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, 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.
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.
Influenza is classified by its surface proteins, hemagglutinin protein (H) and neuraminidase protein (N). For example, the influenza A viruses that are infecting 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 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 November 18, 2024, 53 human cases of HPAI H5N1 have been reported in seven 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. No deaths have occurred. What’s unusual is that the majority of the 2024 cases were in farm workers, and the sources of infections were cows infected with the avian influenza H5N1 virus. Of these 53 human cases, two had no known source of infection. Epidemiologists and public health professionals are keeping a very close watch on the situation for several reasons.
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.
For the latest news about infectious diseases like Avian Influenza, subscribe to our Literature Report email. Select “Lit Report” under the email lists to subscribe.
CDC
USDA
Texas Department of State Health Services
Source: Reuters