The Molecular Mechanisms that Allow Avian Influenza Viruses to Infect Humans
Kyoko Shinya, Kobe University
Influenza A viruses possess 8 segmented RNAs as their genome and are classified into subtypes based on the antigenicity of their surface molecules, hemagglutinin (HA) protein and neuraminidase (NA) proteins; currently, 16 HA subtypes (H1 – H16) and 9 NA (N1 – N9) subtypes are known. Utilizing synthetic pathway of living cells, they make copies of their segmented genomes and produce their progenies. Therefore they need serial infection to susceptive new host for maintaining their life cycles. In the case of influenza A viruses, aquatic birds, like ducks, play the role of maintaining hosts. Actually it is well known that these wild ducks possess numerous viral genes pool. Although they are susceptive for influenza A virus infection, they are usually asymptomatic. It means that ducks coevolute with influenza A viruses and are “natural host” of influenza A viruses.
2. Influenza virus as a pathogen
Although influenza A viruses are non-pathogenic to aquatic birds, they show significant pathogenisity after transmission to the other animal species, including chickens and mammalian. According to massive progenies production in the new hosts, apparent tissue damages and following host responses cause disease symptoms. Most of infections to the other animals are limited to respiratory organs, except for systemic infection of highly pathogenic avian influenza viruses to chickens.
3. Highly pathogenic avian influenza virus infection to humans
Influenza A viruses cause respiratory illness to the humans. Typically, annual human influenza virus infections are characterized by upper respiratory symptoms such as cough and sneezing with generalized fatigue and high fever. By contrast, human infections with highly pathogenic avian influenza viruses result in lower respiratory symptoms, in addition to upper respiratory symptoms. Several findings indicate that highly pathogenic avian influenza viruses grow efficiently in the lower airways of infected individuals
4. Adaptation of avian derived influenza viruses to humans
As described above, all influenza A viruses are believed to originate from aquatic birds. Therefore for efficient growth in the new hosts, it is necessary to pursue adaptation process for the avian derived viruses. During the recent outbreaks of highly pathogenic avian influenza viruses’ infection, we already catch the indications of human adaptation processes. They are actually changing their properties during infection to humans.
5. Properties of recent H5N1 viruses
Recent H5N1 viruses are killing their natural hosts, aquatic birds. It means that recent viruses, which are world wide spreading, are not symbiotic organisms for aquatic birds anymore. It could say that recent H5N1 viruses are “emerging infectious disease” for both of humans and aquatic birds.
“How to prevent new pandemics of influenza viruses?” is one of the big concerns to our research area. In addition to the ordinary preventives and therapeutics, we may need to develop completely new approach to this kind of infectious diseases.
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