Avian influenza, commonly called bird flu, is a viral infection caused by Type A influenza viruses that naturally circulate in wild birds and can spread to domestic poultry, including chickens. The disease comes in two forms: a mild version that may barely affect your flock, and a severe version that can kill nearly every bird within days. The strain circulating most widely in recent years is H5N1, classified as highly pathogenic avian influenza.
Two Forms of the Disease
Avian influenza is classified into two categories based on how much damage it causes: Low Pathogenicity Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza (HPAI). Both are Type A influenza viruses, and the most commonly seen strains start with H5 or H7, named after the surface proteins on the virus particle.
LPAI is the milder form. Chickens with LPAI may show no symptoms at all, or they may have mild respiratory signs like nasal discharge or sneezing, eat less, appear ruffled, and lay fewer eggs. Most birds survive, and many flock owners may not even realize their chickens were infected.
HPAI is a different situation entirely. This form causes severe illness and high mortality. Chickens can die suddenly with no prior symptoms, sometimes within 48 hours of infection. The H5N1 strain that has been circulating through poultry and wild birds since 2022 falls into the HPAI category, and it has devastated flocks worldwide.
Symptoms to Watch For
The signs of avian influenza in chickens depend on which form of the virus is involved. With LPAI, symptoms are subtle: slight sneezing, a runny nose, decreased appetite, and a drop in egg production. These signs overlap with many common poultry illnesses, which makes LPAI easy to miss.
HPAI produces more alarming symptoms. Chickens may show purple discoloration or swelling of the comb, wattles, legs, or other body parts. Egg production drops sharply, and the eggs that are laid may be soft-shelled or misshapen. Coughing, sneezing, and nasal discharge can appear alongside lethargy and loss of appetite. In many cases, though, the first and only sign is sudden death with no warning. A flock that seemed healthy in the morning can have multiple dead birds by evening.
The incubation period for H5N1 averages 2 to 5 days but can stretch up to 17 days, meaning there is a window where infected birds appear perfectly normal while the virus multiplies and spreads.
How It Spreads
Wild birds, particularly waterfowl like ducks and geese, are the primary reservoir for avian influenza. Many wild birds carry the virus without getting visibly sick, shedding it in their droppings, saliva, and nasal secretions as they migrate. Chickens typically become infected through indirect contact rather than face-to-face encounters with wild birds. Virus particles contaminate feed, water sources, soil, and equipment in the farm environment. Boots, clothing, and tools that have contacted contaminated material can carry the virus straight into a coop.
Farm-to-farm spread is another major route. The USDA has identified movement between farms, whether through shared equipment, vehicles, or workers, as a significant driver of outbreaks. A single contaminated pair of boots walking between properties can introduce the virus to a previously healthy flock.
Protecting Your Flock With Biosecurity
Biosecurity is the single most effective defense against avian influenza. The USDA’s “Defend the Flock” program defines biosecurity as everything you do to keep disease-causing organisms away from your birds, your property, and yourself. This includes both physical measures and daily habits.
On the structural side, housing should prevent contact between your chickens and wild birds. Enclosed runs with overhead netting or solid roofing stop wild birds from landing in feeding areas or contaminating water sources. Feed and water should be kept inside covered structures rather than left in the open where wild bird droppings can fall in.
For anyone entering your coop or pen, disposable boot covers are the preferred option. If those aren’t available, a disinfectant footbath at the entrance works, but only if you first scrub off all mud, droppings, and debris with a long-handled brush before stepping into the disinfectant. A footbath full of dirty water does nothing. Keep it clean and refreshed regularly. Dedicated clothing for working with your flock, and washing hands before and after handling birds, further reduces risk.
If you visit other farms, feed stores, or poultry swaps, change your clothes and shoes before returning to your own birds. Limit visitors to your coop area, and never share equipment with other flock owners without disinfecting it first.
What Happens When a Case Is Confirmed
If you notice unusual illness or deaths in your flock, the process starts with contacting USDA or your state veterinarian. Samples are collected and tested. The standard diagnostic method is virus isolation, where swabs from sick or dead birds are tested in a laboratory to confirm live virus. A molecular test called RT-PCR provides rapid results by detecting the virus’s genetic material and identifying the specific strain involved.
Once HPAI is confirmed, the response is swift. Infected birds are typically destroyed within 24 to 48 hours of detection to prevent further spread. The USDA compensates flock owners for birds that must be destroyed and can assist with disposal of carcasses. There is no treatment for avian influenza in poultry. The focus is entirely on containment: eliminating the infected flock, decontaminating the premises, and establishing quarantine zones around the affected property.
Vaccination for Poultry
Five veterinary vaccines against HPAI H5 have been authorized by European regulators as of late 2025. These include both traditional inactivated vaccines and newer vector vaccines designed specifically for chickens. Some vector vaccines can be given to day-old chicks and are intended to reduce mortality, clinical signs, and virus shedding from infected birds.
Despite the availability of these vaccines, farmed poultry are not routinely vaccinated against avian influenza in the EU or the United States. The reluctance stems partly from trade concerns, since vaccinated birds can test positive for influenza antibodies, complicating the ability to distinguish vaccinated flocks from infected ones during surveillance. For backyard flock owners, avian influenza vaccines are generally not commercially available. Biosecurity remains the primary line of defense.
Risk to Humans
Avian influenza can occasionally jump from birds to people, though this remains uncommon. The primary risk factor is direct contact with infected birds or contaminated environments, particularly during handling, culling, or processing of sick poultry. Workers on affected farms face the highest exposure risk.
When human infection does occur, symptoms range from mild flu-like illness and eye inflammation (conjunctivitis) to severe respiratory disease. In some cases, infections produce no symptoms at all. The case fatality rate for H5 and H7N9 infections in humans has been significantly higher than seasonal flu, though total case numbers remain small. Gastrointestinal and neurological symptoms have been reported less frequently.
The reassuring detail is that currently circulating avian influenza viruses have not demonstrated sustained human-to-human transmission. Infections remain isolated events tied to close animal contact. Properly cooked poultry and eggs pose no risk, as the virus is destroyed by heat during cooking.