Bird flu, also known as avian influenza, is a viral disease that primarily infects birds but can sometimes spread to humans as well. There are several different strains of bird flu viruses that vary in severity. The current bird flu strain causing global concern is known as H5N1.
H5N1 is a highly pathogenic avian influenza (HPAI) virus that was first detected in geese in China in 1996. Since 2003, H5N1 has spread from Asia to Europe, Africa, and the Middle East, resulting in the death or culling of hundreds of millions of birds. Currently, H5N1 is endemic in certain countries, meaning the virus persists in the wild bird population year-round. Sporadic human infections with H5N1 occur most years, often associated with direct contact with infected poultry.
Why is H5N1 concerning?
The H5N1 strain has raised global alarms for the following reasons:
- High mortality rate: Over 50% of reported human H5N1 cases have resulted in death. For comparison, the 1918 Spanish flu pandemic had an estimated 2-3% fatality rate, while seasonal flu strains currently have less than 0.1% fatality.
- Zoonotic transmission: H5N1 can cross from birds to mammals, including humans. Human cases remain rare and are from close contact with infected birds. Limited human-to-human transmission has occurred.
- Global spread: H5N1 has a strong ability to mutate and spread geographically amongst wild and domestic bird populations across Asia, Europe, Africa and the Middle East.
- Pandemic potential: Experts warn that if H5N1 mutates further or combines with a human flu strain, it could gain the ability to transmit easily between humans, resulting in a deadly global influenza pandemic.
H5N1 origin and evolution
All known influenza A virus strains, including H5N1, originated in wild aquatic birds. Wild birds act as a reservoir, carrying flu viruses asymptomatically in their gastrointestinal tract. Through migration and mixing of various wild bird species, new influenza strains emerge, mutate and exchange genes over time. This is how novel avian flu strains like H5N1 come about.
The theory is that sometime prior to 1959, an H5 avian flu strain emerged and then diverged into a Eurasian and American genetic lineage. Here is a brief timeline of the evolution of the Eurasian H5 lineage leading to H5N1:
- 1959: H5 Eurasian avian lineage splits from American lineage
- Early 1990s: The Gs/GD lineage of H5 forms, likely in China
- 1996: A highly pathogenic form of the GD lineage (H5N1) is detected in geese in China
- 1997: First recorded transmission of H5N1 from birds to humans during an outbreak in Hong Kong
- 2001-2002: H5N1 re-emerges, spreads across Asia to Europe and Africa
- 2003-early 2022: H5N1 becomes endemic in poultry in parts of Asia and Egypt, and continues to mutate
The root cause of how highly pathogenic H5N1 emerged in the 1990s remains unknown. But most scientists agree its ancestral viruses were likely circulating harmlessly in wild waterfowl for centuries before mutating into more virulent forms. Since 2003, the H5N1 lineage has diversified into many genetic clades and subclades as it spread globally.
H5N1 genetic diversity
There are two major genetic groups of H5N1 circulating in birds:
Clade 2.3.4.4 group
This newer, dominant H5N1 group emerged in China around 2008 and has become widespread across Asia, the Middle East and parts of Europe and Africa. The 2.3.4.4 clade group is highly concerning due to its rapid geographic spread, persistence in wild and domestic birds, and continued evolution.
There are five main 2.3.4.4 subclades:
- A (H5N1 viruses from Cambodia, Vietnam, Japan, Korea)
- B (H5N1 from China)
- C (H5N1 found in Bangladesh, India and Nepal)
- D (H5N1 from China and Indonesia)
- E (H5N1 from West Africa, Europe and the Middle East)
The 2.3.4.4 group remains the predominant H5N1 clade infecting wild birds and poultry today. It accounts for the majority of human H5N1 cases reported since 2009.
Clade 2.2 viruses
This older H5N1 group was detected first in China in 2005. Clade 2.2 viruses remain endemic in poultry in parts of Asia and Egypt. There are two main 2.2 subclades:
- 2.2.1 (H5N1 across Egypt, Nigeria, and West Africa)
- 2.2.2 (H5N1 in Bangladesh and India)
The 2.2 viruses have infected humans less frequently than 2.3.4.4, however Egypt reports sporadic human cases most years. H5N1 2.2 viruses continue to mutate and spill over into wild birds, persisting as a public health threat.
