Highly Pathogenic Avian Influenza (HPAI)

Highly Pathogenic Avian Influenza (HPAI) refers to a severe, contagious viral disease affecting domestic poultry and wild birds, caused by specific strains of the avian influenza virus (AIV). It is characterised by rapid transmission, high mortality, and serious economic and public health implications. HPAI is among the most significant transboundary animal diseases globally, posing challenges to agriculture, trade, and wildlife conservation, and occasionally spilling over to infect humans.

Nature of the Disease

Avian influenza (AI) is caused by influenza type A viruses belonging to the family Orthomyxoviridae. These viruses occur naturally among wild aquatic birds such as ducks, geese, and swans, which often act as reservoir hosts. The viruses can infect domestic poultry, including chickens, turkeys, and ducks, leading to varying disease outcomes.
Avian influenza viruses are classified based on two surface glycoproteins:

• Haemagglutinin (H) – 18 subtypes (H1–H18)
• Neuraminidase (N) – 11 subtypes (N1–N11)

Combinations of these proteins produce different virus strains, such as H5N1 or H7N9.
The disease is divided into two categories based on pathogenicity in chickens:

• Low Pathogenic Avian Influenza (LPAI): Causes mild symptoms or asymptomatic infections.
• Highly Pathogenic Avian Influenza (HPAI): Causes severe disease, high mortality (up to 100%), and rapid spread within and between flocks.

Virology and Evolution

HPAI arises primarily from mutations or recombination events in LPAI viruses, usually of the H5 or H7 subtypes, that increase their virulence in poultry. These mutations typically occur when LPAI viruses circulate in domestic birds over extended periods, gaining genetic changes in the haemagglutinin cleavage site that allow the virus to replicate more efficiently in multiple organs.
The virus is capable of antigenic drift (gradual mutation) and antigenic shift (reassortment with other influenza viruses), leading to the emergence of new variants. Wild birds, especially migratory species, play a critical role in the global dissemination of HPAI, carrying viruses over vast distances.

Transmission

Transmission occurs through multiple pathways:

• Direct contact with infected birds, their secretions, or faeces.
• Indirect transmission via contaminated equipment, feed, water, clothing, or vehicles.
• Airborne spread over short distances in dense poultry populations.
• Wild bird migration, facilitating transcontinental spread.

The virus is hardy in cold, moist environments and can survive for weeks in contaminated litter, feed, or water. It is easily destroyed by heat, sunlight, and common disinfectants.

Clinical Signs in Birds

In poultry, HPAI presents as a rapidly fatal systemic infection. Common signs include:

• Sudden high mortality (often without prior symptoms)
• Severe depression, drooping wings, and lack of coordination
• Swelling of the head, eyelids, wattles, and comb
• Purple discolouration (cyanosis) of combs and legs
• Respiratory distress – coughing, sneezing, nasal discharge
• Drop in egg production and soft-shelled or misshapen eggs
• Haemorrhages in internal organs upon necropsy

The disease can kill infected flocks within 48 hours, making early detection essential for containment.

Global Occurrence and Historical Outbreaks

Since the late 20th century, HPAI has evolved from a sporadic poultry disease into a global panzootic affecting multiple continents.

• H5N1 lineage (since 1996): Emerged in Guangdong, China, spreading widely across Asia, Europe, and Africa. It caused severe poultry losses and occasional human infections with high fatality rates.
• H7N9 (China, 2013): Caused numerous human infections linked to live poultry markets.
• H5N8 and H5N6 outbreaks (2020–2024): Widespread epidemics in poultry and wild birds across Europe, Asia, and North America.
• Recent years have seen record numbers of outbreaks in wild birds and poultry, indicating that HPAI has become enzootic (regularly present) in some wild bird populations.

These global outbreaks have led to the culling of hundreds of millions of poultry, trade restrictions, and massive economic losses for the poultry sector.

