What is overflow? How animal viruses infect humans

Workers prepare ducks for a market in Phnom Penh, Cambodia, on Feb. 24, 2023. Two people in Cambodia were confirmed to be infected with H5N1 avian flu, but it appears to be the result of exposure to infected birds or poultry.  (Photo by TANG CHHIN SOTHY/AFP via Getty Images)

Workers prepare ducks for a market in Phnom Penh, Cambodia, on Feb. 24, 2023. Two people in Cambodia were confirmed to be infected with H5N1 avian flu, but it appears to be the result of exposure to infected birds or poultry.

(Photo by TANG CHHIN SOTHY/AFP via Getty Images)

  • Spillover probably happens every day somewhere around the world, one expert says.
  • The European Center for Disease Control warns that the mutations found in the bird flu virus H5N1 are “worrying”.
  • The US is battling a massive outbreak of bird flu that has spread to mammals.

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In the wake of the COVID-19 pandemic, outbreaks of viruses, such as the avian flu we keep hearing about, may raise concerns about another animal-to-human disease, or “spill over.”

Just this week, the European Center for Disease Control warned that the mutations found in the bird flu virus H5N1 are “worrying”. It could suggest that the potential for avian flu to jump to humans is increasing, the center said.

“The expansion of mammalian species identified as infected with A(H5N1) viruses, as well as the detection of viruses carrying markers for mammalian adaptation in other genes, such as the PB2 correlated with increased replication and virulence in mammals, is concerning” , the center said. said in a report.

“With the wide geographic distribution of bird flu viruses and the high number of detections, including in wild birds and mammals, sporadic human cases infected with HPAI (highly pathogenic avian influenza) viruses cannot be ruled out when humans are exposed to infected sick or dead birds ,” the report said.

The center did say the threat in Europe remains low for the general public, and low to moderate for people in regular contact with birds, but the risk assessments contain “high uncertainty”.

In the US, where there are outbreaks of bird flu in at least 47 states, the virus has spread to minks, foxes, raccoons and bears. And this week, scientists confirmed that the virus had infected some of the 150 New England harbor and gray seals found dead or sick along the coast of Maine last summer. In some seals, the virus had mutations related to mammalian adaptation.

(SEAL DEATH: Maine seals are dying in unusually high numbers)

Last month, two people in Cambodia were confirmed to be infected with H5N1 avian flu, but it appears to be the result of exposure to infected birds or poultry and not human-to-human exposure, according to the U.S. Centers for Disease Control and Prevention. . That strain of the virus is different from the H5N1 viruses now spreading among wild birds and poultry in the U.S., the CDC said.

To learn more about how this “spill over” happens (and how to prevent it), we spoke with Dr. Treana Mayer, a veterinarian and postdoctoral researcher in microbiology at Colorado State University. Here’s our conversation, edited for clarity.

Can you give us a brief definition of what spillover is?

Spillover is an event in which an infectious disease jumps from animal to human. Usually, humans are not typical hosts for the disease. It could be a completely new disease, like when COVID-19 first came to humans, or it could be a known disease in animals that is uncommon in humans.

How common is virus spillover?

It is much more common than scientists previously thought. Overflow probably happens every day somewhere in the world, given how close we live and work to animals and the diversity of viruses out there. However, many of these spillover events do not lead to larger outbreaks, but disappear on their own.

Are all viruses capable of spillover? What makes a virus conducive to spreading?

No, fortunately not all viruses in animals are able to spread to humans. There are many barriers for a virus to enter humans and cause disease. Some viruses are naturally more flexible and can jump between species more easily. Certain viruses can mutate very quickly, including coronaviruses such as the virus responsible for COVID-19 and flu viruses. This means they could eventually gain the ability to infect humans through random changes in their genetic code. Finally, if the virus is normally found in animals more similar to humans, such as primates, the jump to humans is easier for the virus, with less biological distance to travel.

Spillover is a metaphor. How does that metaphor help us understand what can happen?

The dictionary definition of spillover is generally an unexpected consequence of something being too much. If we think of the simplest example of a cup that is too full of water, the water will spill over the edge and splash everything around. We can think of this cup as a population of animals, the water is a disease that can infect humans, and we humans in the splash zone. This is a useful metaphor because the rate and severity of animal infections and how closely we treat those sick animals are all important to the likelihood of spillover.

What can people do to make future viral spills less likely?

There are two main areas we can focus on: first, preventing overflow at its source, which often comes from wildlife. With climate change, habitat loss and land use changes, wildlife diseases are more likely to mutate and spread as the animals are stressed, busy and on the move. We must protect natural spaces and keep wildlife separate from us and our farm animals. And second, improving our ability to detect new overflow events early, before they get out of hand. This includes detecting new diseases in both animals and humans.

Who should worry about overflow happening to them?

The people most at risk of spillover are those most likely to come into contact with large numbers of sick animals without taking protective measures such as good hygiene or wearing gloves and masks. These could be people working with farm animals, in the wildlife trade, or communities encroaching on previously wild areas during animal outbreaks. Spillover will not become a threat to the general public until the disease adapts to our human biology. This usually occurs after repeated overflow, so early detection and prevention of future overflow is critical.

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