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Influenza A and Avian Influenza - H5N1

Avian Influenza (Bird Flu or H5N1)

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 A 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. This has been shown to happen in pigs, leading to entirely new strains of influenza; should these new strains spill over to humans, pandemics can occur. Additionally, an increase in the number of infections with any influenza strain increases 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 surface proteins, hemagglutinin protein (H) and neuraminidase protein (N). For example, the influenza A viruses that infect humans during winter 2024/2025 are labeled H1N1 and H3N2. The number after the ‘H’ and ‘N’ refers to the protein subtype.

The virus currently infecting large numbers of both wild birds and domestic poultry is influenza A subtype 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; therefore, it is often referred to as highly pathogenic avian influenza (HPAI). Avian influenza H5N1 virus has spilled over from just infecting birds into a wide variety of mammals, including cows and pigs. 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 in humans have ranged from asymptomatic or mild disease to severe or critical disease and death. There was a case of human H5N1 in the US in 2022 - fortunately, the illness was mild and we did not see human-to-human transmission. In March 2024 in the US, we saw a second case of spillover of influenza H5N1 into humans, this time from dairy cows.

As of January 6, 2025, 66 human cases of HPAI H5N1 have been reported in 10 US states, plus one case in Canada. In most instances, symptoms have been mild with conjunctivitis being the most common symptom. Other flu-like symptoms, including cough, have been reported. The first death in the US of a person infected with this strain of H5N1 was reported on January 6, 2025.

In North America, the majority of H5N1 cases have been in farm workers who experienced mild symptoms, and the sources of infections were cows infected with the avian influenza H5N1 virus. Still, three human cases had no known source of infection. The World Health Organization and the US Centers for Disease Control and Prevention have stated that at this time, the avian flu risk to the general public remains low. However, 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. This provides many opportunities for the influenza virus to mutate and/or reassort and possibly infect humans more easily.
  • Asymptomatic H5N1 human cases have been observed.
  • With each human case, it is more likely that the H5N1 influenza virus will change and it’s possible that these changes may lead to greater communicability or more severe disease in humans.
  • We are unsure how cows become infected. Each cow was either infected directly by a bird or another cow. Cows infecting other cows is concerning because the virus has adapted and can transmit from mammal to mammal. Cattle and humans frequently interact, increasing the probability of human infections.
  • Cats can be infected with H5N1, either through exposure to wild birds or by drinking raw milk from an infected cow. This also is concerning due to the frequent interaction between cats and humans.
  • The US Department of Agriculture identified two pigs infected with H5N1 at a non-commercial farm in Oregon last October. Sequencing data show that no mutations or reassortment events occurred as a result of these infections. Reassortment is known to happen in pigs, which may lead to changes in the virus to be more easily transmitted from human to human or cause more severe disease. (See section “Influenza A Mutation and Reassortment”)

TEPHI’s Role in Detection and Response

In collaboration with UTHealth Houston School of Public Health and Baylor College of Medicine, TEPHI established and is currently expanding the statewide Texas Wastewater Environmental Biomonitoring (TexWEB) network. While the virus has been detected in wastewater, the source of the virus in wastewater is still unknown. TexWEB continues to monitor H5N1 in wastewater with an eye toward any concerning genetic adaptations that might make it more transmissible to humans.

Situation as of January 6, 2025

  • As of January 6, 2025, CDC has determined that the risk to the general population remains low.
  • 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, beef, and eggs pose no risk as long as they are cooked to the recommended temperatures.
  • Commercial cow’s milk is not a risk since it is pasteurized. The pasteurization process kills the virus.
  • Seasonal influenza vaccination is recommended for all people ages 6 months and older.

Further Up-to-date Information

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