast China and was then traced to R. sinicus and R. affinis, among other horseshoe bats. The close resemblance of the SARSr-CoV to the dangerous human SARS led them to look at disruptions of the bat habitat in association with the bats themselves. This included South and East Asia and China.
Univariate spatial analysis of coronavirus outbreak drivers (A) Sampling points randomly generated within China (dark purple) and outside China (light purple) and bat location points (yellow), weighted by the horseshoe bat species distributions present in East, South & South East Asia; (B) hotspots (red) and coldspots (blue) of livestock density; (C) hotspots of forest fragmentation; (D) hotspots of human settlement.
China Crowded with Livestock
In China, livestock is abundant within the regions with horseshoe bats. In fact, this is a diversity hotspot, relative to the rest of the country, containing higher percentages of domestic poultry, pigs and cattle. The researchers drew a circle with a 30 km radius from each bat location and found that these domestic animals were found at much higher densities than in randomly selected areas.
Loss and Fragmentation of Forests
Forest cover and fragmentation of the forest is again more obvious, statistically speaking, in the region around a random area in China compared to other regions. This shows that forest cover, and cropland density, are lower here, and the forest is broken up into bits and pieces, unlike other areas examined in the same study. This is more significant around the points where the bats are actually seen, compared to randomly picked locations outside China but in the region of distribution of the bats.
Humans have also encroached on horseshoe bat habitats, building their villages and towns there, with high population density. Thus, these two species share the same hotspots of activity. The researchers say, “These results demonstrate that China exhibits stronger signs of human encroachment, livestock density, and forest disturbance of SARSr‐CoV hosting horseshoe bat distributions than other regions. Regions close to forest fragments exhibit lower forest and cropland cover.”
In other words, China is one of the significant hotspots bringing all these factors together - fragmentation, livestock density, and human settlement, some others being in eastern Nepal, Bangladesh, North-east India, and Kerala. The researchers use this knowledge to identify spots of future potential virus spillover. This could occur if one or more of these factors cross the boundary between minor change and a hotspot state.
They conclude that the region of China to the south of the major port Shanghai is probably going to become a hotspot as forests are rapidly being fragmented there. Japan, and the northern part of the Philippines, are also ready to transition for the same cause.
Interacting Factors Increase Epidemic Risk
The geographic area between the hotspot in China and the not so hot spot in Indochina, and that around the Thai hotspot, show signs of increasing human and livestock activity, respectively. This means these regions are “suitable for SARSr‐CoV spillover from wildlife to humans [or] at risk of becoming prone to spillover” as land use patterns and human encroachments increase.
In addition to the ecological impact and the expansion of agricultural land, movement of people or animals, and trade activity, interact with the intrinsic host properties as well as the interventions adopted at this time (biosecurity, wearing personal protective equipment, strict hygiene for meat preparation and consumption) to reduce the final risk.
The researchers call attention to the need to counteract these dangerous tendencies seen in the threatening or potential hotspots, such as by maintaining or rebuilding forests, removing livestock pastures and farms much further away from forest borders and reducing the density of human activity in these buffer zones. Future environmental impact assessments should also consider the risk of turning the region concerned into new hotspots for spillover zoonotic diseases to emerge.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.