H1N1 Influenza A: Human, Avian & Swine Genes
How the Swine Flu Virus Formed with Bird, Pig and Human RNA
Jul 24, 2009
Influenza viruses are in the viral class called Orthomyxoviridae. Influenza viruses are made of eight strands of separate coding material called ribonucleic acid or RNA. Theses viruses are further divided into A, B, and C virus types. Each of these types have subtypes, defined by two proteins that are present on the surface of the virus cells. The different subtypes of hemagglutinin ( H) and neuraminidase (N) proteins define the exact subtype of influenza virus. In influenza A viruses, 15 different subtypes of H proteins and 9 subtypes of N proteins exist. Birds, especially waterfowl, pigs and humans are all susceptible to infection with influenza viruses, but there are certain peculiarities of infection in each species.
Influenza A Viruses that Infect Humans
The most common subtypes that cause infections in humans include H1N1, H1N2, and H3N2. The most common virus subtypes in swine are H1N1, H1N2 and H3N2. By contrast, almost all of the different H and N subtypes exist in bird species. In wild waterfowl such as ducks, influenza viruses infect the gastrointestinal tract rather than the respiratory tract that is most commonly involved in pigs and humans. Influenza infections can cause significant illness in pigs and humans, but generally do not cause significant symptoms in birds. Influenza virus particles are shed in bird droppings in lakes and other water areas inhabited by migratory birds. Waterfowl can also contaminate farm fields with influenza virus particles.
How Novel Influenza Strains Develop with Bird, Swine and Human Genetic Components
When two different influenza A strains infect the same cell of a human, bird or pig, the viruses can exchange genes and create a new viral strain. The hemagglutinin protein is especially important because antibody production to this protein is required to provide immunity.
Antigenic shift is the term used to describe the process by which new H and N proteins form by reassortment of different strands of RNA in cells co-infected by multiple virus types. When a new gene rearrangement such as the novel influenza A H1N1 strain occurs, humans lack immunity and a pandemic can follow.
Pigs Serve as Vehicle through which Bird Influenza Strains can Infect Humans
With a few notable exceptions, avian (bird) origin influenza A viruses do not spread to humans, because these specific strains preferentially use receptors on bird cells to reproduce and human influenza A viruses use unique receptors on human cells to reproduce. However, pigs have swine, avian and human receptors on the surface of their cells. When pigs come in contact with humans and birds at the same time, avian and human influenza strains of viruses can infect pig respiratory cells at the same time. Therefore they can serve as an intermediary for reassortment of avian, human and swine strains of influenza A viruses. The viral strains may adapt to survive in humans. Then the infection spreads by human to human contact through respiratory droplets. Eating pork will not spread influenza A H1N1. It makes sense to keep wild waterfowl (and farm raised turkeys, which can sometimes be infected with avian influenza A viruses), pigs and humans protected from infection when they come in contact together.
The current H1N1 strain is a novel combination of bird, swine and human genetic components. It is NOT the same as other H1N1 swine flu strains as initially believed. For this reason, people do not have natural immunity to the virus, and vaccines to the usual seasonal influenza viruses will also not confer immunity. Several manufacturers are developing vaccines to the unique H1 component of this novel influenza virus strain.
Influenza A H1N1 Pandemic Stage 6 by World Health OrganizationIn June of 2009, the World Health Organization declared pandemic stage 6, which means the H1N1 virus has caused infections in multiple continents around the world. The threat to humans of this virus is felt to be moderate, on the same order as most seasonal influenza virus infections. One peculiarity of this virus is that most infections and deaths have been in young, healthy or pregnant patients, or patients with certain chronic medical conditions rather than the elderly. As always, frequent handwashing and staying away from people ill with fever and upper respiratory symptoms is the best way to avoid infection with this and other respiratory viruses.
Legal Disclaimer:
This article is for informational purposes only and should not substitute for medical advice from your health care provider. The author is not providing personal medical opinion, diagnosis or course of treatment. Do not delay or substitute this information for medical treatment.
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