The COVID-19 pandemic and its driver, the SARS-CoV-2 virus, are directly linked to bats – this is because the coronavirus originated in bats and eventually made the jump to humans and spread worldwide. During the pandemic, the spillover or jumping of bat viruses to humans gained renewed attention especially the questions of why and how this happens and the chances of such spillovers increasing in the future.
These are pertinent questions to assess future dangers. It has been widely thought that habitat loss and increasing human activities in the wild make spillovers more frequent and likely. There have been previous studies as well establishing this fact.
Recently, a paper published in Nature has shed new light on how and why the bat viruses keep infecting people. This research takes the Hendra virus as the subject of study. However, the findings hint that similar factors driving the spillover of the Hendra virus to humans may also influence spillovers of coronaviruses (like SARS-CoV-2, SARS-CoV, etc.) or other bat viruses like Nipah to humans.
Hendra virus originates in the Australian flying foxes, a type of bat endemic to Australia. This deadly virus causes a rare but life-threatening respiratory disease in humans, having a high mortality rate and killing one in every two infected people. This virus spills over to humans from bats via an intermediary animal, usually the horse.
Scientists have known that habitat loss (induced by human activities) has been intricately linked to the spillover viruses. However, pinpointing the specific conditions associated with habitat loss and other ecological lack of balance has been a struggle for scientists, and the new findings are helpful in this regard.
The researchers claim that their detailed investigations and the outcomes enable them to predict in advance – up to two years – when clusters of the Hendra virus spillover may appear.
Commenting on the findings, Emily Gurley, an infectious disease epidemiologist at Johns Hopkins University, said in a statement, “They have identified the environmental drivers of spillover. And they have determined how those events could be prevented.” Gurley is not associated with the new study. Other experts also expressed hope about the findings.
Hendra was first identified in 1994 after an outbreak among horses and people in Brisbane, Australia. Later, studies found that the virus was spread from flying foxes, first to horses through feces, urine, and other such means. Notably, infections occurred in clusters during the winter season in Australia. The second cluster of infection can emerge even after several years. The concern is that such infections have been rising since the early 2000s.
Peggy Eby, the leading author of the research and a wildlife ecologist at the University of New South Wales, Sydney, Australia, has been studying flying foxes for the past 25 years. Eby, with her binoculars, tallies the number of lactating flying fox females who are about to wean their pups. This has been considered a proxy of understanding whether bats experience nutritional stress and also factor in shedding the viruses that can make people ill. The study authors have enumerated three factors that can provoke the spillover of the Hendra virus to humans – food stress, habitat shift and change in climatic conditions.
The researchers found the specific condition of food stress experienced by the bats when the Hendra virus spills over to humans for years. Importantly, the researchers found that the food shortages experienced by the bats are typically followed by years of the climatic condition of El Nino. El Nino brings droughts to the Eastern region of Australia, which can lead to a scarcity of food for the bats.
However, the researchers also found that the trees the flying foxes rely on as their food sources during winter have a large flowering event right after the year of the food crisis, and then there are no spillovers. However, the irony, according to Raina Plowright, a co-author of the study and a disease ecologist the Cornell University, is that “there is hardly any winter habitat left.”
“What’s so exciting about it is that it has led directly to solutions. The study’s approach of looking at the impact of climate, environment, nutritional stress and bat ecology could bring new insights to the study of other pathogens, including Nipah and Ebola, and their viral families,” commented an infectious disease ecologist at the University of Glasgow, Sarah Cleaveland.
Eby and the others in the team collected data about the location and timing of such events along with locations of bat roosts, their health, climate, and habitat loss around 300,000 square kilometers in Southeaster Australia in the period 1996 to 2020. They hereafter used modelling to determine the factors associated with the spillovers.
The researchers observed a visible change in the habitat of the bats. They used to be nomadic, moving from one forest to another in the wild, but in recent years, they have settled in small groups in urban and agricultural areas and thus getting closer to humans. They form new roosts in their new habitats, and the researchers found that these newly established roosts shed the virus almost every winter. In addition, the shedding increases in the years following the food shortage.
The researchers also found that there has been a dramatic increase in the shedding of the Hendra virus in recent years, which has raised the spillover to the horse first.
The researchers suggest that restoring the habitat of the trees that flower during winter, which the flying foxes depend on as their food source, would decrease the migration of the bats to near human habitats. Thus the spillovers can be reduced in future. This can also be the case for other dangerous pathogens, they argue.
Habitat loss of wild animals has increased due to increased human activities in the wild and also by climate change. With this continuing, the danger of newer viruses and other pathogens getting spilt over to humans and thus creating new outbreaks looms large, a new challenge in front of humanity.