Avian Influenza: History and Current Risk

April 1, 2024
Avian Influenza

Avian influenza (“bird flu,” informally) is a disease caused by the Type A influenza virus, which has adapted to target birds but can also infect other animals, including humans. Commonly transmitted through the eyes, respiratory tract, and gastrointestinal tract, the virus can pose problems to wild birds, poultry, and people when outbreaks occur. With several recorded outbreaks in recent history, avian influenza continues to be a salient public health risk.

The first recorded occurrence of avian influenza A virus in humans took place in Hong Kong in 1997, though its presence in birds has a much longer history. In 2003, a major outbreak of extremely contagious avian influenza H5N1 impacted poultry in 8 Asian countries. The infection resulted in a high death rate among humans who were infected, with the primary symptoms being conjunctivitis, fever, pneumonitis, lymphopenia, and diarrhea. After this outbreak, by March 2004, Cambodia, Indonesia, Japan, Laos, China, Thailand, South Korea, and Vietnam slaughtered over 100 million domestic poultry to stop the spread of disease. To efficiently manage the disease, it is critical to measure risk factors that contribute to the spread of avian influenza that exist in commercial supply trains, transportation networks, and live poultry markets (which are hotspots for outbreaks).1,4

To survive and fight antiviral treatments, viruses undergo antigenic changes to fend off the host’s immune response. These occur due to genetic drift caused by point mutations, mainly in the hemagglutinin (HA) gene, or by genetic shift caused by the interchange of the HA ligand with viruses found in avian species. The risk to humans changes as new strains of avian influenza evolve and circulate, as evidenced by the different outbreaks in recent history.1,2,3 Antiviral resistance is an additional element that complicates treatment and containment of the virus. A significant number of H5N1 virus strains present in southeast Asia exhibit genetic resistance to adamantane antivirals such as amantadine and rimantadine. Additionally, there have been documented cases of H5N1 viruses in Vietnam and Egypt that are genetically resistant to oseltamivir.1,4,5

Geographic variation in the predominant strains poses an additional risk factor for managing avian influenza. Asia is home to the H5N1, H5N2, and H9N2 strains, while Africa has two strains, namely, H5N1 and H10N7. The H7N7, H7N3, and H7N2 strains dominate Europe, while North America has reported cases of H7N3, H7N2, and H11N9 strains. Although most strains of the virus are transmitted directly from birds to humans, the highly virulent H5N1 and H7N7 viruses can transmit from person to person. Due to H5N1’s high virulence, researchers predict that it has pandemic potential and could cause the death of 180-360 million people worldwide. In fact, the 1918 Spanish Flu pandemic was triggered by an H1N1 influenza virus with avian origins.5

Avian vaccines are the most effective measure for reducing risk sustainably for both birds and humans. Inactivated virus preparations of vaccines are preferred to the attenuated live influenza vaccine due to the virus’s potential for reassortment and mutations. Chickens that were initially vaccinated with an inactivated LPAIV H7N1 either at three days or three weeks of age with an additional dose at four weeks did not develop the disease. However, administering a plasmid carrying the NP gene from an H5N1 virus did not protect poultry from the H7N7 virus. Because of the high likelihood of reassortment and mutations, a single vaccine that is effective against all strains continues to elude researchers.6 Continued research and vigilance is necessary to reduce the risk of fatal outbreaks of avian influenza and, hopefully, confine them to history.

References

  1. Trampuž, Andrej, et al. “Avian Influenza: A New Pandemic Threat?” Mayo Clinic Proceedings, vol. 79, no. 4, Apr. 2004, pp. 523–30, doi:10.4065/79.4.523.
  2. Zambon, Maria. “The Pathogenesis of Influenza in Humans.” Reviews in Medical Virology, vol. 11, no. 4, July 2001, pp. 227–41, doi:10.1002/rmv.319.
  3. Fouchier, Ron a. M., et al. “Avian Influenza a Virus (H7N7) Associated With Human Conjunctivitis and a Fatal Case of Acute Respiratory Distress Syndrome.” Proceedings of the National Academy of Sciences of the United States of America, vol. 101, no. 5, Jan. 2004, pp. 1356–61, doi:10.1073/pnas.0308352100.
  4. Bridges, Carolyn B., et al. “Risk of Influenza a (H5N1) Infection Among Poultry Workers, Hong Kong, 1997–1998.” The Journal of Infectious Diseases, vol. 185, no. 8, Apr. 2002, pp. 1005–10, doi:10.1086/340044.
  5. Dudley, Joseph P. “Public Health and Epidemiological Considerations for Avian Influenza Risk Mapping and Risk Assessment.” Ecology and Society, vol. 13, no. 2, Jan. 2008, doi:10.5751/es-02548-130221.
  6. Mostafa, Ahmed, et al. “Prevalence and Control of H7 Avian Influenza Viruses in Birds and Humans.” Epidemiology and Infection, vol. 142, no. 5, Jan. 2014, pp. 896–920, doi:10.1017/s0950268813003324.

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