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Influenza A and Avian Influenza - H5N1

Dr. Catherine Troisi explains the avian influenza situation as of May 30, 2024, in the TEPHI Region 6/5 S Vulnerable Populations Working Group meeting. Please keep in mind the video contains information available at the time recorded, and the situation can change rapidly.

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. 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 surface proteins, 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 H5N1 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 with two additional cases in May 2024. The first case, in 2022, was a mild infection, and the person survived. Similarly, in two of the three most recent cases (2024), the infected individuals only experienced an eye infection. However, the third infected individual experienced an eye infection as well as flu-like symptoms including cough. What’s unusual is that the 2024 cases were in farm workers, and the sources of infections were cows infected with the avian influenza H5N1 virus. Public health officials in other states have found dairy cows infected with avian influenza H5N1, but no other human cases have been detected as of May 30, 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 possibly 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 concerning.
  • Although we’ve only seen three cases of humans infected with H5N1 from cows so far this year, as of May 30, 2024, there is a possibility that more cases will occur. With each 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.

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. On March 4, TexWEB detected an H5N1 signal in wastewater using technology that can identify new, as well as every known virus that is in the sample. Since the first detection, TexWEB has observed H5N1 in sites throughout the state. Soon after, TEPHI notified and briefed local, state, and federal partners, including the Texas Department of State Health Services, the Centers for Disease Control and Prevention, and local public health authorities and wastewater treatment facilities. While the virus has been detected in wastewater, the source of the virus 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.

TEPHI’s findings were published as a preprint study on May 10. Click to read the full article or read additional coverage by CNN, Med Page Today, and the Los Angeles Times.


As of May 30, the risk to the general population remains low.

  • The likelihood of human-to-human spread 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 recommended temperatures.
  • Commercial cow’s milk is not a risk since it is pasteurized, which kills the virus. At this time, it’s unclear whether or not raw milk contaminated with H5N1 can infect humans; however, H5N1-contaminated raw milk can infect mice. Additionally, raw milk is not safe to drink as it can carry other pathogens, which pasteurization kills.

Further Up-to-date Information

Texas Department of State Health Services



Additional Resources