Much of the important influenza information is best-understood if you are familiar with some of the key medical terminology related to it. Some of the terms pertain to the flu virus itself. Others pertain to the illness, including its spread, consequences, treatment and prevention. Many of the terms relate to microbiology and infectious diseases in general. But they are pertinent to influenza as well.
Antigenic drift – Is the process of gradual relatively minor changes that occur in the HA and/or NA surface proteins of an influenza virus over time. It is the result of point mutations that occur during the copying of genes as a strain of a virus replicates. Because the changes are gradual and cumulative they can allow a virus to drift from an initial appearance years previously and continue to cause infection several years later despite a population’s immune response to the prior strains. The reason for lost immunity against a mutant strain of a virus is the antibodies the immune system produced against the prior strain(s) no longer recognize the HA and NA antigens on the surface of the virus because of subtle changes in one or both proteins. This process can also account for the recurrence of disease by a subtype of the influenza A virus which had stopped causing disease but resumed because of the emergence of a new strain. Antigenic drift is responsible for seasonal outbreaks and epidemics of flu. It is also the reason for the need to update the flu vaccine seasonally.
Antigenic shift – Is the sudden and major change in antigens on the surface of a subtype of influenza A virus, due to the re-assortment of genes between two or more different virus strains or virus subtypes infecting the same cell of a host organism. For example, pigs are susceptible to infection by pig, human, and bird influenza viruses. Therefore, concurrent pig infection by all three types of viruses is possible. During such a state of mixed infection gene scrambling can occur and give rise to a new virus. If the new virus is capable of infecting humans it can then jump animal species, so to speak, from pigs to humans. Such a jump, along with the unique gene pool of the new virus, is the basis of a pandemic.
The classic model of antigenic shift involving the influenza A virus is the generation of a new virus subtype with a never-before-detected combination of HA and NA proteins on its surface. But in the case of the swine flu pandemic of 2009-2010 the hybrid virus was of the same subtype, but it was a novel virus with an HA or HA and NA combination which emerged that was so different from the same subtype in humans that most people didn’t have immunity against it.
The three pandemics prior to the swine flu one followed the classic model of the generation of a new influenza A virus subtype. Those pandemics were the following:Antigenic shift is best-known to occur with the influenza A virus but, also occurs with some other viruses. It does not occur with the other types of influenza viruses though because the type A virus is the only one that doesn’t infect just humans.
Centers for Disease Control (CDC) – Is the agency of the U.S. government which tracks and investigates public health trends. Its stated mission is “to promote health and quality of life by preventing and controlling disease, injury and disability.”
Epidemic – Is the unusually high incidence of a disease in an area or population. It differs from a pandemic in that even though the disease outbreak is greater than expected based on past experience, it is limited to a particular region. Additionally, the CDC relates it to the number of deaths per week it causes.
Flu-like symptoms – Are many of the symptoms typically seen with the flu that are present with conditions other than the flu. Some of those conditions are the result of infection caused by pathogens other than the flu virus. Others are conditions not caused by infection at all.
Hemagglutinin (HA) – Is the protein on the surface of the influenza virus that plays an important role in its attachment to and entry into a host cell. It is the main immunogen in the influenza vaccine.
Neuraminidase (NA) – Is the protein on the surface of the influenza virus which contains an enzyme that cleaves newly formed viruses from the host cell. That action promotes their release to infect other epithelial cells of the respiratory tract.
Neuraminidase inhibitor – Is an FDA approved class of antiviral drug used to prevent and treat uncomplicated influenza.
Recombination (Re-assortment) – Is the exchange of genes between chromosomes resulting in rearrangement and new combinations of genes. If the gene exchange involves different strains or subtypes of organisms the offspring will have some unique chromosomes different from their parents.
Natural reservoir – Is an organism in which a virus or other microorganism normally lives and multiplies. It does not experience infection or injury. But it is a source of infection of a different organism(s).
Strain – Is a more specific type or subtype of a virus which takes into account any gene mutation which has occurred since initial identification of the virus. For the influenza A virus it is a subcategory of a subtype. With respect to the influenza A virus, the term strain is not a strict synonym of subtype, even though some confusingly use it as such. The reason is a change in the strain of a virus results from mutation, whereas a change in the subtype of the influenza A virus is due to antigenic shift.