HPAI in India

India has experienced multiple outbreaks of HPAI, primarily due to the H5N1 and H5N8 subtypes. The first confirmed outbreak occurred in Maharashtra in 2006, and subsequent events have been reported in states such as Kerala, Madhya Pradesh, Haryana, Rajasthan, and Himachal Pradesh.
The Department of Animal Husbandry and Dairying (DAHD) under the Ministry of Fisheries, Animal Husbandry and Dairying coordinates surveillance and response measures through the National Action Plan for Prevention, Control, and Containment of Avian Influenza. Rapid culling, disinfection, movement control, and compensation for farmers are key elements of containment.

Zoonotic Potential and Public Health Implications

Certain HPAI strains, particularly H5N1, H5N6, and H7N9, have demonstrated the ability to infect humans, usually through direct contact with infected birds or contaminated environments. Human-to-human transmission is rare and inefficient, but sporadic cases raise concerns about potential adaptation.
Human symptoms range from mild conjunctivitis and flu-like illness to severe pneumonia, acute respiratory distress syndrome, and multi-organ failure. The mortality rate in confirmed H5N1 cases exceeds 50%.
Public health authorities monitor such infections closely due to the risk of pandemic emergence if a virus acquires efficient human transmissibility through mutation or reassortment with human influenza viruses.

Diagnosis

Accurate and early diagnosis is essential for control and prevention. Laboratory methods include:

• Reverse Transcriptase Polymerase Chain Reaction (RT-PCR): Detects viral RNA and identifies subtypes.
• Virus isolation: In embryonated chicken eggs or cell cultures.
• Serological tests: Including haemagglutination inhibition and ELISA.
• Genomic sequencing: To monitor mutations and track virus evolution.

National reference laboratories such as the National Institute of High Security Animal Diseases (NIHSAD), Bhopal, conduct confirmatory testing in India.

Prevention and Control

Control of HPAI relies on an integrated approach encompassing biosecurity, surveillance, culling, and vaccination where appropriate.

1. Biosecurity Measures:
   • Restrict movement of poultry and personnel between farms.
   • Maintain hygiene, disinfection, and controlled access to poultry areas.
   • Prevent contact between domestic birds and wild waterfowl.
2. Surveillance and Early Detection:
   • Continuous monitoring of poultry flocks and wild bird populations.
   • Prompt reporting of unusual mortality or production drops.
3. Stamping Out Policy:
   • Immediate culling of infected and exposed birds.
   • Safe disposal of carcasses and disinfection of premises.
4. Vaccination:
   • Used in some countries to protect poultry against specific H5 or H7 strains.
   • Requires close genetic matching and post-vaccination monitoring.
5. Public Awareness:
   • Education of farmers and workers about hygiene and early signs.
   • Avoidance of handling dead or sick wild birds.
6. International Cooperation:
   • Data sharing and coordination through organisations such as the World Organisation for Animal Health (WOAH) and FAO.

Economic and Ecological Impact

HPAI outbreaks have far-reaching economic and ecological effects:

• Poultry industry losses: Massive culling and export bans disrupt trade and livelihoods.
• Wildlife mortality: Certain strains have caused die-offs among migratory birds, affecting biodiversity.
• Food security: Reduced poultry supply impacts protein availability and market prices.
• Government expenditure: Large funds are required for surveillance, compensation, and control operations.

In addition, repeated outbreaks challenge rural livelihoods, particularly small-scale poultry farmers dependent on backyard flocks.

Recent Trends and Global Concerns

Recent surveillance reports show that HPAI viruses have become more persistent in wild bird populations, enabling year-round transmission across continents. Mixed infections and reassortment events continue to generate novel variants such as H5N1 (clade 2.3.4.4b), which have spread widely since 2021.
The unprecedented spread of HPAI in 2022–2024 resulted in record poultry losses across Europe, the Americas, and Asia, as well as spillover into mammals (such as foxes, seals, and even farmed mink), raising concern about potential adaptation to new hosts.