What is Avian Influenza?

Avian Influenza (AI) is an acute infectious disease of birds caused by infection with the avian influenza virus (AI virus, AIV). This virus is rapidly transmissible and highly pathogenic, infecting many species of birds, including chickens, turkeys, ducks, and wild birds. It causes decreased feed intake, rapidly declining egg-laying rates, and mortality (wild birds and ducks may be infected but rarely show clinical manifestations).

Causative Pathogen

• Avian influenza viruses infect poultry (chickens, turkeys, ducks, etc.), and human influenza viruses, swine influenza viruses, and equine influenza viruses also exist.
• Avian influenza viruses have 16 H serotypes and 9 N serotypes, and there are 144 possible serotypes of influenza viruses (16X9). Serotypes are represented by H3N2, H9N2, etc., and have a significant relation with the pathogenicity of the virus
• According to its pathogenicity, avian influenza is classified into highly pathogenic, weakly pathogenic, and non-pathogenic. In Korea, it is classified as a contagious animal disease Type I.

Incubation Period

• The incubation period for HPAI in chickens ranges from a few hours to 3 days, with the OIE setting a maximum of 21 days (3 weeks).
• Incubation periods can vary considerably depending on the colony's size, initial transmission route, specimen management, environment, etc.
• It is categorized into highly pathogenic avian influenza, weakly pathogenic avian influenza, and non-pathogenic avian influenza, and is classified as a contagious animal disease Type I in Korea.

Highly Pathogenic Avian Influenza (HPAI)

• Based on the pathogenicity of the virus, it is classified according to the regulations set by the Office International des Epizooties (OIE), and HPAI is classified as an OIE LIST disease and is also classified as a contagious animal disease Type I in Korea.
• HPAI causes high mortality rates of up to 100% and severe reduction in egg production, resulting in significant economic damage, and is an important disease in cross-border livestock trade.
• Importing livestock products (poultry products) from AI outbreak countries is strictly restricted due to the risk of HPAI. Due to the risk of HPAI, most countries, including Korea, have implemented culling policies and strictly restricted imports of poultry products from countries with HPAI outbreaks.

OIE's Definition of Highly Pathogenic Avian Influenza (HPAI)

75% or higher of the mortality rate

More than 6 out of 8 SPF chickens of 4–8 weeks old die

If it's not H5 and H7 type

• Less than 75% of the mortality rate
• Cytopathic effect (CPE)

In the case of H5 and H7 type

• Cytopathic effect (CPE)
• HA sequences are highly pathogenic

Means of Transmission

• Avian influenza virus can be spread by droplets, water, and other means, with direct contact with feces being the most crucial means of transmission. Direct transmission to other chickens by feces on human feet, feeders, utensils, equipment, and egg surfaces
• Ducks (house ducks, migratory birds), geese, quail, etc., are infected with the avian influenza virus but shed the virus in their feces without showing clinical signs.
• Egg-to-egg transmission does not occur, but fecal contamination of eggshells can transmit the disease, so the movement of eggs and chicks produced in the affected flocks can transmit the disease.
• The virus in feces can survive at least 35 days at 4°C, and 1 gram of virus-contaminated feces can infect approximately one million chickens.
• Birds in the wild, especially waterbirds such as mallards, can be infected with various influenza viruses but they do not cause illness and are common for an infected house duck or goose to show no symptoms.
• In particular, it is important to be cautious about infection by wild birds since the transmission route of infection of domestic birds by northern migratory birds in winter and infection of farm chickens, ducks, etc., is a concern.