Researchers have long known that the number of human infections from the bat-borne Nipah virus fluctuates from year to year. Now, a new study provides insights into the reasons why.
Studying Nipah virus informs COVID-19 research, and may help us avoid novel disease outbreaks in the future because bats are hosts for several diseases that spread from animals to humans. Also, bat-borne viruses have many genetic similarities to SARS-CoV-2, so bats are thought to be the likely host for COVID-19.
Nipah virus outbreaks have occurred in South and East Asia and can be spread by pigs as well as bats. In humans, it can cause symptoms that include respiratory illness and brain swelling. There is no cure for the virus and it kills roughly 70% of the humans it infects.
An international team of researchers that included Luby investigated transmission patterns of the Nipah virus in bats in Bangladesh by analyzing data from a six-year study of bat ecology, blood and immunology and genetic similarities between it and other viruses.
Nipah virus comes and goes with the seasons
As the Q&A explains, the team discovered multi-year cycles in transmission of the Nipah virus -- cycles that likely explain why infections fluctuate over time in humans living near these bat populations.
"We have known for many years that in Bangladesh there is a clear seasonality to human Nipah infections," Luby said. Now the researchers have a better understanding of the reasons for that. The finding that the transmission of Nipah virus in bats rises and falls in multi-years patterns could explain why the number of human cases of Nipah virus have similar ebbs and flows over time.
Their research also points to changes in bats' immune systems as a likely cause of the transmission cycles over time.
Bats infected with the Nipah virus might not shed the virus if their immune response is strong enough to control the infection, Luby explained. But immunity diminishes with time, and after a period of years the virus can overwhelm the immune system causing the bat to shed the virus.
This is important for disease modeling and control efforts, Luby said, "because it means that you do not need to have newly infected animals enter the population in order to reintroduce the virus."
COVID-19 pandemic risk of viral spillover
The findings also could have implications for COVID-19 research.
For some people, Luby said, viruses like the Nipah virus may seem like deadly but distant concerns. But they're not.
"People think of these bat-borne viruses as exotic diseases that are far away," Luby said. "The COVID-19 pandemic illustrates, however, that local spillover of novel viruses can affect the whole world."
As Luby notes in the article, human encroachment on natural ecosystems also increases human exposure to novel viruses from animals.
"Risk of spillovers and even pandemics is a recurring theme in the epidemiology of emerging infections," he said. "If we can preserve natural habitats and natural food sources, then bats and other wild animals will often prefer to leave human settlements alone."