According to new research, restoring natural habitats for wildlife could have another benefit for us: keeping pathogens that originates in the wildlife away from humans.
Based in Australia, the research that when bats have less food and loss of winter habitat in their natural settings, their populations break up and they excrete more virus.
And when the populations break up, bats move near humans, more specifically to agricultural and urban areas.
One of the two companion studies combines various datasets over 25 years. The data includes information on bat behavior, distributions, reproduction and food availability. Other than that, it also provides records of climate, habitat loss and environmental conditions.
Per the study’s prediction, Hendra virus that often develops into a fatal illness in humans, comes from fruit bats to horses and then people.
The researchers discovered that in years when bats could find a lot of food in their natural habitats during winter months, they’ll feed in native forests, away from humans.
Then, a second paper used data from the previous one to show ecological conditions when bats excrete more or less virus.
Different from previous studies which have shown the connection between habitat loss and occurrence of pathogen spillover, the two newer ones reveal a mechanism for such events and provide a method to predict and prevent them.
Viruses from bats to humans
Examples of how viruses can fatally spill from bats to humans include SARS-CoV-2, SARS-CoV-1, Nipah, Hendra and possibly Ebola. In humans, Hendra virus has a 57% fatality rate, and Nipah virus can be up to 100% fatal. Though it should be noted that Nipah transmission in humans is inefficient.
Senior author of both studies Raina Plowright said, “Right now, the world is focused on how we can stop the next pandemic.
“Unfortunately, preserving or restoring nature is rarely part of the discussion. We’re hoping that this paper will bring prevention and nature-based solutions to the forefront of the conversation.”
For the studies, Plowright and colleagues developed datasets from 1996 to 2020 in subtropical Australia and they went through a lot of factors, like the landscapes where they foraged, climate and El Niño events, food shortages period, and reproductive rates, to name a few.
Studying the bats
Then, the scientists created computer models to analyze the data and discovered two major factors that drive pathogen spillover: habitat loss pushing animals into agricultural areas and climate-induced food shortages.
In years after El Niño, buds of trees that bats feed from failed to produce flowers in the subsequent winter, hence the food shortage. Then, forest habitat destruction for farmland and urban development has left few forests that produce nectar for bats in winter.
As mentioned above, food scarcity drives the large populations to fragment and the smaller groups moved to agricultural and urban areas, where weedy species and fig, mango and shade trees offered shelter and reliable but less nutritious food sources than nectar.
Stress from lack of food makes it harder for bats to successfully rear their young. Per the second paper, this stress also makes them shed virus, probably because they needed to conserve energy by directing it away from their immune systems.
It’s also discovered that bats that had moved to newer winter habitats like agricultural areas also shed more virus than bats in traditional winter habitats.
Pathogens could spread in agricultural areas through urine and feces drop where horses are grazing. That may lead to Hendra virus infections. Consequently, horses could act as an intermediary host and occasionally spread the virus to people.
The importance of restoring habitats
Since 2003, researchers have noticed a slow dwindling of large nomadic bats in favor of many smaller roosts in agricultural and urban areas. And within 25 years, the decline has increased fivefold.
Bats are less frequently returning in large numbers to their shrinking native habitats. This could be because forests that provide nectar in winter have been extensively cleared.
The researchers found that when there are remaining eucalyptus trees which bloomed in winter, the bats flocked to those areas. To the authors’ surprise, pathogen spillover completely stopped during those events.
Plowright said, “We put these data into the network models and found that we could predict spillover clusters based on climate, the availability of food, and the location of bats.
“We show that when remaining habitat produces food, spillover stops, and therefore a sustainable way to stop these events could be to preserve and restore critical habitat.”
Preparing by understanding pathogen tolerance in wild animals
Other than restoring the wildlife habitat, another study suggests that understanding host’s ability to tolerate pathogens is also important to prevent future pandemics similar to COVID-19.
Scientists have generally understood that animals with a large, stable and diverse pool of zoonotic pathogens could promote spillover into humans.
However, it’s not as comprehended about how such animals live with these pathogens for so long, as well as what conditions that lead to the point where the pathogen is fit enough to live in other species.
First author of this study Srijan Seal said, “Our understanding of infection and disease has been overtly biased by how we view pathogens that infect us.
“Because pathogens, by their definition, reduce host fitness by causing illness or death, we view host-pathogen interactions as hostile, and ignore the mechanisms that allow the host’s ability to coexist or coevolve with pathogens.”
Since the focus has been to other aspects, Seal added that we fail to understand how some reservoir animals could tolerate disease and how it can be linked to pathogen maintenance and transmission, and how humans are affected by it.
The link between pathogens and hosts
The authors discussed how and why tolerance might evolve naturally during long-term association between hosts and pathogens as an effective strategy for both.
Per the researchers, although activating immunity to respond to infection seems to be the choice that makes sense the most, it’s not the same in coevolution. Host species may fine tune their immune systems to tolerate rather than actively resist infections.
Because when host species activates immunity response, it can rapidly escalate to an uncontrolled inflammatory response that causes long-term harm.
Different from animal hosts, pathogens require a number of sequential steps to spill over into humans, with each step coinciding with certain conditions. The conditions include infectivity of the reservoir host, and being close enough to a large and dense human population.
Before the steps can occur, the authors noted, the most important condition is the maintenance of a large and diverse zoonotic pathogen pool. So, it’s likely that the pathogens adapt to allow their host to tolerate them, increasing the infectious period and pathogen burden which prepares the stage for a spillover.
These processes can also support a genetically diverse pathogen pool by allowing more mutations and genetic exchanges to arise between circulating strains. As a result, there can be new variants of viruses that are more infectious to humans and other hosts than previous strains.
Given the fact that there are additional traits that a pathogen might need to survive in a new host, the authors surmise that there can also be another situation. Hosts can, in fact, harbor multiple pathogens thriving together.
Then, the increased competition can lead to the rapid evolution of traits that enable pathogens to jump into new hosts. This suggests that
Senior author Imroze Khan concluded, “Host immune strategies, ecology and pathogen prevalence all play crucial roles in allowing spillover, but studying them in isolation is far from ideal given the complex interactions involved.”
According to the researchers, there should be more studies on the host so patterns and processes of pathogen prevalence and infection outcomes in the wild can be more understandable.
“This is relevant not only to the present crisis created by the COVID-19 pandemic and emerging infections, but will provide a newer understanding of other important aspects of public health, such as infectious disease control.”