Five years after COVID-19 changed our world, another virus is raising global health concerns. H5N1 bird flu, long watched by health authorities in India and across Asia, has taken an unexpected turn in North America, revealing new capabilities that could have worldwide implications.
The virus's recent unusual behaviour in the United States and Canada demands global attention for the simple reason that it is doing something never seen before by prior bird flu strains. Since March 2024, H5N1 has infected
Five years after COVID-19 changed our world, another virus is raising global health concerns. H5N1 bird flu, long watched by health authorities in India and across Asia, has taken an unexpected turn in North America, revealing new capabilities that could have worldwide implications.
The virus's recent unusual behaviour in the United States and Canada demands global attention for the simple reason that it is doing something never seen before by prior bird flu strains. Since March 2024, H5N1 has infected dairy cattle across America – the first time it has spread widely in cows anywhere in the world. Earlier this year, California declared a state of emergency after the virus affected 660 dairy farms in the state, which produces more milk than any other in America. This unprecedented spread in mammals shows the virus is adapting in ways that concern health experts worldwide.
The virus's impact extends far beyond farms. Across North America, it has caused deaths among wild animals in startling numbers. Just this month, a wildlife sanctuary in Washington lost 20 big cats, including tigers and lions, to H5N1 infection. The virus has also killed seals along coastlines, foxes in forests, and bears in national parks. This widespread impact on mammals is unprecedented in H5N1's known history.
H5N1's story begins in Hong Kong in 1997, where it first infected humans during a poultry outbreak, causing six deaths and leading to the culling of 1.5 million chickens. Since then, it has caused over 900 human infections globally, with a mortality rate exceeding 50%. Most severe cases occur through direct contact with infected birds.
Yet mysteriously, recent cases have typically been mild. For example, of the 65 people infected in the United States in 2024, most experienced only mild symptoms, particularly dairy workers exposed to infected cattle. Scientists believe this mildness might be due to better detection of mild cases that previously went unreported, or because the B3.13 strain circulating in dairy cattle is less adapted to causing severe human disease.
However, two severe cases in late 2024 have caught medical experts' attention. On November 2, a teenager in British Columbia, Canada developed conjunctivitis that rapidly progressed to severe respiratory distress, requiring mechanical ventilation. Extensive testing failed to identify the infection’s direct source. However, genetic analysis linked the strain to bird flu circulating among wild birds in Canada.
Just weeks later, on December 13, Louisiana reported America's first severe case of a man who had contact with backyard poultry. Both cases involved the D1.1 strain, which shows concerning mutations that enhance its ability to bind to human lung cells.
The key to understanding H5N1's current threat lies in its molecular structure. The virus struggles to spread between humans because it relies on avian-specific receptors that are rare in human upper airways. While recent mutations allow better binding to human lung cells (explaining the severe cases) the virus still cannot efficiently replicate in the nose and throat crucial to human-to-human transmission. This limitation has prevented human-to-human transmission and, so far, a pandemic.
What can the public do right now?
The US Centers for Disease Control and Prevention (CDC) has issued clear guidance emphasising avoidance of sick or dead birds and animals, consumption of only pasteurised dairy products, and protection of pets from wild bird contact. These precautions are particularly relevant in countries like India, where traditional farming practices and live bird markets create potential exposure risks.
As we enter 2025, the international community faces a critical challenge. H5N1 is not spreading between humans, but its evolution in different species increases the chance it could. The virus could potentially combine with human influenza strains, particularly in intermediate hosts like pigs, creating a more dangerous variant.
Over the past two decades, India has successfully contained H5N1 since its first major outbreak in 2006, through rapid detection and response systems, thereby protecting both its massive poultry industry and public health. Similar challenges face other Asian nations with dense poultry populations, such as Indonesia and Vietnam, where H5N1 has historically caused significant outbreaks.
Efforts to develop vaccines tailored to H5N1 are underway. Candidate vaccine viruses have been produced by organisations like the CDC and WHO to target currently circulating H5N1 strains. While no mass immunisation campaigns are planned yet, manufacturers will be able to scale up production if the virus shows signs of efficient human-to-human transmission.
What is clear is that the threat is not going away anytime soon. Through coordinated monitoring, rapid sharing of information, and proactive preparedness, the global community can reduce the risk of H5N1 becoming the next pandemic.
Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal
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