As a lifelong Midwesterner, I’m accustomed to the short-lived, false springs of January and February. I know to save gleeful cries of “spring is here!” until the trees bud and I can hear the buzzing trill of red-winged blackbirds and the calls of other birds returning from their winter homes. But this spring, the return of birdsong is not all good news.
In January 2022, the state of South Carolina reported a case of highly pathogenic avian influenza in a wild bird—the first detected case of this virus subtype in the United States since 2016. Since then, the outbreak has spread. Two weeks ago my home state of Wisconsin reported its first case in a commercial chicken flock of nearly 3 million birds, one of the largest US flocks affected so far.
This virus belongs to the influenza A H5N1 subtype and is classified as “highly pathogenic”, which specifically refers to its ability to kill chickens and other domesticated poultry species. While highly pathogenic H5N1 strains pose limited threats to wild birds (and to human health), infected poultry have high death rates—nearly 100%. Most commercial flocks infected with highly pathogenic H5N1 are “depopulated” (i.e., killed) and removed from commercial food supply chains, posing major economic losses for the agricultural industry and potential food price hikes and shortages for consumers.
Highly Pathogenic Avian Influenza Strains
Like other influenza viruses, A(H5N1) is characterized by the type of hemagglutinin (H) and neuraminidase (N) proteins present on the virus’s surface. Hemagglutinin proteins bind to receptors on the cell surface and facilitate viral infection, and neuraminidase enzymes aid viral release from infected cells.
While most influenza A viruses are considered “low pathogenicity”, all known highly pathogenic influenza A viruses display the H5 or H7 hemagglutinin subtypes. The enhanced ability of highly pathogenic H5- and H7-bearing viruses to infect hosts is thought to be due in part to a cleavage site on the hemagglutinin protein that allows viral replication to take place beyond the respiratory and gastrointestinal tracts (1–3).
When birds are infected with A(H5N1), they can shed the virus in their saliva, nasal discharge and feces. Surfaces and bodies of water contaminated with the virus can also infect other birds. Wild waterfowl species appear to be natural reservoirs of the virus, which hinders eradication (4).
Typical biosecurity measures for commercial and backyard poultry flocks mainly focus on preventing flocks’ contact with materials—ponds, feed, clothing—that might be contaminated with the virus. If disease is detected in a flock, the flock is depopulated. Routine testing of wild waterfowl through cooperation with hunters is also an important part of the USDA’s Animal and Plant Health Inspection Service’s avian influenza biosecurity program.
The strain driving the current US outbreak belongs to the Eurasian lineage goose/Guangdong/1/1996 lineage H5 clade 18.104.22.168b, which has become the most common H5N1 strain detected globally. A pre-print released by the USDA National Wildlife Research Center in February suggests that, based on genetic analysis of virus samples collected in South Carolina, US cases spread from Europe through birds with trans-Atlantic migrations (5). If so, this is the first documented trans-Atlantic transmission of this virus strain (6).
Avian Flu and Humans
The CDC lists the current A(H5N1) outbreak as a low risk to humans. Current and past variants of A(H5N1) have demonstrated low transmission from birds to humans. Most human cases have been in people who work in the poultry industry and have a high level of exposure to infected birds.
However, because of the tendency for influenza viruses to mutate, highly pathogenic A(H5N1) represents a potential future human health risk. Recognizing the potential public health impact of avian influenza, the CDC is developing various vaccines for several avian influenza subtypes. The current A(H5N1) virus in circulation has been found to be “nearly identical” to one of these vaccine candidates. In the chance that sustained human-to-human transmission was detected, vaccines like this one could be manufactured and deployed.
In the meantime, surveillance efforts in bird flocks continue. For people in the poultry industry or who have backyard flocks, standard biosecurity practices are the most tractable steps to take to protect their birds and their own health.
- Senne, D. A., et al. (1996) Survey of the Hemagglutinin (HA) Cleavage Site: Sequence of H5 and H7 Avian Influenza Viruses: Amino Acid Sequence at the HA Cleavage Site as a Marker of Pathogenicity Potential. Avian Dis. 40, 425–437.
- Rott, R., et al. (1995) Influenza Viruses, Cell Enzymes, and Pathogenicity. Am. J. Respir. Crit. Care Med. 152, S16–S19.
- Boggs, J., et al. (2010) Highly Pathogenic H5N1 Influenza Viruses Carry Virulence Determinants Beyond the Polybasic Hemagglutinin Cleavage Site. PLoS One 5, e11826.
- Sonnberg, S.; Webby, R. J.; Webster, R. G. (2013) Natural History of Highly Pathogenic Avian Influenza H5N1. Virus Res. 178, 63–77.
- Bevins, S. N., et al. (2022) Intercontinental Movement of H5 22.214.171.124 Highly Pathogenic Avian Influenza A(H5N1) to the United States, 2021. [bioRxiv preprint] https://doi.org/10.1101/2022.02.11.479922
- Caliendo, V., et al. (2022) Transatlantic Spread of Highly Pathogenic Avian Influenza H5N1 by Wild Birds From Europe to North American in 2021. [bioRxiv preprint] https://doi.org/10.1101/2022.01.13.476155
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