Avian influenza (commonly called bird flu) is an infectious disease caused by influenza A viruses that mainly affect birds, especially wild waterfowl and domestic poultry. Birds are the natural hosts of these viruses, but in rare cases the infection can spread to humans and other animals, including pigs and cats. Human cases are uncommon but can be severe or even fatal, raising concerns about the potential for a future pandemic if the virus adapts to spread easily from person to person.
The impact of avian influenza on farming and public health depends on the strain of the virus and how it spreads. Outbreaks can cause large numbers of bird deaths, trade bans, and the need to destroy infected or exposed flocks. These events create serious economic losses for the poultry industry and can threaten food supplies. At the same time, health systems must monitor for possible infections in people.
Avian influenza viruses are divided into subtypes based on two proteins on their surface: hemagglutinin (H) and neuraminidase (N). Well-known subtypes such as H5N1, H7N9, and H5N6 have caused major outbreaks in birds and occasional human infections. Viruses are also classified by their level of severity. Highly pathogenic avian influenza (HPAI) can spread rapidly and kill most infected birds, sometimes wiping out entire flocks. Low pathogenic avian influenza (LPAI) usually causes only mild illness, but it can change into more dangerous forms under certain conditions.
In humans, symptoms can range from mild to severe. Common early signs include fever, cough, sore throat, muscle aches, and fatigue. Severe infections may progress to pneumonia, breathing difficulties, organ failure, or death. Other possible symptoms include eye infections, headaches, and digestive issues such as nausea, vomiting, or diarrhea. Mortality rates in human cases, particularly those caused by H5N1 and H7N9, are much higher than those seen with regular seasonal flu.
Treatment usually involves antiviral drugs such as oseltamivir (Tamiflu) and zanamivir (Relenza), which work best when taken early in the illness. In serious cases, patients may require hospitalization and breathing support.
Prevention focuses on avoiding contact with sick or dead birds, practicing good hygiene, using protective equipment when handling poultry, and cooking poultry products thoroughly. Some countries use poultry vaccines as part of their control measures. Examples of vaccines for avian influenza include specially engineered vaccines that use harmless viruses, such as fowlpox or Newcastle disease virus, to deliver a gene from the avian influenza virus. Other vaccines are made using virus-like particles (VLPs), which are laboratory-created structures that resemble the virus but do not cause infection. These vaccines can be tailored to protect against specific subtypes of avian influenza.
Classification
The classification of avian influenza viruses is based on two key surface proteins, hemagglutinin (H) and neuraminidase (N), which play central roles in viral entry and release from host cells.
Hemagglutinin and Neuraminidase Subtypes
The hemagglutinin (H) protein enables the virus to attach to and enter host cells, while neuraminidase (N) facilitates the release of newly formed viral particles. Variations in these proteins form the basis for subtype classification. To date, 18 hemagglutinin subtypes (H1–H18) and 11 neuraminidase subtypes (N1–N11) have been identified in influenza A viruses. These proteins can combine in multiple ways, resulting in a wide variety of subtypes such as H5N1, H7N9, and H9N2, many of which have been associated with outbreaks in birds and occasional spillover infections in humans. Most subtypes circulate primarily in wild birds, although some have adapted to domestic poultry or other animals.
Pathogenicity Categories
In addition to subtype classification, avian influenza viruses are grouped according to their pathogenicity, or ability to cause disease in birds.
- Low pathogenic avian influenza (LPAI) viruses generally cause mild illness in birds, often limited to respiratory symptoms, ruffled feathers, or a temporary drop in egg production. Many LPAI infections go undetected, particularly in wild bird populations where the virus may circulate with minimal impact.
- Highly pathogenic avian influenza (HPAI) viruses, in contrast, can produce severe disease and rapid death in domestic poultry, sometimes killing entire flocks within days. These viruses are defined by specific genetic markers and by their ability to cause high mortality in standardized laboratory tests. The most notable HPAI subtypes are H5 and H7, which have been responsible for major global outbreaks and significant economic losses.
