Viruses can spread from bats to humans more often than expected

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September 21, 2021 – When humans and other species mix and viruses move between them, experts call it “spillover”. As people move and seek new habitats for wild animals to live in, and as climate change pushes the boundaries of those habitats, scientists predict that we will see more of these effects.

Coronaviruses, which are common in bats, are no exception. But most of the time it is assumed that an intermediate animal bridges the transmission of the virus from bats to humans. For example, Middle Eastern respiratory syndrome, or MERS coronavirus, likely passed from bats to camels and then from camels to humans.

Most people who were infected with MERS developed severe respiratory illnesses, including fever, cough, and shortness of breath, and about 3 or 4 in 10 people with MERS have died.

Investigators who have looked at the controversial issue of how SARS-CoV-2 – the coronavirus that causes COVID-19 – made the jump from bats to humans have asked the broader question of how often such jumps happen especially directly between bats and humans, and their assessment is striking.

According to a preprint study published online on September 14th that has not yet been peer-reviewed, up to 400,000 people in South and Southeast Asia could ingest SARS-related coronaviruses directly from bats each year. The study focused on South Asia and Southeast Asia because of the high overlap between humans and bats.

Undetected infections

Most cases of these “undetected spillovers,” as the study authors call them, don’t ping the public health radar because they just fizzle out. The infections go undetected and cause mild or no symptoms or symptoms similar to those of common viruses. The human immune system simply suppresses them most of the time, leaving antibodies to the virus as evidence of victory.

In the work, which has yet to be peer-reviewed, the researchers, led by Peter Daszak, PhD, a UK zoologist and President of the EcoHealth Alliance, used multiple sources of data to arrive at their estimate.

One of these was geographic information about where bats and humans intersected in their habitats. Another source was human blood samples with tell-tale antibody signs for fighting a coronavirus and information about how long those antibodies persisted. And investigators also gathered information about how often bats and humans encounter each other.

When they used all of this information in their calculations of the risk that humans could become infected with a virus from a bat, they came up with an estimate of 400,000 such encounters per year.

Given that their work is only estimates and has many limitations, the authors hope that the results can help epidemiologists and infectious disease experts in surveillance. Maps where these risks are highest could help focus resources on identifying clusters of infections before they spread.

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sources

MedRxiv: “A Strategy to Assess the Spillover Risk of Bat SARS-Associated Coronaviruses in Southeast Asia.”


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