News & Events

The mercurial nature of influenza has made this virus a formidable foe throughout the centuries. With its talent for mutation, close monitoring of influenza is critical to contain outbreaks of new strains. Over the past year, avian influenza, or “bird flu,” has been in the news. Let’s take a closer look at the history of avian influenza, the current status of H5N1 virus and what this may mean for the future. 

Two types of influenza viruses regularly infect humans: influenza A and influenza B. Both types cause seasonal influenza epidemics, and circulating strains of both are included in annual flu vaccines. While influenza B viruses primarily infect humans, influenza A viruses can infect many types of animals, including livestock, poultry, and wild birds. Avian influenza is used to describe influenza A viruses that commonly circulate in wild birds and poultry. Some avian flu viruses can infect mammals, including humans. When people are infected with avian influenza viruses, they can experience serious disease since human immune systems don’t necessarily recognize the proteins on the surface of avian viruses. 

Influenza A viruses, including avian flu, are often referred to by the types of proteins present on their surface. Hemagglutinin (HA) and neuraminidase (NA) are both proteins found on the surface of flu viruses. The type of HA and NA on the surface of a flu virus helps determine the tissues and types of animals that it can infect. Both HA and NA are targeted by the human immune system when it responds to an infection. 

Multiple types of HA and NA proteins exist, so they are further designated by a number next to the H or N, respectively, when discussing flu. For example, seasonal influenza A viruses are H1N1 and H3N2 viruses. Many avian viruses that are able to infect humans contain HA proteins designated as H5, H7, or H9. Historically, the most common causes of human avian influenza infections have been H5N1 and H7N9 viruses.  

As most people are aware, influenza viruses change regularly as they replicate. That is one of the reasons we get annual influenza vaccines, but these changes can also result in the ability of a particular influenza virus to infect different animals, cause more severe disease, or better evade the immune system. Changes to influenza viruses occur in two major ways, as first described by Dr. Maurice Hilleman. The first, called antigenic drift, refers to changes that occur over time during viral replication, such as the changes from one influenza season to the next.

Antigenic shift, on the other hand, occurs when an animal is infected with two different types of influenza virus at the same time and the two viral genomes mix to instantly create a new influenza virus. Antigenic shift is the type of change that sets the stage for a pandemic. For new influenza viruses to cause a pandemic, three things are needed: the ability to spread from animal to person (i.e., infect people), the ability to cause illness in people, and the ability to spread among people. Both antigenic drift and antigenic shift occur in avian influenza viruses.

Find out more about the concepts of antigenic drift and antigenic shift in this animation.
 

The Fowl Plague

Avian influenza has long been recognized as a disease of poultry. In fact, when the disease was first described in 1878, it was named the “fowl plague.” Almost 100 years later, it was discovered that fowl plague was caused by influenza A virus, and by 1981 it was renamed “avian influenza.” For decades, H5N1 avian influenza viruses have been recognized as a problem on poultry farms, periodically causing mass infections and requiring flocks to be culled. In 1997, H5N1 flu was first recognized as a human pathogen during an outbreak in Hong Kong. Importantly, no human-to-human transmission was recorded, and researchers did not observe experimental transmission between other mammals. In 2003, H5N1 virus reemerged and spread in migratory birds, causing die-offs of wild birds, outbreaks on poultry farms, and illness among people. Over the next two decades, H5-containing influenza viruses continued to circulate in wild bird populations around the world, causing more infections in people than any other type of avian influenza virus. 
 

The Current H5N1 Situation in the U.S. 

H5N1 virus has been continuously circulating in birds around the world since 2021. Historically, H5-containing viruses have infected some mammals but minimal mammal-to-mammal spread had been observed. However, recently transmission between animals has been detected, indicating an increased ability for the virus to spread among mammals. For example, spread among minks on fur farms has been confirmed as has transmission between seals in the wild.

In March 2024, H5N1 influenza grabbed the attention of scientists when it was detected in dairy cattle in Texas. This was surprising because influenza A viruses don’t typically infect cattle or spread among a herd. The virus is thought to have been introduced into cattle in late 2023 when farmers began to notice cattle becoming ill and producing less milk. In the months between introduction and detection of the virus in cattle, H5N1 flu spread rapidly through U.S. dairy herds, likely the result of unwitting interstate transport of infected animals. As of July 2025, H5N1 virus has been detected in 1077 herds across 17 states. 

Influenza viruses are typically spread through respiratory secretions; however, the H5N1 virus spreading among cattle appears to be doing so through infected milk and contaminated milking equipment. In cows, the infection is primarily localized to the mammary glands and may result in the cows experiencing a high fever and reduced milk production. The infected mammary glands can then produce milk that contains high levels of infectious virus. Thankfully, milk pasteurization kills infectious virus, rendering the milk safe to consume. 