Current global H5N1 situation
Here is an overview of the current H5N1 situation around the world as of October 2022:
Asia
Both 2.3.4.4 and 2.2 H5N1 clades widely circulate in wild and domestic birds across Asia. Intense circulation persists in China, Vietnam, Indonesia and India. Sporadic human infections with H5N1 occur annually, primarily in China, Cambodia, Vietnam, Bangladesh and India. Rare instances of human-to-human transmission have occurred.
Europe and Middle East
The 2.3.4.4 H5Nx clade caused massive wild bird outbreaks across Europe between 2021 and early 2022. Affected countries included the UK, France, Germany, Spain, and Greece. Millions of farmed birds were also culled. Currently the outbreak appears contained. Sporadic H5N1 outbreaks continue in poultry in Bulgaria and Russia.
In the Middle East, clade 2.3.4.4 H5N1 has caused recurrent outbreaks in wild birds and commercial poultry in Saudi Arabia, Israel and Iran in 2022. Egypt reports ongoing H5N2 outbreaks in poultry and some human cases.
Africa
Nigeria has reported multiple H5N1 outbreaks in poultry and wild birds in 2022. The 2.3.4.4 viruses appear dominant. West and Central Africa suffer ongoing circulation of the older 2.2.1 H5N1 in poultry. Human cases remain rare in Africa.
North America
In early 2022, a highly pathogenic H5N1 virus was detected in wild birds and poultry in Canada and multiple US states for the first time since 2014-2015. The virus appeared similar to the Eurasian H5Nx 2.3.4.4 clade. Control measures appear to have contained the outbreaks. No human infections were reported.
Human infections with H5N1
Here are key points about human cases of H5N1 infection:
- Global total since 2003: around 850 cases
- Deaths: approximately 450 fatalities, over 50% mortality rate
- Most cases result from close contact with infected live or dead poultry
- Rare cases of human-to-human transmission have occurred
- Average age of patients is around 18 years old
- Symptoms include pneumonia, acute respiratory distress, multi-organ failure
- Antiviral medications like oseltamivir can improve survival if given early
While the virus does not yet spread easily between people, experts continue to warn that future mutations or re-assortment with human flu strains could enable human-to-human transmission and start an H5N1 pandemic. Ongoing surveillance and vaccine development for H5N1 remains a global public health priority.
H5N1 pandemic concerns
Experts offer several reasons why an H5N1 pandemic threatens global health security:
- The 2.3.4.4 viruses are entrenched across several continents in wild and domestic birds, increasing risk of spread to humans.
- Constant virus evolution increases chances of a mutation enabling human transmission.
- Lack of pre-existing immunity in humans means entire global population is theoretically susceptible.
- Current seasonal flu vaccines do not protect against H5N1.
- H5N1 causes more severe human illness compared to seasonal flu, taxes healthcare systems.
- Many countries lack pandemic preparedness plans, surveillance capabilities.
Mitigation measures against an H5N1 pandemic could include:
- Stockpiling antiviral medications like oseltamivir
- Advancing H5N1 vaccine development for human use
- Enhancing surveillance in wild and domestic bird populations
- Culling infected poultry flocks
- Public health campaigns on hygiene and avoiding sick/dead birds
- Preparedness to close schools, workplaces, limit public gatherings
- Planning to ensure functioning supply chains and healthcare capacity
Conclusion
The H5N1 avian flu virus remains a significant global health threat. Since emerging in the 1990s, these viruses have become entrenched in wild and domestic bird populations across Asia, Europe, Africa and the Middle East. While H5N1 does not currently transmit easily between humans, its continued spread, mutation and lethality in human cases keeps pandemic fears high. Ongoing surveillance and preparedness measures are vital to mitigate a potential H5N1 pandemic. Public health officials advise people avoid direct contact with wild birds and poultry in countries reporting H5N1 outbreaks. Any human cases should be aggressively contained. With vigilance and continued research, the world hopes to stay one step ahead of the shifting threat posed by avian influenza.