New Study Highlights Importance of Bird Flu Surveillance in Live Bird Markets

CDC Update

January 31, 2024 – A new CDC co-authored study published in Influenza and Other Respiratory Viruses describes findings from avian influenza A(H5) virus surveillance in live bird markets (LBMs) across Vietnam over a nearly six-year period (March 2017- September 2022). The study found almost 40% of samples taken from LBMs in Vietnam were positive for influenza A viruses with the prevalence of H5 bird flu increasing in 2022. Active surveillance (regular testing) was better at detection than passive surveillance (receiving reports), especially when poultry did not show signs of illness. This study underscores how LBM surveillance remains a vitally important activity for situational awareness around bird flu, bird flu mitigation, and pandemic preparedness activities.

LBMs, where large numbers of live birds are bought or traded, are commonplace in many parts of the world, including Vietnam. LBMs are settings where many different avian influenza viruses can be found, and this raises the risk for human infections or new avian influenza viruses emerging through genetic reassortment (mixing of virus genes). For this study, two methods were used to capture the percent positivity and geographic distribution of avian influenza A(H5) detections:

• Passive surveillance of influenza A(H5N1), A(H5N6), and A(H5N8) where outbreaks were reported to the Vietnamese Department of Animal Health.
• Active surveillance of influenza A, A(H5), A(H5N1), A(H5N6), and A(H5N8) viruses where 30 poultry oropharyngeal swab specimens and five environmental samples were collected monthly at each LBM.

From March 2017 to September 2022, 128,870 specimens were collected at 167 LBMs in 24 provinces. Almost 40% of pooled samples (samples pooled into groups of five) were positive for influenza A. Influenza A(H5) virus was detected in LBMs at least once in more than 90% of provinces and more than half of LBMs sampled in Vietnam. The prevalence of A(H5) in LBMs also increased during 2022 to 6%. During the same time period, there were 246 A(H5) outbreaks in poultry; most outbreaks were of influenza A(H5N6) followed by A(H5N1) and A(H5N8). Active surveillance captured more influenza A(H5) virus activity than what was passively reported through outbreaks.

In addition to finding year-round circulation of A(H5) virus in poultry and LBM environmental samples in Vietnam, the study authors also found differences in geographic and annual temporal patterns of avian influenza:

• Influenza A(H5N1) and A(H5N6) viruses circulated across all sampled regions.
  • Higher A(H5N1) positivity rates were observed in LBMs in the Southern region
  • Higher A(H5N6) positivity rates were observed in LBMs in the Central region
• Influenza A(H5N8) virus was only detected in LBMs in the Northern and Southern areas.
• Influenza A(H5) positivity rate was higher in LBMs in the Southern provinces. (This may reflect the higher number of domestic ducks living in these areas.)

These study findings further support the value of partnerships to enhance global influenza surveillance. The first collaboration between Vietnam's National Institute of Hygiene and Epidemiology (NIHE) and CDC began in 1999 with sero-epidemiology studies of H5 and H9 among poultry workers in LBMs in Hanoi, and this work built capacity at NIHE which was critical in the response to the SARS outbreak of 2003. From 2003 to 2022, 127 infections and 64 deaths with highly pathogenic avian influenza (HPAI) A(H5) viruses have been detected in people living in Vietnam. Since 2013, avian influenza surveillance has been conducted in provinces in Vietnam with active LBMs through a collaboration between the Vietnamese Ministry of Agriculture and Rural Development, Department of Animal Health (DAH), and CDC.

The current risk to the U.S. public from HPAI A(H5) viruses in Vietnam remains low. However, because of the potential for influenza viruses to rapidly evolve and the wide global prevalence of HPAI A(H5) viruses in wild birds and poultry outbreaks, continued sporadic human infections are anticipated. Continued comprehensive surveillance of these viruses in LBMs and frequent risk assessments, are critical to a continuous public health risk assessment, along with ongoing preparedness efforts. Global influenza surveillance, both epidemiologic and virologic, is the foundation of influenza preparedness and response for influenza, including avian influenza. More information on avian flu is available on CDC's avian influenza website.