Cross-species filovirus spillover – a dormant threat
Antibodies against filovirus have been detected in the blood samples of bats and the native human community hunting them, leading scientists to hypothesize that cross-species filovirus transmission from bats to humans had occurred in the past.
SINGAPORE | 1 November 2019
Infectious diseases, diseases caused by living organisms like viruses and bacteria, affect people, domestic animals and wildlife alike, with many pathogens being able to infect multiple species. In the mid-1900s, the commercial use of antibiotics led to an age where humans can no longer fear deadly consequences from infectious diseases such as pneumonia and smallpox. Today, despite the rising concern of antibiotic resistance, it is not the bacteria that cause our everyday illnesses that are our largest cause for concern, but rather, the increasing emergence of mostly viral, zoonotic diseases from wildlife which have the potential to cause fatal outbreaks of epidemic proportions.
A collaborative study by the National Centre of Biological Sciences (NCBS) in India, Duke-NUS Medical School in Singapore, and the Uniformed Services University of the Health Sciences (USUHS) in the USA has revealed that virus spillover – the transmission of viruses from one species to another – may be occurring between bats and humans in a small village in India.
Far away in the state of landlocked state of Nagaland, India, Mimi Village is home to three clans – the Whourr, the Merr and the Bomrr. The Bommr clan of the Longpfurii Yimchungii sub-tribe have an unusual yearly tradition that has persisted for more than seven generations – bat hunting in the Bomrr Lapkhun cave. Every October, the locals climb the cliff to reach the caves with firewood, block all the openings and light a fire to smoke the bats. The bats either fall dead or try to fly out, and those that come out alive are killed with sticks. The fallen bats are then picked up from the floor, cooked and eaten with rice, or stored as medicine and seasoning.
“The hunters are often scratched and bitten by bats trying to escape the smoky cave, exposing them to virus shed by the bats,” Said Pilot Dovih, the study’s lead author from NCBS.
For the study, 85 individuals participating in an annual bat harvest in Mimi village were provided a paper-based survey to record their gender, age, occupation and number of times involved in the bat harvest. Blood of consenting volunteers was collected in a serum separation tube, while bat blood samples from E. spelaea, the Cave Nectar bat, and the R. leschenaultii, a species of fruit bat, was collected via cardiac puncture after being sacrificed by the harvesters.
The results of the study indicated that two species of bats and the locals involved in the bat hunting activity have been exposed to viruses in the family Filoviridae, which includes Ebola and Marburg viruses. Antibodies against two and three distinct filoviruses have been detected in the blood samples of the locals and the bats respectively, indicating that cross-species virus transmission is likely to have occurred in the past. On closer observation, the pattern of reactivity of anti-filovirus antibodies carried by the humans were similar to that found in the Cave Nectar bat, leading researchers to conclude that these bats were the most probable source of the viral spillover event. However, it was noted that no viral genetic material was detected in any of the samples.
Despite these bone-chilling findings, Dr Ian Mendenhall, Principle Research Scientist from Duke-NUS’ Emerging Infectious Disease Programme, the senior author of the study, wants to ensure that the results to this study does not result in demonization of bats. “These animals are an essential part of our ecosystem because they are major pest control, pollination and seed dispersal agents,” Said Dr Mendenhall, who has studied animal reservoirs of infectious disease for the past 20 years. Instead, he strongly advocates in protecting the bats’ natural habitats.
This study reaffirms the importance of virus surveillance at wildlife and human interfaces where the risk of virus spillover may be the highest. This includes rural populations in South-East Asia, and native tribes that roam the forests and are in close proximity to wildlife.
Filoviruses, including ebolaviruses and marburgviruses, are pathogens with epidemic potential. Yet, despite proving that bats and locals in eastern Nagaland have been exposed to filoviruses, there has been a historical absence of outbreaks of filovirus haemorrhagic fever in this region. The team has expanded their sampling range in efforts to discover and characterise filoviruses across Southeast and South Asia, hoping to better understand why we have yet to witness a major bat-borne filovirus outbreak in the region.
Henceforth, the team aims to investigate another similar bat-hunting site in another part of India. However, due to time and cost constraints, the researchers are in the midst of developing more effective field and laboratory techniques for the quick detection of viral spillovers. The team will also be investigating viral sequence data to better understand the evolutionary history of these viruses, and their potential to cause outbreaks.
The research paper was published in October 2019 on PLOS Neglected Tropical Disease.
This article was contributed by Michelle Tan Min Shuen, an editorial intern at World Scientific Publishing Co. and a contributing writer for Asia-Pacific Biotech News. She is from Nanyang Girls' High School, has a keen interest in chemistry and the life sciences, and pursues taekwondo in her free time.