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H5N1 vs H3N8: Which Bird Flu Poses the Greater Risk to Humans?

Chickens roam as bird flu grows

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While the H5N1 bird flu has been dominating the news (and headlines about human infections,) a new bird flu has quietly entered the chat. The H3N8 bird flu just took its very first human victim. Could there be more? And what’s the difference between H3N8 vs H5N1? 

The term “bird flu” or “avian flu” actually refers to any influenza Type A viruses that infect birds. Normally, these don’t cause infections in humans, but every now and then they may after close contact with sick birds, the CDC shares. The big concern is that one of these may one day mutate to allow for prolonged human-to-human transmission. The more prolific a virus is in the wild and the more often it contacts humans, the more likely this is to one day happen. And the more it spreads between mammals, the more likely it is to be able to mutate into a form that’s more readily transmissible to humans. That’s one of the reasons why experts try so hard to keep these viruses in check. We don’t have natural immunity to these, and some carry pretty high death rates from what we know so far. 

So what’s the difference between H3N8 and H5N1? 

Infections in People: H3N8 Is Far Below H5N1

H3N8 has infected only three people so far: one in March 2023 and two in 2022. There have been no reports of human-to-human transmission. H5N1, meanwhile, has a broader history of human inefection, affecting more than 880 people since the 1980s. While there have been no H3N8 cases in the U.S. in people, there was one case of H5N1. 

Here’s a look at the H3N8 human infections: 

  • The first was a child in China in 2022 exposed to backyard ducks and chickens, who recovered after hospitalization
  • The second was a child in China in 2022 exposed in a wet market who only had mild symptoms.
  • The third was a 56-year-old female in China, first reported on March 26
    • She died on March 16, 2023 and had “multiple underlying conditions,” WHO said.  
    • She had a history of exposure to live poultry and wild birds around her home. No one close to her developed symptoms. It appeared that she may have been exposed at a wet market. 
    • WHO noted: “Based on available information, it appears that this virus does not have the ability to spread easily from person to person, and therefore the risk of it spreading among humans at the national, regional, and international levels is considered to be low.”

And here’s a look at H5N1’s broader history of human infection.

  • In the U.S., there’s only been one reported case of human infection of H5N1 so far. This was in April 2022 and may have been a contamination of nasal passages and not an actual infection.
  • In total, 880 people have been infected since 1987 wordlwide, with a case fatality rate of about 50%.
  • There have been sporadic moments of cluster groups, such as 20 infections and 7 deaths in Hong Kong from 1997-2003. 
  • An 80-year-old man had an asymptomatic case in England in December 2021 (he raised ducks that became sick) 
  • A human case in Spain (asymptomatic poultry worker) in September 2022
  • Spain reported a second human case at the same farm
  • Ecuador reported its first human infection in a child who became critically ill after backyard poultry exposure in 2023
  • Cambodia reported 2 human infections (including 1 fatality) in February 2023. (These appear to be different from what is circulating in wild birds in the U.S.) 
  • Chile reported a human infection in March 2023. The source of the investigation is being investigated.
  • There have been 11 human cases of H5N1 globally since January 2022. All but the Chile infection were confirmed to be from poultry exposure.

There have also been probable cases of limited human-to-human transmission of H5N1 over the years, making it more concerning. These were limited to households with close exposure or hospitals. 

Symptoms of H5N1 are like many other respiratory infections: from mild upper respiratory symptoms to severe pneumonia, encephalitis, and organ failure. There have even been occasional asymptomatic cases, including one in the UK in 2021. 

Other Bird Flu Viruses Have Been Reported in Humans

It’s worth noting that other bird flu viruses are reported in humans too. In fact, H7N9 viruses actually have accounted for the highest number of human infections, even more than H5N1, CDC reported. H5N6 also has quite a few.

Here are some examples. 

