Hidden deep
One acronym scares the world: H5N1. But bird flu is still an animal disease that rarely affects humans. And human-to-human transmission is even rarer, if at all. Why exactly?
It started in December 2003 somewhere in the deepest part of Asia. But it didn't take long for the pathogen to find its way to Europe, including Germany. The bird flu virus with the cryptic abbreviation H5N1 is now also our guest.
Despite all the nervousness that is now spreading in this country, chickens (and their keepers) in particular should be worried: bird flu is still an animal disease. An animal disease that can also affect humans. The World He alth Organization (WHO) is aware of 184 cases to date, 103 of which were fatal. Most recently, people in Azerbaijan and Egypt died from H5N1 infection.
Bird flu viruses are constantly changing
(Yoshihiro Kawaoka) Virus experts assume that human bird flu victims were infected through direct contact with infected animals. A transmission of the pathogen from person to person cannot be ruled out, but has not yet been proven beyond doubt.
Two working groups, the researchers led by Yoshihiro Kawaoka from the University of Wisconsin in Madison [1] and Thijs Kuiken and his colleagues from the Erasmus Medical Center in Rotterdam [2], wanted to know why H5N1 spreads so rapidly in poultry widespread, but remarkably reserved in humans. Both research groups found the answer hidden deep in the human lungs. But first things first.
Influenza viruses, including our bird flu pathogen, infect cells by docking with their surface protein haemagglutinin (this is what the "H" in H5N1 stands for) to a specific molecule on the cell membrane - sialic acid. Haemagglutinin is then split, the viral genome penetrates the cell, takes over command and forces the conquered cell to produce new pathogens. When the new viruses leave their place of birth, they can entrain sialic acid molecules from the cell surface, which would lead to clumping with the viral haemagglutinin. To prevent this reaction, which is fatal for the virus, it uses its enzyme neuraminidase (the "N" in H5N1) to cut off the sialic acid.
Sialic acid is not the same as sialic acid. Depending on how the acid is linked to the sugar galactose, there are two variants known by biochemists as SA-alpha-2,3-Gal and SA-alpha-2,6-Gal. Avian influenza viruses prefer SA-alpha 2,3-gal while human pathogens stick to the 2,6 version.
It has now been shown that in the human body, cells with SA-alpha-2, 6-gal occur primarily in the upper airways such as the nasal mucosa and the trachea. Human flu pathogens find plenty of prey here and can multiply accordingly. A runny nose and violent coughing attacks from the victim are very welcome for the virus to spread quickly.
SA-alpha-2,3-Gal, on the other hand, is mainly found in the bronchioles and alveoli of the lungs. A bird flu virus that has found its way into the depths of the lungs will feel extremely comfortable here and multiply vigorously. However, it has a harder time infecting other people because it cannot easily leave its victim's lungs again.
So as long as H5N1 doesn't change its preference for SA-alpha-2, 3-gal, there's little danger. However, it is likely to become critical if the virus also takes a liking to the sialic acid version of the upper respiratory tract. Nothing would stand in the way of a rapid spread from person to person - even a worldwide pandemic.
"No one knows if the virus will become a pandemic strain, but avian flu viruses are always evolving," warns Kawaoka. But the researcher also wants to reassure: "Certainly, the H5N1 virus must accumulate several mutations to become a pandemic strain."