Genetic Variation and Evolution
The classification system also reflects the dynamic nature of influenza A viruses. Because of their segmented RNA genome, these viruses can undergo two major types of change:
- Antigenic drift, which refers to small, gradual mutations in the H and N genes that accumulate over time. These changes can alter the virus enough to evade immunity developed from prior infections or vaccinations.
- Antigenic shift, which occurs when gene segments are reassorted between different influenza viruses infecting the same host. This process can generate entirely new subtype combinations, raising concerns about the emergence of strains capable of infecting humans and spreading efficiently.
History and Origin
Avian influenza viruses have been present for centuries, with their association with disease in domestic birds first recognized in the late 19th century. The earliest well-documented epidemic of highly pathogenic avian influenza (HPAI) was reported in Italy in 1878, where large numbers of poultry experienced sudden and severe mortality. At the time, the disease was termed “fowl plague” due to its devastating impact on flocks, and its viral nature was not yet understood. Later studies in the early 20th century confirmed that the causative agent belonged to the influenza A group of viruses.
The natural origin of avian influenza is traced to wild waterfowl, such as ducks, geese, and swans, which act as reservoirs for the virus. These species typically carry the virus in their intestinal tract without showing serious signs of illness. Because of their migratory behavior, waterfowl play a critical role in the global distribution of influenza A viruses, introducing them into new regions and transmitting them to domestic poultry populations.
Throughout the 20th century, sporadic but significant outbreaks of avian influenza were recorded worldwide, particularly in regions with dense poultry farming. With the rise of intensive poultry production after the mid-20th century, the economic consequences of outbreaks became more severe. Control efforts often required the mass culling of infected or exposed birds, resulting in major financial losses and trade restrictions.
The link between avian influenza and human infection was first firmly established in the late 1990s. In 1997, an outbreak of the H5N1 strain in Hong Kong resulted in 18 confirmed human cases, six of which were fatal. This marked the first known instance of direct bird-to-human transmission of HPAI and highlighted the virus’s potential to cross the species barrier. Since then, additional strains such as H7N9 in China and H5N6 in parts of Asia have caused sporadic but often severe human infections, intensifying global concerns about pandemic risk.
Global Distribution
Avian influenza viruses have a worldwide distribution, with infections documented on every continent except Antarctica. The global spread of these viruses is closely linked to the movements of migratory waterfowl, which act as natural reservoirs and can carry the virus across continents without showing significant signs of illness. When these wild birds interact with domestic poultry or come into contact with shared water sources, transmission to farmed birds can occur, often triggering localized outbreaks.
The occurrence of avian influenza varies by region. Highly pathogenic avian influenza (HPAI) outbreaks are reported most frequently in Asia and Europe, where high-density poultry farming and extensive trade networks create conditions that favor rapid transmission and wide-scale economic losses. In Asia, repeated outbreaks of subtypes such as H5N1, H5N6, and H7N9 have had significant impacts on both animal health and public health due to occasional human infections. Europe has experienced recurring HPAI events, particularly in association with migratory bird flyways that connect Eurasia and Africa.
In Africa and the Americas, avian influenza outbreaks have generally been less frequent, but notable episodes have occurred. For example, outbreaks of H5N1 have been recorded in parts of North and West Africa, affecting both poultry and wild bird populations. In the United States and Canada, large-scale HPAI outbreaks linked to wild bird introductions have resulted in millions of poultry deaths or cullings, with substantial economic costs to the agricultural sector. South America has also reported sporadic outbreaks in poultry, often associated with migratory bird routes along the Pacific and Atlantic coasts.
Seasonal patterns influence the virus’s circulation. In many temperate regions, outbreaks are more likely to occur during the cooler months, when virus survival in the environment is enhanced and migratory bird movements peak. However, in tropical and subtropical areas, avian influenza may circulate year-round, with outbreaks often linked to rainfall patterns, farming practices, and live bird markets.
| Region | Prevalence | Common Subtypes | Notes |
| Asia | High | H5N1, H7N9 | Frequent outbreaks, human risk |
| Europe | Moderate to High | H5N8, H5N1 | Seasonal outbreaks |
| Africa | Low to Moderate | H5N1 | Localized outbreaks |
| Americas | Sporadic | H5N2, H5N8 | Migratory bird transmission |
Transmission
Avian influenza is caused by specific viral strains that infect birds and can sometimes spread to other species. Transmission occurs primarily through direct or indirect contact, involving various pathways within and beyond avian populations.
