What Is Spillover? How Animal Viruses Infect Humans

What Is Spillover? How Animal Viruses Infect Humans

Workers prepare ducks at a market in Phnom Penh, Cambodia, on February 24, 2023. Two people in Cambodia were confirmed to have been infected with H5N1 bird flu, but it appears to have been a result of exposure to infected birds or poultry.

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

  • Spillover is likely happening every day somewhere around the globe, an expert says.
  • The European Centre for Disease Control warns that mutations found in the H5N1 avian flu virus are “concerning.”
  • The U.S. is experiencing a massive bird flu outbreak that has spread to mammals.

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Following the COVID-19 pandemic, outbreaks of viruses such as the bird flu we keep hearing about might make us concerned about another disease jumping, or “spilling over” from animals to humans.

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

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

“With the wide geographical distribution of avian influenza viruses and high number of detections also in wild birds and mammals, sporadic human cases infected with HPAI (highly pathogenic avian influenza) viruses cannot be ruled out whenever people 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 those in frequent contact with birds, however, the risk assessments contain “high uncertainty.”

In the U.S., which is experiencing an outbreak of bird flu in at least 47 states, the virus has spilled over 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 Maine coast last summer. In some of the seals, the virus had mutations associated with adaptation to mammals.

(S​EAL DEATHS: Maine Seals Dying In Unusually High Numbers)

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

To learn more about how this “spillover” happens (and how to prevent it), we talked with Dr. Treana Mayer, a veterinarian and post-doctoral fellow in microbiology at Colorado State University. Here’s our conversation, edited for clarity.

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

Spillover is an event when a contagious disease jumps from animals to people. Usually, people are not a typical host for the disease. It could be a totally new disease, like when COVID-19 first jumped to people, or it could be a well known disease in animals that’s unusual to find in people.

How common is spillover with viruses?

It is much more common than scientists previously thought. Spillover is likely happening every day somewhere around the globe, given how close we live and work with animals and the diversity of viruses out there. Many of these spillover events don’t lead to bigger outbreaks, however, instead fizzling out on their own.

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

No, thankfully, not all viruses in animals are capable of spillover to people. There are a lot of barriers for a virus to be able to enter and cause disease in people. Some viruses are naturally more flexible and can jump between species more easily. Certain viruses can mutate very quickly, including coronaviruses like the one responsible for COVID-19, and influenza viruses. This means they could eventually gain the ability to infect humans through random changes in their genetic code. Lastly, if the virus is normally found in animals more similar to people, like primates, the jump to humans is easier for the virus, with less biological distance to cross.

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

The dictionary definition of spillover generally is an unexpected consequence of something in excess. If we think of the simplest example of a cup being overfilled with water, the water will spill over the rim and splash anything nearby. We can think of this cup as a population of animals, the water being a disease capable of infecting people, and us humans in the splash zone. This is a useful metaphor because the rate and severity of animal infections and how closely we interact with those sick animals are all important for the likelihood of spillover to occur.

What can humans do to make future viral spillover less likely?

There are two main areas we can focus on: first, preventing spillover at the source, which often originates in wildlife. With climate change, habitat loss, and land-use changes, diseases from wildlife are more likely to mutate and spill over as the animals are stressed, crowded, and on the move. We need to protect natural spaces and keep wildlife wild, separate from us and our agriculture animals. And second, improving our ability to detect new spillover events early, before it gets out of control. This includes the detection of new diseases in both animals and people.

Who should be worried about spillover happening to them?

The people who are at highest risk of spillover are those most likely to contact large numbers of sick animals without taking protective measures like good hygiene or wearing gloves and masks. This could be people working with agriculture animals, in the wildlife trade, or communities encroaching on previously wild lands during animal outbreaks. Spillover only becomes a threat to the general public if the disease becomes adapted to our human biology. This usually occurs after repeated spillover, which is why early detection and prevention of future spillover is vitally important.


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What Is Spillover? How Animal Viruses Infect Humans

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