Human Infection with Avian Influenza A(H3N8) Virus Reported by China

CDC Update

April 14, 2023 – The National Health Commission of the People's Republic of China reported a confirmed case of human infection with avian influenza A(H3N8) virus "H3N8 bird flu" on March 27, 2023. The patient was an adult with multiple underlying medical conditions from Guangdong Province who became ill on February 22, 2023, was hospitalized with severe pneumonia on March 3, 2023 and later died on March 16, 2023. This is the third human infection with H3N8 bird flu virus and first fatality ever reported. The previous two human infections with H3N8 virus were also reported in China, during 2022. H3N8 viruses are a different influenza A virus subtype and unrelated to H5N1 viruses currently spreading among wild birds and poultry in the United States and globally. Based on what is currently known, this human case of H3N8 virus infection is not thought to pose a risk to the health of the U.S. public at this time.

Background:

• H3N8 viruses were first detected in wild birds in the 1960s and have been detected in other animals.
• H3N8 viruses have been sporadically detected in poultry in China and some have been found to be genetically closely related to the human cases reported in 20221.

Sporadic human infections with bird flu viruses have occurred with different bird flu virus subtypes. Over the past two decades, H7N9 viruses have caused the highest number of human infections with bird flu viruses, and H5N1 viruses have caused the second highest number of human infections. Most human infections with avian influenza A viruses have resulted from direct contact with, or close exposure to, infected birds/poultry.

Information on Human Infections with Avian Influenza A(H3N8) Virus:

According to a statement from the Guangdong Provincial Center for Disease Control and Prevention, the third human infection with H3N8 virus occurred in an adult in China who reportedly had a history of exposure to live poultry and wild birds (which had been observed around the home). An investigation did not find any additional human infections. According to a report from the World Health Organization, "Environmental samples were collected from the patient's residence and the wet market where the patient spent time before the onset of illness. The results of testing showed that the samples collected from the wet market were positive for influenza A(H3)"2. Officials in China believe the risk of the virus spreading further at this point is low ^[1].

Both of the human infections with H3N8 viruses reported in 2022 were in children. The first infection was in a child who had exposure to backyard chickens and wild ducks in April 2022⁵. The patient became critically ill and was hospitalized for their illness. The second infection occurred in May 2022 in a child who visited a wet market where live poultry were present⁶. That child had mild symptoms3. All close contacts of the first two reported cases tested negative for influenza A viruses, and both children recovered.

United States Situation:

H3N8 bird flu has never been detected in the United States in people; however, H3N8 viruses of a different genetic lineage have been detected in U.S. wild birds and some mammals in the past. In 2011, there was an outbreak of H3N8 viruses among harbor seals in New England that caused deaths in 162 seals.

In the United States, current bird flu preparedness and surveillance activities are largely focused on H5N1 bird flu, which is widespread in U.S. wild birds and has caused extensive outbreaks in U.S. poultry, with sporadic spillover to some mammals. The current risk to the U.S. general public from H5N1 bird flu viruses remains low at this time; however, it is important to remember that risk depends on exposure, and people with more exposure might have a greater risk of infection. CDC has guidance for specific groups of people with exposure to poultry, including poultry workers and people responding to outbreaks of avian influenza A(H5N1) in poultry. Additional information on protective actions around birds, including what to do if you find a dead bird, is available as well.

Currently, CDC does not recommend against travel to any countries due to bird flu viruses. CDC recommends that travelers to countries and states with bird flu outbreaks in poultry or people:

• Do not visit poultry farms, bird markets, or other places where live poultry are raised, kept, or sold, if possible.
• Do not eat raw or undercooked poultry products and wash your hands after touching uncooked poultry.
• Practice good hygiene and cleanliness.
• Visit a health care provider if you become sick during or after travel.

CDC is following global developments related to avian influenza closely and will continue to provide further updates to the situation and update guidance as needed.

This spotlight was first posted on April 10 and subsequently updated on April 14 to include additional information from two WHO reports, including the fact that the patient died from their illness and had multiple underlying conditions that would have placed them at higher risk of developing serious complications from influenza A virus infections.