H5N1 is a subtype of avian influenza, or bird flu, that primarily circulates among wild birds and poultry. While it predominantly affects avian species, this virus can, on rare occasions, cross the species barrier to infect humans. Though human cases are infrequent, they are a significant public health concern due to the virus’s potential to cause severe illness and fatality. The H5N1 virus has been circulating globally in bird populations since its emergence in 1996, with a variant causing widespread outbreaks since 2020.
Human Transmission and Risk Factors
The most common route for H5N1 transmission to humans involves direct or close contact with infected birds, such as poultry, or environments contaminated by their bodily fluids. This includes interactions in settings like live bird markets or backyard farms where sick or dead poultry are handled. Preparing sick or dead poultry for consumption also presents a risk of infection.
More recently, transmission from infected mammals, particularly dairy cattle, has raised concerns. Human cases have been reported following exposure to infected dairy cows, with unpasteurized milk identified as a potential source. Contact with environments where infected animals are housed also poses a risk.
Certain occupations carry a higher risk of exposure. These include poultry farm workers, veterinarians, and those involved in culling operations or working with infected animals. While limited human-to-human transmission has been suspected, sustained spread of H5N1 between people has not been observed.
Symptoms and Health Impact in Humans
Upon H5N1 infection, individuals may initially experience symptoms resembling common influenza, such as fever, cough, sore throat, and muscle aches. Conjunctivitis (eye infection) has also been reported. Gastrointestinal symptoms like diarrhea and abdominal pain are more commonly associated with H5N1 infections compared to other flu types.
The illness can progress rapidly, leading to severe respiratory problems. These complications include pneumonia, acute respiratory distress syndrome (ARDS), and multi-organ failure. The virus primarily replicates in the lower respiratory tract, leading to viral pneumonia.
H5N1 infections in humans have a high case-fatality rate, around 50% globally, varying by country. Indonesia, for example, has reported an 84% case-fatality ratio among its confirmed cases. Despite this severity, the total number of laboratory-confirmed human infections worldwide since 2003 remains relatively low, with nearly 900 cases and over half being fatal as of early 2025.
Diagnosis and Medical Interventions
Diagnosing H5N1 infection in humans typically requires laboratory tests, as initial symptoms can mimic other respiratory illnesses. Molecular testing, such as reverse transcription-polymerase chain reaction (RT-PCR), is recommended to confirm the virus and identify its subtype from nose or throat samples. Deep respiratory specimens, like tracheal swabs, may provide better virus detection than upper respiratory samples.
Treatment for H5N1 primarily involves antiviral medications, with oseltamivir (Tamiflu) being a common option. Early administration of antiviral therapy is advised for symptomatic patients with suspected or confirmed infection, as it can significantly improve outcomes. While existing antivirals may not always be fully effective against severe H5N1 infections, baloxavir marboxyl (Xofluza) has shown benefits for ocular infection routes.
Beyond antivirals, supportive care manages symptoms and complications, including intensive care for respiratory or multi-organ failure. Candidate vaccines against H5N1 have been developed for potential human use in pandemic preparedness, though seasonal flu vaccines do not protect against this avian influenza strain.
Global Surveillance and Pandemic Preparedness
International and national health organizations, such as the WHO and CDC, conduct active surveillance of H5N1 strains in animal and human populations. This monitoring aims to detect genetic changes that could alter its transmissibility or severity in humans. The WHO’s Global Influenza Surveillance and Response System (GISRS) identifies and tracks circulating influenza viruses to advise on public health risks.
Pandemic preparedness efforts involve strategic actions to mitigate the impact of a potential widespread H5N1 outbreak. These actions include stockpiling antiviral drugs, such as oseltamivir, for rapid treatment. Pre-pandemic vaccines are also developed and stockpiled for deployment if the virus gains efficient human-to-human transmission.
Response plans are continuously updated to outline public health measures, including isolation guidelines and contact tracing. H5N1 is closely monitored due to its potential to mutate for more efficient human-to-human spread, which would signal a new influenza pandemic.