Since March 2024, 41 cases of H5N1 influenza have been confirmed in farm workers, though blood tests suggest additional people have been infected but they did not have symptoms. Thus far, people infected with the H5N1 virus circulating in dairy farms have mostly experienced mild disease with pink eye as a common symptom. This is likely due to raw milk splashing into the eyes of farm workers. Importantly, splashing milk is unlikely to get deep into a person’s lungs, so severe respiratory infections have not occurred. Cats, rodents, raccoons, and opossums have also been infected, likely from drinking raw milk. Scientists have shown in the lab that mice can become infected with the virus after drinking raw milk from infected cows, highlighting the importance of milk pasteurization for preventing further H5N1 virus infections. 

In addition to the H5N1 virus circulating in dairy cattle, a different strain of H5N1 virus has been circulating among birds in North America. In the U.S. one person developed severe disease and died as a result of infection with this second version of H5N1 influenza. Between the two types of H5N1 circulating in the U.S., 70 people have been diagnosed with avian influenza infections from February 2024 to July 2025. Some cases in people have not been linked to cow or bird exposures, indicating a potential third, as-yet unknown source of infection. Due to the mostly mild disease associated with currently circulating H5N1 flu viruses in the U.S., infections among people may be underreported. Cases have slowed over the summer, but H5N1 virus continues to circulate in cattle and birds.
 

What to Watch for Moving Forward

The current risk from H5N1 virus to people is low; however, it is important to continue monitoring the situation and preparing for the possibility that this could change. Specifically, a version of the virus with an increased ability to spread and cause illness could still emerge. Moving forward, four things should be considered when monitoring the situation: 1) changes to the virus; 2) potential for disrupted food supplies; 3) preparedness of vaccines and treatments; and 4) policies that can help prevent or stem an epidemic. 

Changes to the virus: The virus has already shown signs of adaptation that increase its ability to infect mammals. Continued adaption could enable the virus to better replicate in people, which could position it to more easily spread or cause severe disease. In an effort to prevent the world from being surprised by rapidly spreading H5N1 flu and to allow for time to ramp up prevention measures, scientists should be closely monitoring the specific viruses that are circulating to quickly identify additional signs of adaptation. Even with close monitoring for antigenic drift, the virus can dramatically change through antigenic shift. In fact, three of the last four flu pandemics resulted from antigenic shift. Thorough surveillance could aid in rapidly detecting a new virus, allowing for a swift response from scientists and policymakers. 

Potential for disrupted food supplies: Even if the virus does not adapt to more readily infect and spread among people, changes to the virus could still affect people. Specifically, should the virus adapt to more easily spread among, or cause severe illness in, currently affected livestock, or if it gains the ability to infect other livestock, food supplies could be disrupted. As noted, farmers have confirmed decreased milk production from infected cows. The U.S. also experienced high egg prices earlier in the outbreak, due to infections among poultry. If herds need to be culled or if production decreases, further financial or supply effects could be experienced. In this area, continued monitoring will be important. 

Preparedness of vaccines and treatments: The U.S. has H5N1 flu vaccines stockpiled, but given that the virus continues to change, vaccines in the stockpile are not perfectly matched to currently circulating strains. Several H5-based, HA-containing vaccines are in various stages of clinical development, but continued research and funding will be necessary. Further, it would take some time to produce updated vaccines. 
In addition to vaccines, treatments will also be important to prevent severe disease and decrease loss of life until vaccines become available. Treatments are also important for those not protected by vaccination. It is possible that some existing antivirals could be effective since many current options used to treat seasonal influenza work by blocking parts of influenza virus that are conserved (i.e., the same) across influenza A viruses. However, the scope of their effectiveness would need to be determined at the time. 

Policies to prevent or stem a pandemic: While some policymakers have suggested that allowing uncontrolled spread of H5N1 influenza in poultry and livestock would be beneficial for generating a population of immune animals that survive the infection, scientists have noted several concerns about this approach. First, uncontrolled spread would increase the number of animals and people exposed to and infected with the virus in a short window of time. This could lead to dramatic changes in food supply and overwhelmed medical institutions, respectively. Second, the more animals and people that become infected with H5N1 influenza, the greater the chance for the virus to drift or even shift, increasing the risk for new, more dangerous versions. Rather than allowing for uncontrolled spread, several policy-related measures can help. First, vaccinating farm workers against both seasonal and H5N1 influenza could help protect people and reduce the risk for wider spread among the population. Second, use of vaccines in poultry could also help prevent spread among flocks. However, potential economic impacts would need to be considered if vaccination would affect exportation. Third, for livestock without vaccine availability or options, heightened infection control measures could be put in place. Finally, culling of infected flocks could help prevent the virus from spreading more broadly among wild animals and livestock. 

While cases of H5N1 in cattle have slowed over the summer, influenza is a seasonal virus, so it’s possible that cases will again increase during the fall and winter. Therefore, it’s critical to continue monitoring this situation and ensure that those in power are also monitoring the situation and planning ahead as preparedness is not something that can be done after an emergency has arrived. 
 

Related resources 

Influenza: The Disease & Vaccines (Webpage, VEC at CHOP)

Antigenic Drift: How the Influenza Virus Adapts (Animation, Vaccine Makers Project)

H5N1 Influenza (Webpage, USDA)