Seal Flu Shows How Viruses Hop From Birds to Mammals

(Michael Cameron/NOAA Fisheries Service via AP, file)

(CN) — A new study examines the way that viruses mutate to hop between birds and mammals, giving possible insights into the spread of future viruses.

Zoonotic viruses, the technical term for illnesses which spread between animals and humans, are a matter of particular interest at the moment because current evidence suggests that SARS-CoV-2 — the virus which causes Covid-19 — has a zoonotic source. The World Health Organization suggests that while SARS-Cov-2 originated with bats, transmission of the virus to humans happened through another animal species more likely to be handled by humans.

In 2014, an avian influenza virus caused an outbreak in harbor and gray seals in northern Europe, killing over 10% of the population. The strain of avian influenza virus responsible for the outbreak, of the H10N7 subtype, is believed to have been caused by a seal coming into contact with birds or their droppings, but how it passed between seals is unknown.

“Transmission from seal to seal is likely to have occurred via aerosols or respiratory droplets, most probably whilst the seals [were] resting on land. However, direct contact transmission between seals can also not be excluded because seals are highly social and interact with each other regularly,” says study author Sander Herfst, an assistant professor of molecular virology and virus evolution at Erasmus MC.

The outbreak provided an opportunity for researchers to study how influenza A viruses, which are known for their high interspecies transmission, may jump across mammal species. In a study published Wednesday in the journal Cell Host & Microbe, Herfst and his colleagues describe how they collected both avian and seal-adapted variants of the virus that caused the 2014 outbreak and measured their transmissibility between ferrets, which were used for the study because viruses that are spread through the air between ferrets are likely also transmissible between humans and other mammals.

“We found that the seal-adapted virus was efficiently transmitted through the air via aerosols or droplets between ferrets, whereas the avian virus was not,” Herfst says. “These findings suggest that the mutations the avian virus underwent once it took hold within the seal population have allowed it to become transmissible via the air between mammals.”

Researchers found key mutations in the virus hemagglutinin, a protein on the surface of influenza viruses that plays an important role in binding to host cells. These changes, the study found, affected the stability of the hemagglutinin and in addition led to the virus preferring to bind to mammal virus receptors in the respiratory tract.

The study’s findings show that there are consistent viral mutations that allow avian influenza to become transmissible between mammals. This may eventually help in monitoring the spread and mutation of zoonotic diseases.

“It is important to monitor and predict which of the various zoonotic viruses have the potential to emerge in humans and start outbreaks or even pandemics,” Herfst warned. “With such monitoring, it might be possible to create a preemptive approach to future virus outbreaks, thus shortening or even preventing pandemics.”

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