What to Know About the H3N8 Bird Flu Virus

Avian influenza A(H3N8) is a subtype of influenza virus that primarily circulates among bird populations. First detected in wild birds during the 1960s, it is now one of the most common subtypes found in wild aquatic birds globally. While it mainly affects avian species, the virus has demonstrated the ability to cross the species barrier, a process known as zoonotic transmission, and cause infections in humans. These infrequent instances of human infection underscore the need to understand the virus’s behavior in both its animal hosts and the human population.

H3N8 Virus in Animal Hosts

The H3N8 influenza virus originates in wild bird reservoirs and is frequently identified in migratory waterfowl, such as ducks and shorebirds. In these birds, it causes mild or no signs of illness, which allows the virus to be carried along migratory routes over vast distances. In domestic poultry like chickens, H3N8 infections are also low pathogenic, meaning they do not cause severe disease and can circulate undetected.

The H3N8 virus has shown a capacity for interspecies transmission, leading to outbreaks in mammal populations. It has established stable lineages in both horses and dogs, causing respiratory illness. A 2011 outbreak among harbor seals in New England, caused by a different H3N8 genetic lineage, resulted in significant mortality. These spillover events into mammals highlight the virus’s adaptability.

The circulation of H3N8 in live poultry markets in Asia is a focal point for its evolution. Genetic analyses show that H3N8 viruses in these markets can undergo reassortment, swapping genetic material with other avian influenza viruses. This genetic mixing has produced novel genotypes, including the virus responsible for human infections. The close contact between animals and humans in these environments increases the opportunity for zoonotic transmission.

Human Cases and Transmission Dynamics

All confirmed human infections with the avian H3N8 virus have been reported in China, with the first case identified in April 2022 and subsequent cases in May 2022 and March 2023. These cases have been sporadic and are not linked, suggesting independent spillover events from animals. Investigations of close contacts found no evidence of further spread, indicating the virus does not currently transmit easily between humans.

Transmission to humans is consistently linked to direct or indirect contact with infected poultry. Exposures have occurred in settings such as backyard poultry flocks and live bird markets where individuals handle infected birds or are in contaminated environments. For instance, one case involved a child exposed to backyard chickens, while another had visited a wet market where live poultry were present.

The genetic makeup of the H3N8 viruses that infected humans is of avian origin. It shows a novel combination of genes from Eurasian avian H3, North American avian N8, and internal genes from the H9N2 Eurasian lineage. While these viruses can replicate in human cells in lab studies, they have not acquired the adaptations for efficient transmission among mammals. The risk of sustained community spread is considered low.

Symptoms of H3N8 Infection

When H3N8 crosses into human hosts, the clinical presentation has ranged from mild, flu-like illness to severe, life-threatening disease. The second case identified in China, a young child, experienced only mild symptoms and recovered. In contrast, the first and third reported cases developed severe pneumonia requiring hospitalization. The first patient was admitted to an intensive care unit but ultimately recovered.

The most severe outcome was seen in the third case, a woman with multiple underlying health conditions who developed severe pneumonia and died weeks later. Common symptoms reported across the cases include fever and respiratory difficulties, which can progress rapidly in severe instances. The variation in severity highlights the unpredictable nature of zoonotic influenza infections in humans.

Monitoring and Prevention Measures

Global health organizations like the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) monitor H3N8 in wild and domestic birds. These surveillance systems track the virus to detect genetic changes and assess zoonotic risk. This monitoring allows for the early detection of viruses that could threaten human health. National authorities also investigate human cases to prevent further spread.

To prevent outbreaks in domestic birds, biosecurity measures are recommended for poultry farms and backyard flock owners. These measures aim to prevent the virus’s introduction from wild birds or contaminated sources. Practices include controlling access to poultry areas, cleaning equipment, and preventing contact between domestic and wild birds. Farmers should also report any unusual flock illness or deaths to veterinary authorities.

The general public can minimize exposure risk by taking several precautions. Individuals who work with poultry should use personal protective equipment and practice good hygiene. General recommendations include:

• Avoiding direct contact with wild birds and sick or dead poultry
• Ensuring that poultry and egg products are thoroughly cooked
• Washing hands frequently, especially after contact with birds or their environments
• Avoiding live bird markets and poultry farms when traveling to regions with known H3N8 activity