Avian Influenza Virus Can But Rarely Jumps to Humans

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(1) What To Know About Bird Flu.Center for Disease Control and Prevention, undated
https://www.cdc.gov/flu/pdf/avia ... xposure-Handout.pdf

first two paragraphs:

"About Bird Flu
Bird flu is a disease caused by certain flu viruses that usually spreads between birds, not people. Infected birds can spread the virus through their mucous, saliva or feces. People rarely get bird flu, but when they do, it’s most often through direct unprotected contact (no gloves, protective
wear, facemasks, respirators or eye protection) with infected birds. People can become infected by breathing virus in droplets in the air or possibly dust, or by touching surfaces contaminated with infected bird mucous, saliva or feces and then touching their eyes, mouth or nose. Human illness
from bird flu has ranged from no symptoms at all to severe illness, resulting in death.

"Types of Birds That Can be Sick with Bird Flu
Wild birds that can be infected with bird flu viruses include waterbirds, like ducks, geese and swans, and shorebirds, like storks. Bird flu can spread from wild birds to poultry, like chickens and turkeys. While most wild birds can be infected with bird flu viruses without being sick, poultry, like chickens and turkeys, can get very sick and die from certain bird flu viruses. If you have poultry, your birds can get bird flu if they have contact with infected wild birds or share food, water sources and other environments with them. Most common songbirds or other birds found in the yard, like cardinals, robins, sparrows, blue jays, crows or pigeons, do not get infected with the bird flu viruses that can be dangerous to poultry or possibly spread to people.

(2) Frequently Asked Questions about Avian Influenza (Bird Flu). CDC, undated.
https://www.cdc.gov/flu/avianflu/avian-flu-faq.htm

"What is the risk of bird flu viruses to human health?
Human infections with bird flu viruses are rare but can occur, usually after close contact with infected birds. The current risk to the general public from bird flu viruses is low; however, it is important to remember that risk depends on exposure, and people with more exposure might have a greater risk of infection. There is existing federal guidance around bird flu exposures for different groups of people, including people with occupational or recreational exposure, such as hunters [297 KB, 2 pages] and poultry producers, and also for the general public, as well as health care providers.


Note:
(a)
(i) neuraminidase
https://en.wikipedia.org/wiki/Neuraminidase("is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: Hydrolysis of α-(2→3)-, α-(2→6)- * * * glycosidic linkages of terminal sialic acid residues" and the sugar moiety next to it (neuraminic acid) )
(ii) neuraminic acid
https://en.wikipedia.org/wiki/Neuraminic_acid
("The N- or O-substituted derivatives of neuraminic acid are collectively known as sialic acids * * * The name 'neuraminic acid' was introduced by German scientist E Klenk in 1941, in reference to the brain lipids from which it was derived as a cleavage product")

Wiktionary for "neuraminic acid" says its etymology is "neuro- +‎ -amine +‎ -ic."
(iii) sialic acid
https://en.wikipedia.org/wiki/Sialic_acid
("The term 'sialic acid' (from the Greek for saliva, σίαλον - síalon) was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this group is N-acetylneuraminic acid. * * * In humans the brain has the highest sialic acid content")
(A) sialic
https://www.merriam-webster.com/dictionary/sialic
(pronunciation)
(B) All you have to know is a neuraminidase cleaves the bond between a neuraminic acid (which is a kind of sialic acid) and the next sugar moiety -- in a polysaccharide.

(b) Byrd-Leotis L et al, The Interplay between the Host Receptor and Influenza Virus Hemagglutinin and Neuraminidase. International Journal of Molecular Sciences, 18: 1541 (2017).
https://www.mdpi.com/1422-0067/18/7/1541
(i) The first figure (caption: graphic abstract) shows an influenza virus with, on its membraneous surface, green hemagglutinin homotrimers (each made up of three identical monomers of hemagglutinin) and blue-and-purple neuraminidase homotetramers (each made up of four identical monomer of neuraminidase). This figure also demonstrates, at 6 o'clock direction, that AFTER hemagglutinin attaches itself to a sialic acid at the end of a polysaccharide, the neuraminidase next to it (hemagglutinin) will cleave the sialic acid 9reoresented by a purple diamond) from the next (galactose 半乳糖 here, represented as a white circle).

It is called hemagglutinin, because presence of influenza virus and (mammalian) red blood cells will cause red blood cells to clump and settle down (at the bottom of a fluid). That is because various hemagglutinin trimers of ONE influenza virus may bind to different red blood cells.
(ii) The paragraph right before Figure 1 stated, "The linkage of the sialic acid to the penultimate galactose is considered to be a determinant of species [of host] specificity, with avian viruses characterized by binding α2,3-Sia and mammalian viruses by binding to α2,6-Sia."

In figure 1, sialic acid (marked by both purple ring and purple text) and galactose (marked by green ring and green text) are connected by carbon 2 on sialic acid (carbon 1 of sialic acid is that in -COOH atop carbon 2) to carbon 3 OR 6 of galactose (whose carbon 1 is connected to -O-R).
(c) Gamblin SJ et al, Hemagglutinin Structure and Activities. Cold Spring Harbor Perspective in Medicine, 11: a038638 (2021).
http://perspectivesinmedicine.cs ... /11/10/a038638.long
(i) Abstract: "Hemagglutinins (HAs) * * * recognize sialic acid–containing, cell-surface glycoconjugates as receptors but have limited affinity for them, and, as a consequence, virus attachment to cells requires their interaction with several virus HAs. Receptor-bound virus is transferred into endosomes where membrane fusion [fusion of endosomal membrane (from host) AND virus membrane] by HAs is activated at pH between 5 and 6.5, depending on the strain of virus.
(ii) Figure 1 is the 3-dimensional hemagglutinin homotrimer. But you do not have to know.
(iii) caption of Figure 2 stated in part:

"Avian and human receptor analogs complexed with Group 1 and Group 2 HAs represented by H1 [eg, H1N1] and H3 HAs. The saccharide composition of the receptors is shown schematically below each panel. The two types of receptors are distinguished by the α-2,3 (avian) and α-2,6 (human) linkages between sialic acid and the second saccharide, galactose. Different avian species also prefer oligosaccharides with different linkages between Gal-2 and GlcNAc-3. Wild ducks, for example, prefer Gal-2 b1,3-linked GlcNAc-3, whereas domesticated poultry prefer Gal-2 b1-4 GlcNAc-3-linked sialosides (Gambaryan et al. 2008)." (emphasis added).

(A) Sia-1 (meaning sialic acid in position 1; represented as a rectangle), followed by Gal-2 and NAG-3 (in that order; GlcNAc stands for N-acetylglucosamine).
(B) All in the above quotation expressed "prefer[rence]." Thus jump in species barriers MAY occur for influenza virus.
(C) "Gal-2 b1,3-linked GlcNAc-3" means Gal-2 and GlcNAc-3 are linked by beta 1,3 glycosidic bond. A glycosidic bond forms between two monosaccharide by joining two hydroxyl (-OH) groups and eliminating a molecule of water H2O. The glycosidic bond can be either alpha or beta (which would be too deep for you to understand). The 1,3 refers to carbon 1 of the left-hand monosaccharide for glycosidic bond with carbon 3 of the right-hand monosaccharide.