By Stan Deresinski, MD, FACP, FIDSA
Clinical Professor of Medicine, Stanford University
SYNOPSIS: A strain of influenza A virus that is highly pathogenic for birds is capable of infecting humans for whom it poses a potential risk as it continues to evolve and spread.
SOURCE: Adlhoch C, Baldinelli F. Avian influenza, new aspects of an old threat. Euro Surveill 2023;28:2300227.
The 1918 influenza epidemic (“Spanish flu”) was caused by an H1N1 influenza virus carrying genes of avian origin. This observation informs current concerns about the spreading pandemic of influenza in birds.
Since 2020, poultry farms in parts of Europe have been devastated by more than 14,000 outbreaks of infection by highly pathogenic avian influenza (HPAI) A(H5) viruses belonging to clade 2.3.4.4b. As a consequence, approximately 96,000 birds were culled. This occurrence is the result of a drastically altered epidemiology of HPAI with the emergence of clade 2.3.4.4b.
In the summer of 2021, HPAI was found in wild birds in some breeding areas of northern Europe, breaking the previous seasonal pattern of its presence only in the cooler months of the year and associated with migratory birds overwintering. At the same time, A(H5) HPAI clade 2.3.4.4b spread along migratory paths and was detected in Asia, while spreading in Europe, and extending to Greenland and Iceland as well as North America and, subsequently, to South America (as far as the southern tip of Chile) and Africa. More than 40% of pelicans, an already threatened population, in Peru and Chile died.
This year, as of May 12, at least 22 California condors, a species that has been on the edge of extinction and the subject of many salvage efforts over recent decades, were tragically reported to have died due to infection from HPAI. This has led to initiation of a clinical trial of a previously conditionally licensed inactivated vaccine in North Carolina in North American vultures, a species closely related to the condor.1
Transmission of A(H5) HPAI clade 2.3.4.4b to free-living mammals also occurs, with examples including minks in Spain, dolphins in South America, and harbor and gray seals in New England. Infection of humans also can occur, but it has been rare and most often associated with farmed poultry activities and without evidence of human-to-human transmission.2 Human infections generally have been mild, although severe infection and even death are possible.
COMMENTARY
This virus already has caused great economic damage, which is likely to worsen. It also may have the potential to lead to extinction of some species, such as the California condor, a species for which vaccination is planned. (Question: How do you get a bird with a 15-foot wingspan to sit still long enough?)
While A(H5) HPAI clade 2.3.4.4b can infect humans, illnesses observed to date have been mild and the lack of human-human transmission is encouraging. However, evidence indicates that the virus continues to evolve, largely through recombination, raising the possibility of it developing enhanced transmissibility to humans as well as the ability to transmit between humans. It also could become more virulent for humans. Current evidence indicates that clade 2.3.4.4b virus is susceptible in vitro to oseltamivir, zanamivir, and baloxavir, but this also could change.
REFERENCES
- Hackett D. Bird flu vaccine authorized to save California condors. Precision Vaccinations. May 16, 2023. https://www.precisionvaccinations.com/bird-flu-vaccine-authorized-save-california-condors
- Centers for Disease Control and Prevention. Current U.S. bird flu situation in humans. Last reviewed April 28, 2023. https://www.cdc.gov/flu/avianflu/inhumans.htm
