Avian Influenza (AI) is a viral disease primarily affecting birds, circulating globally in wild and domestic populations. It is caused by Type A influenza viruses, members of the Orthomyxoviridae family, characterized by two surface proteins: Hemagglutinin (H) and Neuraminidase (N). Subtypes are designated based on the combination of these proteins (e.g., H5N1 or H7N9). Viruses are categorized by their potential to cause disease, known as pathogenicity, which separates them into two distinct groups.
Defining Highly Pathogenic Avian Influenza
Avian influenza viruses are classified as either Low Pathogenic Avian Influenza (LPAI) or Highly Pathogenic Avian Influenza (HPAI) based on their ability to cause severe illness in domestic poultry. LPAI strains typically result in few or no clinical signs, such as ruffled feathers or a slight drop in egg production. HPAI viruses cause severe systemic disease and high mortality rates, posing a significant concern for animal health.
Classification relies on established criteria developed by international organizations, such as the World Organisation for Animal Health (WOAH). One method determines the Intravenous Pathogenicity Index (IVPI) by injecting the virus intravenously into susceptible chickens. A virus is defined as highly pathogenic if it kills at least 75% of the inoculated test chickens or produces a high IVPI score.
Alternatively, classification is determined by molecular characteristics, focusing on the genetic sequence of the hemagglutinin (HA) protein. High pathogenicity is associated with a specific amino acid sequence at the HA cleavage site, allowing the virus to be cleaved by a wider range of host enzymes. This feature enables the virus to spread beyond the respiratory and intestinal tracts to infect multiple internal organs, leading to systemic failure. All naturally occurring HPAI viruses identified belong only to the H5 or H7 hemagglutinin subtypes.
This molecular signature predicts the potential for a virus to cause widespread disease, regardless of its current clinical severity. Some H5 and H7 LPAI viruses are considered notifiable because they can mutate into HPAI strains after circulating within poultry. The HPAI designation is a measure of inherent virulence potential, guiding regulatory and control measures worldwide.
Transmission and Spread Dynamics
The natural reservoir for all influenza A viruses, including HPAI, is wild aquatic birds, particularly waterfowl and shorebirds. These birds often carry the virus with few or no signs of illness, shedding it primarily through feces into the environment. Migratory routes facilitate the long-distance movement of the virus, allowing it to spread across continents.
Transmission to domestic poultry, referred to as a spillover event, occurs when wild birds come into direct or indirect contact with commercial or backyard flocks. Indirect spread is common and involves contaminated sources, such as water, feed, or equipment exposed to infected wild bird droppings. Within poultry operations, the virus spreads rapidly between birds through respiratory secretions and direct contact.
The virus’s ability to cross the species barrier and infect non-avian hosts is known as zoonotic spillover. This occurs when mammals, including humans, have close or unprotected contact with infected animals or contaminated environments. Recent outbreaks have demonstrated HPAI’s capacity to infect a growing number of mammalian species, including foxes, seals, mink, and dairy cattle.
For many carnivorous mammals, infection occurs primarily through consuming infected wild birds or poultry carcasses. Detection of the virus in species like dairy cattle and domestic cats indicates that the virus continues to evolve, adapting to new hosts and transmission pathways. While the risk of human-to-human spread remains low for currently circulating strains, the increasing frequency of mammalian infections raises concerns about further viral adaptation.
Clinical Impact and Manifestation
Avian Hosts
In domestic poultry, HPAI infection is characterized by rapid onset, often resulting in sudden, unexplained death. Mortality rates in susceptible birds, such as chickens and turkeys, can reach 90% to 100% of the flock within 48 hours. Birds that survive immediately show severe, systemic signs of illness affecting multiple body systems.
External symptoms include a lack of energy, depression, and loss of appetite, accompanied by significant drops in egg production. The infection causes noticeable physical changes, such as swelling of the eyelids, head, comb, and wattles. These areas may also display purple or dark discoloration due to circulatory distress.
Internally, the virus causes widespread organ failure. Clinical presentations include respiratory distress, characterized by gasping and nasal discharge, and severe digestive issues like diarrhea. Neurological signs, such as tremors, a twisted neck, or incoordination, indicate that the infection has reached the central nervous system.
Human and Mammalian Hosts
In humans, infection with HPAI viruses, such as the H5N1 subtype, can range from mild symptoms to life-threatening disease. Initial symptoms often resemble common influenza, including fever, cough, fatigue, and sore throat. Infection can rapidly progress to severe respiratory illness, including pneumonia, acute respiratory distress, and high fatality rates.
The overall severity in humans is not strictly correlated with the HPAI designation, which is based on virulence in birds. However, certain HPAI strains, like the Asian lineage H5N1, have historically caused severe illness in approximately half of reported human cases. Neurological complications and multi-organ failure have been documented.
In non-human mammals, the clinical presentation varies widely by species. In highly susceptible species like mink and domestic cats, HPAI can cause severe illness, including neurological signs and respiratory distress, often leading to sudden death. Conversely, in dairy cattle, the infection may manifest as a sudden drop in milk production, fever, and reduced feed intake, with many animals recovering.
Global Surveillance and Prevention
Effective control of HPAI relies on comprehensive global surveillance and preventative measures, guided by the One Health approach, which recognizes the interconnectedness of human, animal, and environmental health. Surveillance programs in wild birds serve as an early warning system to detect the introduction and spread of viral strains. Laboratory diagnostic capabilities are enhanced to ensure rapid and accurate identification of HPAI, especially the H5 and H7 subtypes.
Biosecurity is the primary defense against HPAI in domestic flocks. Measures focus on preventing contact between wild birds and domestic poultry, including housing flocks indoors and restricting access to outdoor areas. Personnel and equipment moving between facilities must follow cleaning and disinfection protocols, such as sanitizing footwear and vehicles, to avoid carrying the virus on contaminated surfaces.
In response to a confirmed outbreak, rapid action is taken to contain the virus and prevent further spread. This often involves a stamping out policy, which mandates the humane culling of all infected and exposed birds on the affected premises. Affected farms are cleaned and disinfected before restocking is permitted. International cooperation and timely reporting of outbreaks to organizations like WOAH are necessary to track viral evolution and inform global risk assessments.