Viruses May Jump from Bats to People More Often Than Realized

Sept. 21, 2021 -- When humans and other species intermingle and viruses move between them, experts call that "spillover." Scientists expect more spreadovers in the future as people move to new areas where wild animals can live and change their habitat boundaries.

CoronavirusesThese viruses, which are very common in bats are not unusual. Most often, however, the transmission of the virus to humans is bridged by some intermediary animal. For example, the Middle East respiratory syndrome, or MERS, coronavirus likely moved from bats to camels, and then from camels to people.

Most people infected with MERS developed severe respiratory illness, including fever, coughing, and shortness of breath, and about 3 or 4 out of every 10 people with MERS have died.

The controversial topic of SARS CoV-2 (the coronavirus that causes COVID-19) and how it made the leap between bats, humans, has prompted investigators to look at the bigger question: how frequently do such leaps take place, especially when they are happening directly between people and bats.

According to a preprint study posted online on Sept. 14, which hasn't been peer-reviewed yet, as many as 400,000 people each year in South and Southeast Asia might pick up SARS-related coronaviruses directly from bats. Because of high overlap between human and bat populations, the study was limited to South Asia and Southeast Asia.

Undetected Infections

Most instances of these "undetected spillovers," as the study authors call them, don't ping public health radar because they simply fizzle out. These infections go untreated and cause mild to no symptoms, or symptoms similar to common viruses. The human immune system simply quashes them most of the time, leaving behind antibodies to the virus as evidence of the victory.

The work is still being vetted and the experts led by Peter Daszak PhD, British zoologist president of EcoHealth Alliance used multiple data sources to reach their conclusion.

Another source was information on the locations of bats and people in common habitats. A second source were human blood samples that contained the antibodies of people who had been fighting the coronavirus. They also provided information on how long these antibodies lasted. The investigators collected data on the frequency that bats and humans come into contact with each other.

They used all this data to calculate the chance that humans could contract a virus by a bat. This led them to their conclusion of approximately 400,000 encounters per year.

The authors acknowledge that the work they did is only an estimate and has many limitations. They hope their findings will help epidemiologists and experts dealing with infectious diseases to monitor the situation. These maps could be used to help target resources for capturing infected clusters and preventing them from spreading.