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H5N1 and H7N9 Avian Influenza A Viruses

Influenza A viruses continuously challenge the poultry industry and human health due to antigenic shift and drift. Of the 16 haemagglutinin (HA) subtypes in influenza A viruses, extensive human epidemics have occurred with only three: H1, H2, and H3. Avian Influenza infections in people are rare but possible. Avian influenza refers to diseases caused by other influenza type-A viruses, most often by the H5, H7, and H9 subtypes. A number of avian influenza virus (AIV) strains have proven fatal in a small proportion of the people they infect, most of whom have had direct contact with poultry. A zoonotic AIV of subtype H5N1 emerged in humans in Hong Kong in 1997. More recently, in February 2013, a novel H7N9 AIV emerged.

Cumulative numbers of H5N1 and H7N9 AIV genomes available in public databases. Fig.1 Cumulative numbers of H5N1 and H7N9 AIV genomes available in public databases. (Lam, 2018)

Epidemiology

AIV can infect people when enough virus gets into a person’s eyes, nose or mouth, or is inhaled. Most infected people develop a fever, cough, runny nose, loss of appetite, etc. In the worst cases, the AIV may cause pneumonia, respiratory distress syndrome, and even death. One epidemiological similarity of H5N1 and H7N9 is that are both derived from poultry or related to a poultry environment. Another similarity is their seasonal distribution. The third similarity is that both avian viruses exact a disproportionate health toll on children compared with adults.

  • Pathogenicity. Human H5N1 cases report higher severity of disease and higher levels of contact with sick or dead birds. H7N9 is asymptomatic in birds and found at lower prevalence rates in poultry. The fatality risk among hospitalized H5N1 human infections was higher than that among hospitalized H7N9 infections. Thus, H5N1 is a high-pathogenicity AIV (HPAIV) while H7N9 is classified as a low-pathogenicity AIV (LPAIV).
  • Poultry exposure history. For the H7N9 group, the transmission of H7N9 is mainly among poultry in urban settings. However, H7N9 has not been found to any significant degree in wild birds, in waterfowl, or on rural farms. For the H5N1 group, unlike H7N9, H5N1 circulates in wild birds and infects poultry in backyards and small farms in rural areas. Direct avian-to-human H5N1 virus transmission is the predominant means of human infection.
  • Transmission. Generally, direct avian-to-human H5N1 and H7N9 virus transmission is the predominant means of human infection.
  • Geographical distribution. H7N9 was the major circulating type of avian influenza in east and south China, while H5N1 was predominantly identified in Southeast Asia and northern Africa.
  • Population Distribution. In general, H7N9 was the major strain identified in the male population >60 years old, while H5N1 was most often found in young adults, without a sex bias.

Emergence and distribution of AIV in China. Fig.2 Emergence and distribution of AIV in China. (Su, 2015)

Feature of H5N1 and H7N9

  • Viral receptor. For the H5N1 virus, H5N1 prefers to bind to α-2, 3-linked sialic acid receptors located predominantly in the human lower respiratory tract. For the H7N9 virus, H7N9 viruses demonstrate an affinity to both α-2,3- and α-2,6-linked sialic acid receptors. A further animal test proved that H7N9 viruses infect both upper and lower respiratory cells.
  • Multibasic cleavage site. Although the human disease is severe, the absence of the multibasic cleavage site in the HA indicates that H7N9 would be considered an LPAIV. However, the multibasic cleavage site is seen in the HA protein of HPAIV (H5N1) and is associated with greater viral replication and systemic spread, manifesting as a much more severe disease in poultry and mammals.

Prophylaxis and Treatment

Human infections with AIV usually can be treated with the same prescription drugs that are used to treat human seasonal flu viruses. Two neuraminidase inhibitors, Tamiflu (oseltamivir phosphate) and Relenza (zanamivir) are influenza antiviral drugs used by health authorities against recently circulating influenza A viruses, including H7N9 influenza viruses. However, neuraminidase (NA) inhibitors-resistant influenza viruses are arising in clinical settings. In addition to antiviral drugs, inactivated and live attenuated vaccines are available. Vaccination is the most effective and cost-effective healthcare intervention to prevent influenza infection.

Creative Biolabs is committed to providing a full range of antibodies engineering services for our global customers. Focus on the virology area, we have the ability to provide customized anti-H5N1/H7N9 antibodies. Our experienced scientists are specialized in tailoring the most appropriate solutions according to special custom demands and expectations. Please feel free to contact us.

References

  1. Lam, T.T. and Pybus, O.G. Genomic surveillance of avian-origin influenza A viruses causing human disease. Genome Med. 2018, 10(1): 50-54.
  2. Su, S.; et al. Epidemiology, evolution, and recent outbreaks of avian influenza virus in China. J Virol. 2015, 89(17): 8671-8676.

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