Bats are reservoirs of many microorganisms responsible for infectious diseases. One such is a type of virus called filovirus, which causes severe haemorrhagic fever in humans and primates.
We know of ten different varieties of filoviruses, including Ebola, the Marburg virus and the Mengla virus.
In South and Southeast Asia, there is no historical record of filovirus haemorrhagic outbreaks. A new study presents the first evidence of filovirus exposure in humans (apart from Reston virus transmission in the Philippines) in this region. The study states that humans likely acquired the infection from bats.
“From our study we find that there is some indication of spillover disease that is happening from wildlife to humans,” says Pilot Dovih, a doctoral candidate at the National Centre for Biological Sciences (NCBS), Bengaluru, and a member of the research team.
The finding has significant public health implications as epidemic breakouts of zoonotic infectious diseases have become regular around the world.
“The ever increasing conflict at the human-animal interface and impending climate changes enhance spillover of pathogens from animals,” G. Arunkumar, director of the Manipal Institute of Virology, told The Wire. In this context, he argues it is important to understand the biodiversity and its health effects in India to identify and mitigate public health threats. “Hence this is a very important first step.” Arunkumar was not part of the research team.
Scientists undertook this work over a four-year collaboration between NCBS and the Duke-National University of Singapore (NUS) Medical School, Singapore, along with others in the US and China.
The team had earlier come to know of the Bomrr clan’s annual bat harvest in October in Nagaland. Sensing an opportunity to examine both bats and people at the same time, the team visited the cave where the Bomrr smoked out bats.
“These sorts of situations are quite difficult to find,” Ian Mendenhall, a principal research scientist at the Duke-NUS Medical School, said. This is because although it would be possible to screen blood for antibodies from animals at an abattoir, it would be difficult to connect exposure to a specific species. “Here we have the same individuals harvesting the same two species of bats, one day per year, every year,” Mendenhall said. “This is a very unique cohort and we are fortunate to study these interactions.”
When the clan’s members smoke the cave, they’re exposed to the bats’ blood, saliva and faeces as the hunters and the hunted come in direct contact.
During the 2017 bat harvest, the team drew blood from 85 willing donors. They also drew blood from 16 bats of the Eonycteris spelaea species and 30 of the Rousettus leschenaultii species. And they collected kidney, lung and spleen samples from 34 E. spelaea and 69 R. leschenaultii bats.
The team carried these samples back to their labs and screened them for filoviruses. They found five out of 85 humans had developed antibodies – which isn’t active virus but a neutralising immune response to one – against filoviruses. They also found antibodies in one and four of the E. spelaea and R. leschenaultii bats, respectively.
When the researchers chemically analysed filovirus strains from the humans and the bats, they found the samples from E. spelaea displayed the same reactivity pattern as that from the humans. This suggested that bats and humans had exchanged the virus but the virus hadn’t affected the humans.
“This may be because these viruses lack the capacity to replicate in humans and thus don’t transmit human to human, or there are ecological barriers where humans aren’t encountering bats often,” Mendenhall said. But, according to him, “the results are evidence of humans being exposed to filoviruses and generating an antibody response, so there may be an opportunity for sustaining transmission.”
David T.S. Hayman, a professor of infectious disease ecology at Massey University, New Zealand, thinks the study adds two things to what we know. “First, there may be more than one type of virus circulating in this region of Asia, but it is not clear what they are yet, because this study detected antibodies and not the viruses themselves. Second, key … groups of people can be at risk of infection.” Hayman wasn’t involved in the study.
Mendenhall also argued that the geographical footprint of filoviruses is larger than previously estimated, and the genetic diversity of filoviruses greater than expected.
“This study is like a surveillance case that can lead us to find what other things are there in the bats and help us prepare for the things that are circulating in the bats,” Dovih said. “This study shows that prevention is better than cure.”
The study was published in the journal PLoS Neglected Tropical Diseases on October 31, 2019.
Vrushal Pendharkar is a freelance science writer.