Transmission Among Birds
The spread of avian influenza among birds is primarily through direct contact with infected individuals or their secretions. Infected birds shed virus particles in their saliva, nasal secretions, and feces, contaminating shared environments such as water sources, feeding areas, and roosting sites.
Wild waterfowl, especially ducks and geese, act as natural reservoirs of the virus. They often carry and shed influenza viruses without showing signs of illness, enabling long-distance transmission along migratory flyways. When these wild species intermingle with domestic poultry, they can introduce new strains into farms and live bird markets.
Transmission is further facilitated in environments where birds are housed in close proximity, such as intensive poultry farms, backyard flocks, or crowded live animal markets. In such settings, the virus can spread quickly through contaminated feed, water, cages, equipment, clothing, and footwear of farm workers or traders. Airborne transmission through fine droplets or dust particles may also occur, particularly in enclosed or overcrowded facilities.
Zoonotic Transmission
Although primarily an avian disease, avian influenza can occasionally infect humans and other mammals. Transmission from birds to humans is considered rare but serious, requiring close and prolonged exposure to infected poultry or heavily contaminated environments, such as live bird markets. Most documented human cases have been linked to direct handling of sick or dead birds, slaughtering, defeathering, or preparation of raw poultry products.
Human infections are most frequently associated with specific subtypes, notably H5N1, H7N9, and H5N6. These viruses are not well adapted for sustained human-to-human transmission, which has limited their ability to cause large outbreaks or pandemics.
Transmission to Other Animals
Avian influenza viruses can also infect a variety of non-avian species, including pigs, cats, dogs, and certain wild mammals. In pigs, infection is of particular concern because they can be infected with both avian and human influenza viruses, creating opportunities for genetic reassortment that may give rise to novel strains with pandemic potential. Cats and other carnivores may become infected after consuming raw meat from infected birds.
Symptoms and Diagnosis
Avian influenza presents distinct signs depending on the host species and the virus strain. Diagnosis relies on clinical observation combined with laboratory testing to confirm infection.
Clinical Signs in Birds
Infected birds may display sudden death with no prior symptoms, especially in highly pathogenic strains. Common signs include coughing, sneezing, nasal discharge, and swollen wattles or combs.
Other symptoms are reduced egg production, pale or discolored combs, lethargy, and loss of appetite. Diarrhea is frequently observed. Neurological signs such as tremors or paralysis can occur in some cases.
Mortality rates vary but can be high in domestic poultry flocks. Wild birds often serve as carriers with mild or no symptoms.
Clinical Signs in Humans
Human cases typically arise from close contact with infected birds. Symptoms resemble seasonal flu, including fever, cough, sore throat, and muscle aches.
Severe cases can develop into pneumonia, acute respiratory distress syndrome (ARDS), or multi-organ failure. Eye infections and gastrointestinal symptoms may also occur depending on the virus subtype.
The incubation period ranges from 2 to 8 days. Early diagnosis is critical for treatment and containment.
Testing Methods
Laboratory confirmation uses virus isolation, real-time reverse transcription-polymerase chain reaction (RT-PCR), and serological assays. RT-PCR is preferred for its speed and specificity.
Molecular Detection
The most widely used and reliable diagnostic approach is real-time reverse transcription polymerase chain reaction (RT-PCR). This method detects viral RNA in clinical samples with high sensitivity and specificity, enabling rapid confirmation of infection. RT-PCR assays are designed to identify conserved gene regions of influenza A viruses as well as subtype-specific markers, allowing laboratories to distinguish between different hemagglutinin (H) and neuraminidase (N) subtypes. Because of its speed and accuracy, RT-PCR is considered the gold standard for both human and avian testing.
Virus Isolation
Virus isolation remains an important reference method in laboratory settings. This involves inoculating clinical samples into embryonated chicken eggs or cell cultures, where viable virus can replicate and be further characterized. Although more time-consuming than molecular assays, virus isolation provides live virus strains that can be studied for genetic sequencing, antigenic analysis, and vaccine development.