  • H5N6 has infected more than 80 people in China since 2014, including a 2021 case and five cases in China in 2020.
    • Laos reported a mild infection in a child in March 2021.
    • China reported 36 human cases with 18 deaths in 2021.
    • Then China reported seven cases from poultry exposure from January-April 2022, including 1 death.
    • And four human cases from poultry exposure, wiht 1 death, from May-September 2022. Case fatality is about 30%. 
  • H5N8 infected one person (a poultry worker) in Russia in 2020. 
  • H9N2 infected a child moderately from indirect poultry exposure in Hong Kong in 2020, and another child moderately in Senegal in 2019.
    • Five additional cases were reported in China in 2020 (four in children and one adult, all mild.)
    • Then in 2021, 24 cases were reported in China, including 19 mild cases in children, 2 mild cases in adults, 2 severe adult cases, and 1 adult death.
    • Then in 2022, China reported four mild cases from January-April.
    • Cambodia reported a one-day hospitalization case in a child who recoverd in March 2022.
    • China reported another 4 cases in humans from May-Sept. 2022.
  • H10N3 infected an adult in China in May 2021, who recovered after being critically ill. They visited a live poultry market one week prior. Another subtype of H10N3 infected a human in China in June 2021.
  • H10N3 had an unknown exposure from May-September 2022 in a man in China, who recovered after being critically ill. 
  • H3N8 infected its first human — in China — in April 2022
  • H7N7 has reported limited, probable human-to-human transmission in the past. 
  • H7N9 has reported limited, probable human-to-human transmission in the past. 

Infections in Wild Birds & Mammals: H3N8 vs H5N1

CDC's map of H5N1 in birds
CDC’s map of H5N1 in birds

H3N8 is not currently circulating among wild birds in the U.S., while H5N1 is circulating widely among mammals here in the States. 

It’s worth noting that H3N8 viruses of a different lineage than those infecting humans in China have been found in birds and mammals in the U.S. in the past. In 2011, 162 seals in New England died from H3N8. 

H5N1 is currently circulating among wild birds and mammals in the U.S. 

  • 6,542 wild birds detected in the U.S. as of April 13: See a map here.
  • 58,653,031 poultry affected since January 2022. 47 states are affected. 

Here are some more specific examples of animal cases of H5N1: 

  • Sea lions in Peru were infected in February 2021. 
  • U.S. reported its first cases in wild birds siince 2016 in January 2022. 
  • An outbreak in turkeys in the U.S. was reported in a commercial facility in February 2022
  • More cases in mammals were reported in the U.S. and other countries from May-September 2022, including: 
    • Eight U.S. states with reports in foxes
    • Two bobcats in Wisconsin
    • 1 coyote pup  in Michigan
    • Raccoons in Washington and Michigan
    • Skunks in Idaho (and Canada) 
    • A Mink (in Canada) 
    • 10 seals in May from June-July 2022
    • Also seen in: otters, lynx, polecat, badger (all in Europe) and raccoon dogs in Japan
  • 2023 animal exposures included: a major outbreak in Peru in seal lions and pelcicans. As of March 10, 16 countries in Latin America and the Caribbean, along with the U.S. and Canada, and most of the rest of the world, have seen outbreaks in wild bird or poultry,. 

Here are some interesting statistics about how other bird flus are affecting mammals:

  • H5N8 infected swans, seals, and a fox in the UK in late 2020. 
  • H5N8 infected seals in UK, Germany, and Denmark in 2021
  • H7N7 has caused seal epidemics
  • H4N5 has caused seal epidemics
  • H4N6 has caused seal epidemics
  • H3N3 has caused seal epidemics
  • H10N7 has caused seal epidemics

These are a lot of numbers to process. But in summary, although a human death from H3N8 was just reported, it’s still H5N1 that is the bigger concern at the moment. 

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    Stephanie Dwilson started Post Apocalyptic Media with her husband Derek. She's a licensed attorney and has a master's in science and technology journalism. You can reach her at [email protected].

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