AIV stands for avian influenza virus, commonly called bird flu. It’s a type A influenza virus that naturally circulates among wild birds, particularly waterfowl and shorebirds, and can occasionally spill over into poultry, other mammals, and humans. The virus has drawn global attention because certain strains can be deadly: between 2003 and April 2024, H5N1 alone caused 889 confirmed human cases worldwide, with a case fatality rate of 52%.
How Avian Influenza Strains Are Named
Every avian influenza virus carries two key proteins on its surface. The first, hemagglutinin (H), helps the virus latch onto and enter cells. The second, neuraminidase (N), helps newly made virus particles break free and spread. There are 16 known H subtypes (H1 through H16) and 9 N subtypes (N1 through N9), and they can combine in many ways. That’s where names like H5N1, H5N2, H7N2, and H7N8 come from. Each combination behaves a little differently in terms of which species it infects and how severe the illness can be.
High Pathogenicity vs. Low Pathogenicity
Not all avian influenza viruses are equally dangerous, even to birds. Scientists split them into two categories based on how much damage they cause in poultry: highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI).
HPAI strains are mostly limited to the H5 and H7 subtypes. In susceptible poultry flocks, HPAI can kill up to 100% of infected birds. The virus spreads throughout the entire lung and can reach multiple organs quickly. Chickens infected with HPAI typically show depression and ruffled feathers within 24 hours.
LPAI strains tend to cause milder respiratory symptoms in birds: runny eyes, nasal discharge, and sinus swelling. The virus stays localized around the larger airways rather than spreading through the whole lung. Interestingly, research has shown that the total amount of virus in the lungs is similar between HPAI and LPAI infections. The critical difference is where the virus goes. HPAI spreads deep into the smallest airways and toward the air sacs, which is what makes it so lethal.
How It Spreads
Wild birds are the natural reservoir. They can carry the virus with few or no symptoms and shed it in their saliva, nasal secretions, and droppings. From there, the virus moves through several routes.
- Direct contact: Touching infected birds or other animals, including visiting live poultry markets, is the most common way people have been infected.
- Contaminated surfaces: The virus can survive on surfaces contaminated with bird mucus, saliva, or feces. Touching those surfaces and then touching your eyes, nose, or mouth can lead to infection.
- Airborne particles: Dust and small droplets containing the virus can become airborne, especially in poultry barns or other enclosed spaces. Breathing these in is another route of exposure. The closer you are to an infected animal, the higher the risk.
- Food products: It’s possible to become infected by consuming something contaminated with live virus, including unpasteurized (raw) milk from infected cows.
The 2024 Outbreak in Dairy Cattle
In March 2024, H5N1 was detected in U.S. dairy herds for the first time. Infected cows were confirmed across more than a dozen states, including Texas, Michigan, Colorado, Idaho, Iowa, Kansas, and others. The virus was found in respiratory secretions and in raw milk from sick cows.
A small number of human infections followed, mostly among farmworkers who had close contact with diseased cattle. Pasteurized milk remains safe because the pasteurization process inactivates the virus. The concern centers on raw, unpasteurized dairy products, which health authorities have long advised against consuming due to pathogen risks, and this outbreak added another reason to that list.
Symptoms in Humans
Most human cases of bird flu in the United States so far have been mild, particularly among people with known animal exposure. Eye redness and irritation (conjunctivitis) has been the most common symptom in recent U.S. cases. Other mild symptoms include low-grade fever (100°F or higher), cough, sore throat, runny or stuffy nose, body aches, headache, and fatigue. Diarrhea, nausea, and vomiting are less common.
Severe cases look very different. High fever, shortness of breath, difficulty breathing, altered consciousness, and seizures can all develop. Complications include pneumonia, respiratory failure, acute kidney injury, multi-organ failure, sepsis, and brain inflammation. The global fatality rate for confirmed H5N1 cases over the past two decades sits at 52%, though that number likely overestimates the true rate because mild cases are less likely to be tested and counted.
Treatment
Avian influenza in humans is treated with antiviral medications, the same class of drugs used for seasonal flu. Treatment works best when started within 48 hours of symptom onset, though it can still help later in severe or complicated cases. For uncomplicated infections, a typical treatment course lasts five days. Some newer antivirals require only a single dose.
For anyone hospitalized or showing signs of severe illness, antiviral treatment is started immediately, without waiting for lab confirmation. The key takeaway is that early treatment matters. The sooner antivirals are started, the better the clinical outcome.
How the Virus Is Tracked Globally
The World Health Organization runs the Global Influenza Surveillance and Response System (GISRS), a worldwide network of labs that monitors how influenza viruses are evolving. National labs in participating countries sequence the genes of circulating strains, which allows scientists to track transmission patterns, spot mutations that could make a virus more dangerous, and detect resistance to antiviral drugs.
Virological data flows into a platform called FluNet, while epidemiological data goes into FluID. Genetic sequences are shared through the GISAID database, giving researchers around the world near-real-time access to information about how avian influenza strains are changing. A dedicated Influenza Virus Tracking Mechanism specifically monitors H5N1 and other strains with pandemic potential. This surveillance infrastructure is the early warning system that public health agencies rely on to detect whether an avian influenza virus is gaining the ability to spread efficiently between people, a scenario that has not yet occurred but remains the central concern.