Serological Assays
Serological testing is used to detect antibodies generated in response to avian influenza infection. Common techniques include the hemagglutination inhibition (HI) assay, enzyme-linked immunosorbent assay (ELISA), and microneutralization assays. These methods are particularly valuable for epidemiological surveillance, helping identify prior exposure in both bird and human populations. However, serology is less useful for diagnosing acute infection, as antibodies develop only after several days to weeks.
Rapid Antigen Detection
Rapid antigen tests have been developed to provide point-of-care screening in field or clinical settings. These tests typically detect viral proteins in swab samples and can deliver results within minutes. Despite their convenience, they are generally less sensitive and specific than molecular methods, which limits their role in confirming cases. As such, rapid antigen tests are often used only as preliminary tools, with RT-PCR or virus isolation required for definitive diagnosis.
Sampling Strategies
The choice of sample depends on the host species. In birds, the most commonly collected specimens are tracheal and cloacal swabs, as the virus is shed through both respiratory and intestinal routes. In humans, diagnostic samples include nasopharyngeal or throat swabs, sputum, and blood, depending on the clinical presentation. Proper sampling technique and biosafety precautions are critical to ensure reliable results and protect laboratory personnel.
Prevention and Control Measures
Effective prevention and control of avian influenza require targeted actions to reduce virus exposure, limit its spread, and eliminate infected populations. Strategies include strict hygiene, vaccination, isolation of suspected cases, and removal of affected birds.
Biosecurity Practices
Biosecurity is the foundation of avian influenza prevention. It encompasses farm-level measures that minimize the introduction and spread of pathogens. Effective practices include:
- Restricting entry to poultry facilities, limiting visitors, and maintaining visitor logs.
- Disinfecting vehicles, equipment, and footwear; requiring workers to wear protective clothing; and enforcing thorough handwashing before and after handling birds.
- Securing poultry houses to prevent contact with wild birds, rodents, and insects that may act as mechanical carriers of the virus.
- Ensuring feed is uncontaminated and using treated or clean water sources to prevent indirect transmission.
- Monitoring flock health, mortality, and production trends to allow early detection and rapid response in case of infection.
Strict adherence to these measures significantly reduces the likelihood of virus introduction from migratory birds, contaminated environments, or human activity.
Vaccination Strategies
Vaccination is one of the main tools used to control avian influenza in poultry. The goal is to build flock immunity, reduce illness and deaths, and lower the amount of virus birds shed into the environment. Vaccination is mainly applied in areas where the disease is common or during outbreaks, and the type of vaccine used is chosen based on the strains of avian influenza circulating in that region.
Vaccines can be administered through injection, sprays, or drinking water, depending on the vaccine and the size of the farm. While vaccines can protect birds from getting very sick and reduce virus spread, they do not always prevent infection completely. Vaccinated birds may still carry and spread the virus without showing symptoms. Because of these challenges, vaccination programs require government approval, strict monitoring, and must be paired with strong biosecurity and ongoing surveillance.
Several types of vaccines are in use. Inactivated vaccines, made from killed virus, are the most widely used in poultry. Live recombinant vector vaccines use engineered viruses, such as Newcastle disease virus or fowlpox, to deliver influenza genes and stimulate immunity. Virus-like particle (VLP) vaccines mimic the virus without containing genetic material, providing protection without risk of infection. Adjuvanted vaccines include immune-boosting substances to enhance protection and durability.
For humans, there are no routine avian influenza vaccines. However, some vaccines targeting strains such as H5N1 and H7N9 are stockpiled by governments and reserved for emergency use in case of a pandemic.
Quarantine Protocols
Quarantine involves isolating new or sick birds to prevent virus introduction or spread within poultry populations. Newly acquired birds are typically quarantined for 2-3 weeks before introduction to established flocks. Sick or symptomatic birds must also be isolated immediately to prevent further transmission.
Isolation reduces contact between infected and healthy birds, limiting transmission. Facilities for quarantine should be separate with dedicated equipment and personnel.
