This year, the world has been witnessing one of the worst-ever documented outbreak of the highly pathogenic avian influenza H5N1 killing millions of birds. The virus, which is known to cause severe disease and death in birds, has also been detected in mammalian species and also in humans.
This has put health authorities on high alert regarding the implications of the large outbreak on public health.
High mortality in birds
Although avian influenza has different subtypes, H5N1 is a highly pathogenic subtype that causes mortality in birds. Since 2022, the virus has infected over 100 million birds across the globe, resulting in the deaths of over 50 million and culling of millions of poultry. Unlike previous outbreaks of highly pathogenic subtypes of avian influenza, H5N1 is heavily impacting wild bird species, including many which were on the verge of extinction.
While it is difficult to ascertain how many wild birds have been affected by the virus, a significant impact has been seen in eagles, pelicans, geese, waterfowl, gulls, falcons and shorebirds, in addition to the highest possible impact on poultry seen till date, at least in the U.S.
The impact of H5N1 on wild bird populations has varied depending on several factors, such as level of exposure, geographical locations and migratory patterns of the affected species.
High mortality in wild birds due to the virus could lead to significant ecological consequences, including vulnerability of predators and alterations in species composition in affected ecosystems, and therefore a possible impact on biodiversity not just limited to avian species.
It has raised concerns regarding the spread of the virus among critically endangered avian populations.
In recent weeks, reports suggest that at least 20 California condors, a species that was on the verge of extinction since 1980s, have succumbed to H5N1 avian influenza. With around 300 condors estimated to be remaining in the wild, this would roughly account for a significant 7% of the species. H5N1 has also killed a large number of bald eagles and Caspian terns in the U.S. since January 2022, along with thousands of cranes in Israel. Last year, H5N1 hit a colony of the endangered African penguins in South Africa, killing at least 30 penguins. An uncontrolled spread of the virus could, thus, be catastrophic to the already fragile populations of endangered birds across the world, leading to the mass extinction of several species.
Spread to animals
The highly contagious H5N1 virus can also occasionally spillover from birds to animals through direct or indirect contact with infected birds or their droppings.
Worryingly, there have been several reports on spillover of H5N1 to mammals during the current outbreak from different countries, infecting species such as sea lions, minks, foxes, wild bears, and skunks, apart from domestic animals such as dogs and cats.
In 2023 alone, H5N1 caused the deaths of over 3,000 sea lions in Peru. In a recent yet-to-be peer-reviewed study, scientists found that the virus could efficiently spread between ferrets in the laboratory. The only known cases of the virus spreading between mammals were reported in minks that were raised in close confinement in a farm in Spain.
The transmission of H5N1 from birds to mammals is rare, but when it does occur, it can be a cause for concern, as the virus could accumulate mutations and acquire the ability to potentially initiate human outbreaks. H5N1 has a high mortality rate of over 60% in humans and is primarily transmitted to humans through close contact with infected birds or animals, either through handling infected poultry or exposure to contaminated environments.
In the recent months, a few sporadic cases of human H5N1 infections have also been reported from Ecuador, Cambodia, and more recently in Chile. Given the ongoing threat of an influenza pandemic, monitoring the spread in mammalian populations is important in areas of close contact between humans and animals. However, since the virus does not yet transmit efficiently among humans, the World Health Organization (WHO) has assessed the risk of H5N1 to humans to be low.
However, the large and unabated outbreak in avian species and not so rare mammalian spillovers could potentially provide the virus a chance to adapt for mammalian transmission.
As the current H5N1 outbreak continues unabated with devastating impact on avian population globally, and with significant ecological and economic consequences, the time has never been better to initiate efforts for preparedness towards building better, efficient vaccines for avians and humans and genomics surveillance to map the continued evolution of the virus. Enhanced biosecurity measures are also needed to protect both animal and public health.
(Bani Jolly and Vinod Scaria are researchers at the CSIR Institute of Genomics and Integrative Biology (CSIR-IGIB). Opinions expressed are personal)