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Influenza A and Avian Influenza - H5N1

Avian Influenza

 

Influenza A Mutation and Reassortment

Influenza A is an RNA virus, and like other RNA viruses such as SARS-CoV-2, it mutates or changes easily and can obtain new characteristics. However, unlike other RNA viruses, influenza can change in an additional way – reassortment. Its genetic material consists of eight separate pieces, and “reassortment” of those pieces can occur when two different influenza viruses infect the same cell and a new type of influenza virus is made -- some segments from the first virus and some segments from the second virus combine into a third virus. Additionally, increased infections with any influenza strain increase the probability that a new strain of influenza A virus will emerge through mutation or reassortment.

What Species Does Influenza A Infect?

Influenza A virus can infect many mammals and birds, but different strains of influenza A are typically limited to one species. However, there can be spillover, where one species infects another with its specific virus strain. Sometimes, this can result in the original virus adapting to the new species and spreading throughout the new species. For instance, the influenza virus that caused the pandemic in 2009 is thought to have originated in pigs, then infected a few humans, and mutated in those humans, making it spread very easily from person to person.

Avian Influenza: H5N1

Influenza is classified by its hemagglutinin protein (H) and neuraminidase protein (N). For example, the influenza A that infected humans during the past winter months is influenza A (H1N1). The influenza virus currently infects large numbers of both wild birds and domestic poultry is influenza A (H5N1). This influenza A found in birds – commonly referred to as avian influenza – is a highly pathogenic strain in birds, which means it causes high rates of death when birds are infected. Avian influenza H5N1 virus has spilled over from just infecting birds into a wide variety of mammals. Since 1997, more than 900 human cases of influenza A (H5N1) that spilled over from birds have been reported in 23 countries, with more than half of these cases resulting in death. Symptoms have ranged from mild disease to severe or critical disease and death. Fortunately, we have not seen sustained human-to-human transmission.

Current News

In March 2024 in the US, we saw a second case of spillover of influenza H5N1 into humans. The first case, in 2022, was a mild infection, and the person survived. Similarly, in the most recent case, the person experienced an eye infection and has recovered. What’s unusual is that this 2024 case in Texas was in a dairy farm worker, and the person got their infection from a cow infected with the avian influenza H5N1 virus. Public health officials in other states have found dairy cows infected with highly pathogenic avian influenza (HPAI) avian influenza H5N1, but no other human cases have been detected as of April 17, 2024.

Epidemiologists and public health professionals are keeping a very close watch on the situation for several reasons.

  • This is the first time we’ve found avian influenza in so many different types of mammals, and this provides many opportunities for the influenza virus to mutate and/or reassort and infect humans more easily.
  • We are unsure how cows become infected. Each cow was either infected directly by a bird or cows are infecting other cows, which is more worrisome.
  • Although we’ve only seen one case of humans infected with H5N1 from cows as of April 17, 2024, there is a possibility that more cases will occur. With each case, it is more likely that the influenza H5N1 virus will change to infect humans more easily.

Outlook

Right now, there is no need to panic.

  • Should the virus change to spread more easily from person to person:
    - Drugs such as antivirals currently used to treat the human influenza strains are effective against influenza H5N1.
    - Scientists can quickly develop a vaccine that works against the influenza H5N1 virus, although the current influenza vaccine for humans does not protect against influenza H5N1.
  • Poultry meat and eggs pose no risk as long as they are properly cooked.
  • Commercial cow’s milk is not a risk since it is pasteurized, which kills the virus. However, we don’t know whether raw milk can infect humans or not, as raw milk can carry other pathogens.
  • While influenza H5N1 has been found in dairy cattle, it has not been found in beef cattle. Even if it becomes present, properly cooking meat would destroy the virus.

Further Up-to-date Information

Texas Department of State Health Services

CDC

USDA

Additional Resources