During quarantine, birds are monitored closely for symptoms like respiratory distress and drops in egg production. Strict control of movement in and out of the quarantine area is essential to prevent contamination.
Culling and Eradication Efforts
During confirmed outbreaks, culling infected and exposed birds remains one of the most effective containment measures. This process involves the humane destruction of poultry, followed by secure disposal of carcasses through incineration, rendering, or deep burial under biosecure conditions.
Outbreak response often includes:
- Establishing protection and surveillance zones to prevent spread.
- Clearing affected flocks within a defined radius of the outbreak.
- Thorough cleaning and decontamination of facilities, equipment, and surrounding areas.
- Introducing new birds only after adequate downtime and negative environmental testing.
Impact on Public Health
Avian influenza presents specific risks to human health, particularly among those in close contact with infected birds. The virus’s ability to spread among humans remains limited, but its mutation potential poses ongoing concerns. Exposure during work in poultry industries is a significant factor in transmission risks.
Human Health Risks
Human infection with avian influenza typically occurs through direct or indirect contact with infected birds, their secretions, or contaminated surfaces. Live bird markets, poultry farms, and slaughterhouses are among the primary environments where zoonotic transmission occurs.
Certain subtypes of avian influenza, particularly highly pathogenic strains such as H5N1 and H7N9, are associated with elevated case fatality rates. While overall human infections remain relatively uncommon compared with seasonal influenza, the mortality rates for avian influenza infections can reach between 30% and 60% in confirmed cases. The rarity of efficient human-to-human transmission has prevented large-scale epidemics
Pandemic Potential
The principal public health concern associated with avian influenza lies in its capacity to evolve into a strain capable of sustained transmission among humans. Influenza viruses possess a segmented genome, which enables genetic reassortment when different strains co-infect the same host. Reassortment between avian and human influenza viruses, or adaptive mutations within avian strains, could theoretically generate a novel influenza virus with both high pathogenicity and efficient transmissibility in humans.
Historical experiences with influenza pandemics, including the 1918 H1N1 pandemic, underscore the consequences of such viral adaptation. Although no avian influenza virus has yet achieved widespread human-to-human transmission, the continuing circulation of highly pathogenic avian influenza in poultry and wild birds provides opportunities for viral evolution. For this reason, the World Health Organization (WHO), the Food and Agriculture Organization (FAO), and the World Organisation for Animal Health (WOAH, formerly OIE) coordinate global surveillance efforts to monitor mutations, detect zoonotic spillover events, and assess pandemic risk. Vaccine development and stockpiling are part of preventive strategies.
Occupational Exposure
Individuals employed in sectors involving frequent contact with poultry are at greatest risk of avian influenza infection. Workers in poultry farms, live bird markets, abattoirs, and veterinary services are particularly vulnerable due to their direct handling of birds and potential exposure to virus-laden dust, feathers, or bodily fluids. Occupational exposure accounts for the majority of documented human cases of avian influenza, particularly in regions where backyard poultry farming and live bird trade are widespread.
Personal protective equipment (PPE), including masks, gloves, goggles, and protective clothing, plays a critical role in minimizing infection risk. Training in hygiene measures such as regular handwashing, disinfection of equipment, and controlled culling practices further reduces occupational hazards.
Economic and Environmental Effects
Avian influenza causes significant disruption to economic activities related to poultry and impacts ecosystems through its spread in wild bird populations. It also triggers international trade responses, affecting markets and regulatory measures worldwide.
Impact on Poultry Industry
The poultry industry bears the most direct and severe economic burden from avian influenza outbreaks. Standard control measures typically involve the mass culling of infected flocks and the pre-emptive destruction of at-risk birds to contain viral spread. These interventions often result in the loss of millions of birds during major outbreaks, generating immediate financial losses for poultry farmers. Beyond mortality and culling, outbreaks disrupt egg and meat production, decrease feed efficiency, and damage long-term flock productivity.
