H5N1 Avian Flu: Missouri Case Raises New Questions
First U.S. detection without animal exposure prompts heightened surveillance and renewed focus on pandemic preparedness
What’s going on with Avian Flu?
On September 6, 2024, the Missouri Department of Health reported a significant case of avian influenza A (H5) in a hospitalized patient. Here’s why this case matters:
It’s the first U.S. case of H5 bird flu where the patient had no known exposure to infected animals.
It’s the first time the national flu surveillance system detected H5 bird flu.
The lack of animal exposure raises questions about new transmission routes.
The patient, who had underlying health conditions, has since recovered after receiving antiviral treatment. No other cases have been detected among their close contacts. This is the 14th human case of bird flu reported in the U.S. this year. So, while it’s important to stay informed, there’s no need for panic.
Influenza 101
Virology is a complex subfield of immunology that comes with A LOT of terminology, so let’s review some of it. Influenza viruses are typically classified into four types: A, B, C, D, according to differences in biological properties. A combination of Influenza A and B viruses typically cause seasonal flu epidemics of diseases in people, while Influenza A viruses (the most common type in humans) are the only type known to cause flu pandemics, or global epidemics. Influenza C virus infections are rare and generally thought to cause mild illness, while Influenza D viruses primarily affect only cattle.
When we say “Avian Flu” we are referring to a type of Influenza A virus that is primarily adapted to birds, but can sometimes infect humans. The H and N in Influenza A virus subtypes refers to two key surface proteins on the viral envelope (kind of like the outer shell of an M&M). Viruses are not alive in the classic sense, so although they have genetic material, they cannot replicate on their own. Instead, these sneaky pathogenic particles “hijack” host cells and use their infrastructure to replicate and create more virus particles that infect the host and cause the symptoms of viral infection that make us sick. How easily a virus can hijack cells is determined by its H and N proteins.
Hemagglutinin is a protein that helps the virus attach to and enter host cells, because viruses cannot replicate on their own. There are 18 different subtypes of hemagglutinin (H1 through H18) each with its own unique infective properties. Neuraminidase is an enzyme also located on the surface of a virus that is essential for the release and spread of new virus particles. There are 11 different subtypes of neuraminidase (N1 through N11), each with its own unique enzymatic properties influencing how effectively the virus spreads. The combination of different hemagglutinin and neuraminidase proteins determines the specific subtype of influenza A as well as the type of host cell it can infect (i.e. human, bird, pig, cattle). These combinations of H and N also dictate how easily influenza can infect host cells and replicate, which also impacts how contagious and severe the viral strain is. Avian flu is classified as H5N1 meaning that the virus has hemagglutinin subtype 5 and neuraminidase subtype 1.
Understanding the Virus: What’s a Clade?
The exact viral clade in this Missouri case is still unknown, but experts suspect it might belong to clade 2.3.4.4b, a group of related virus strains known to cause outbreaks in birds and poultry. A clade refers to a group of viruses that share a common ancestor and genetic traits. Determining the clade helps public health experts understand the virus’s behavior—whether it’s changing, becoming more transmissible, or staying stable. Genetic sequencing is underway to confirm if this case involves a known clade or something new.
Best-Case Scenario
If the virus belongs to clade 2.3.4.4b, it’s likely the virus is behaving as expected. This clade has been circulating in birds and poultry since 2021, and while human cases are rare, they generally involve direct contact with animals. In this scenario, the virus is already on health officials’ radar, and existing vaccines might offer some protection.
Worst-Case Scenario
A worst-case scenario would be a new or mutated clade that can spread more easily among humans. If this is the case, we might face a situation where current vaccines are less effective, requiring new strategies and rapid vaccine development to prevent a pandemic.
Context of H5N1 in the United States
Here’s what we know about H5N1 in the U.S. so far:
14 human cases since 2022, linked to dairy cows and poultry.
Over 100 million poultry affected and 9,831 wild birds detected with H5N1.
The virus has affected 196 dairy herds in 14 states as of September 2024.
Public Health Response and Next Steps
The CDC is closely monitoring this case. There’s no sign of human-to-human transmission, and the risk to the public remains low. However, to stay prepared, officials are:
Investigating the source and potential new transmission pathways.
Conducting contact tracing to rule out human-to-human spread.
Testing and monitoring people exposed to animals.
H5N1 Vaccine Information
Yes, there is an H5N1 vaccine, but it’s different from the seasonal flu shot you get each year. The H5N1 vaccine is stockpiled for use in an emergency and is usually given to high-risk groups like poultry workers. This year’s flu shot is trivalent which means it protects against three strains of flu (H1N1, H3N2, and one B strain), but it doesn’t protect against H5N1.
What Can We Do to Reduce Risk?
Even though the risk to the general public is low, there are a few simple steps you can take to protect yourself, especially in areas where H5N1 has been found:
Avoid contact with wild birds: Don’t handle sick or dead birds.
Cook poultry thoroughly: Ensure your poultry reaches 165°F to kill viruses.
Practice good hygiene: Wash your hands regularly, especially after handling birds or poultry.
Avoid raw milk products: Stick to pasteurized milk and dairy products to reduce exposure to possible infections.
Stay informed: Keep an eye on local public health updates and follow any guidance on avian flu.
What About Cheese?
Cheese made from pasteurized milk poses no risk for H5N1 transmission. Raw milk cheese, however, could carry a higher risk if the milk comes from infected animals. Although there are no confirmed cases of H5N1 spreading through raw milk cheese, it’s a good idea to stick with pasteurized products, especially if you live in or near outbreak areas.
What About Masks?
Masks, especially N95s, can help reduce the risk of inhaling particles contaminated with the virus in high-risk settings. For people working with poultry or in areas with active outbreaks, wearing a mask can prevent respiratory exposure. While the overall risk is low, it’s a simple, precautionary step you can take in environments where bird flu might be present.
A Broader Public Health Perspective
This case serves as a reminder of the importance of emergency preparedness. If a more transmissible strain emerges, it could challenge our public health systems, which are still recovering from COVID-19. Surveillance, rapid testing, and stockpiling vaccines are essential, but current measures may not be enough for another pandemic. Experts agree that we’re not fully prepared for a large-scale outbreak, and this situation emphasizes the need for continued investment in pandemic preparedness, including stronger global collaboration and faster vaccine development.
In the meantime, stay calm but be vigilant.
Yours in [unbiased] science,
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