Influenza
- Influenza is a virus that actually has hundreds of different strains. The virus mutates frequently, but the strains are classified into one of three main categories—A, B, or C.
- The virus can be transmitted by direct contact with infected individuals, via contaminated objects (also called fomites) and by inhalation of virus-laden aerosols.
- An unexpected emergence of a new and highly virulent influenza virus strains can result in a world-wide pandemics with high morbidity and mortality – such as the “avian flu” in 1997 and “swine flu” in 2009.
- Human influenza viruses are single-stranded RNA viruses. The main targets of the virus are the columnar epithelial cells of the respiratory tract, i.e. trachea, bronchi and bronchioles.
- Infectivity of influenza virus particles depends on the pH, temperature and salinity of the water, as well as the UV irradiation.
Influenza types
- Influenza is caused by three types of RNA viruses called influenza types A, B and C (considered different genera), which all belong to the family Orthomyxoviridae.
- The disease, colloquially called “flu” in humans, is generally caused by the viruses A and B.
- Subtypes of influenza A and B viruses can be further characterized into strains. There is a plethora of different strains of influenza B viruses and of influenza A subtypes, and new strains of influenza viruses can appear and replace older strains.
- Influenza type A viruses are known to infect people, birds, pigs, horses, whales, seals and other animals, but wild birds represent the natural hosts for these viruses.
- Only a fraction influenza A subtypes (i.e. H1N1, H1N2 and H3N2) are currently in general circulation among people.
- Influenza B viruses are responsible the same spectrum of disease as influenza A. And, influenza B viruses do not cause pandemics.
- Influenza C viruses are different in comparison to influenza A and B. They cause a mild respiratory illness and are not thought to cause epidemics.
Influenza A
- Influenza A is the group that most commonly causes illness in humans.
- All influenza A viruses are further broken down into H and N subtypes. So, any influenza virus that is described as “H#N#” (such as H1N1) is influenza A virus.
- There are 16 H subtypes and nine N subtypes, but only three combinations have actually caused highly contagious illness in humans.
- Other combinations have been found to infect other species (such as birds and pigs), but they have not caused widespread human infections.
- The three combinations that cause almost all outbreaks of the flu in humans are H1N1, H2N2 and H3N2.
- Even in these subtypes, the influenza virus can mutate and change each year. For this reason, influenza viruses are also named using:
- The host of origin (swine, chicken, etc., or no host if it is of human origin)
- The geographical location of origin (Hong Kong, Alberta, etc.)
- Strain number
- Year of discovery (or isolation)
Influenza B
- Influenza B is less common but still causes outbreaks of seasonal flu.
- One or two strains of influenza B are included in the seasonal flu vaccine every year to protect people from the strain(s) that researchers believe are most likely to cause illness during the upcoming flu season.
- The quadrivalent flu vaccine contains two strains of influenza B but the traditional trivalent flu vaccine only contains one.
- Influenza B is not broken down into subtypes like influenza A is, but it is broken down into individual strains.
- Typically, two strains of influenza A and one strain of influenza B are included in the seasonal flu vaccine. Quadrivalent flu vaccines contain two strains of influenza A and two strains of influenza B.
- Influenza B can cause outbreaks of seasonal flu but they occur less frequently than outbreaks of influenza A.