Producers also incur substantial expenses related to enhanced biosecurity protocols, continuous surveillance, laboratory diagnostics, and vaccination programs. Fear of infection among consumers can further reduce demand for poultry products, leading to price declines and prolonged revenue losses. For small-scale and subsistence farmers, these impacts can be devastating, as poultry often serves as a primary source of income, nutrition, and food security. Recovery trajectories vary, with some sectors requiring months or years to regain pre-outbreak production levels, depending on outbreak severity, compensation mechanisms, and national disease control strategies.
Effects on Wildlife
Avian influenza is maintained in wild bird populations, particularly among migratory waterfowl, which serve as natural reservoirs for the virus. While many species carry the virus asymptomatically, certain strains, especially highly pathogenic avian influenza (HPAI), can cause severe disease and mortality in wild birds. Large die-offs have been reported in sensitive species, including swans, geese, and raptors, with consequences for local and global biodiversity.
The ecological implications extend to altered population dynamics and disrupted food webs, as mortality in keystone species can influence ecosystem balance. Additionally, the virus’s persistence in wetlands and other habitats frequented by migratory birds complicates control efforts, as environmental contamination may facilitate ongoing transmission. Conservation programs face heightened challenges in protecting endangered species during outbreaks, particularly when avian influenza coincides with other ecological stressors such as habitat loss or climate change.
Trade Restrictions
International trade in poultry and poultry products is highly sensitive to avian influenza outbreaks. Countries experiencing outbreaks frequently encounter import bans or restrictions imposed by trading partners, with measures ranging from regionalized bans to nationwide prohibitions. Such restrictions aim to limit the risk of cross-border disease transmission but often result in substantial economic losses for exporting nations.
Producers and exporters face reduced market access, surplus domestic supply, and downward pressure on prices. Export-dependent economies may experience significant disruptions to foreign exchange earnings and broader agricultural sectors linked to poultry production. At the policy level, the World Organisation for Animal Health (WOAH, formerly OIE) provides guidelines for science-based trade measures, encouraging regionalization and compartmentalization approaches to allow disease-free zones to continue trade while controlling outbreaks elsewhere.
Recent Outbreaks
Recent avian influenza activity highlights several critical cases and expanded efforts to monitor the virus.
Notable Recent Cases
In early 2025, multiple outbreaks of the H5N1 strain were reported across Southeast Asia, with significant poultry losses in Vietnam and Thailand. These outbreaks caused economic disruption and heightened concerns over zoonotic transmission.
Europe saw cases of H5N8 affecting wild birds and poultry farms, particularly in Germany and the Netherlands. Surveillance indicated virus persistence in migratory bird populations, raising risks of seasonal outbreaks.
The United States reported isolated H5N2 detections in wild waterfowl in the Pacific Northwest during spring migration. These cases prompted precautionary measures to protect commercial poultry operations.
Global Monitoring Efforts
Organizations like the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) coordinate international avian influenza surveillance. They compile outbreak data, track virus mutations, and issue early warnings to affected regions.
Regional networks use real-time reporting systems combined with field sampling to identify emerging strains promptly. Satellite tracking of migratory birds supports understanding of virus spread pathways.
Countries invest in laboratory diagnostics and biosecurity training to improve detection and response capabilities. Collaborative data sharing enhances global preparedness and helps prevent large-scale epidemics.
Treatment
Avian influenza management involves targeted antiviral therapies and vaccination efforts. Research focuses on improving drug efficacy and developing effective vaccines. Studies continue to assess virus mutations and transmission patterns.
Current Therapeutic Approaches
The mainstay of treatment for human avian influenza infections is the use of neuraminidase inhibitors, particularly oseltamivir (oral) and zanamivir (inhaled). These drugs act by blocking the neuraminidase enzyme on the surface of influenza viruses, thereby limiting viral replication and reducing the duration and severity of illness. Treatment is most effective when initiated within 48 hours of symptom onset, although in severe cases, later administration may still provide clinical benefit.
Supportive care is also a crucial component of management. This may include:
- Oxygen therapy for patients with respiratory distress.
- Intravenous fluids or oral rehydration to maintain hydration.
- Antipyretics and analgesics for fever and pain relief.
- Antibiotics if secondary bacterial infections such as pneumonia develop. In severe cases, patients may require mechanical ventilation or admission to intensive care units.