Influenza A Virus
Family: Orthomyxoviridae   Genus: Influenzavirus
Influenza – known as the "flu" - is one of the oldest and most common diseases known to man.
It can also be one of the deadliest.

        Influenza A is responsible for epidemics of respiratory illness that occur almost every winter.  One or two strains seem to dominate at any given season.  Influenza outbreaks can range from worldwide pandemics to more frequent epidemics. Influenza type A viruses undergo two kinds of changes: antigenic drift which is a series of mutations that occur over time and cause a gradual evolution of the virus.   Antigenic shift results is an abrupt change in hemagglutinin or neuraminidase proteins making a new subtype of the virus. Type A viruses are divided into subtypes based on differences in hemagglutinin (H) and the neuraminidase (N). The current subtypes of influenza A are designated A(H1N1) and A(H3N2).

        Influenza A infections are characterized by abrupt onset of fever, sore throat, dry coughing, mylagia, headache, malaise, extreme fatigue, and muscle soreness.   Most deaths caused by influenza are among the elderly and immunodeficient and are the result of secondary infections such as bacterial pneumonia.  The incubation period for Influenza A is about 1-4 days, and the syndrome usually lasts for about 3-7 days. Influenza A is transmitted via the respiratory route.

        If an individual has been infected with a similar strain within the past few years, then antibodies might neutralize the infecting virions.  Anti HA antibodies such as IgA and IgG have been shown inhibit uncoating of the virus.  However, if preexisting antibodies do not protect against the infection, activated macrophages and natural killer cells help recover from the infection.
        A polyvalent inactivated vaccine is developed each year depending on the most common strains of influenza present during the last season.  It is not cost effective and necessary to immunize the entire community, so only high risk cohorts (such as elderly, people with chronic debilitating diseases, and the immunodeficient) are usually vaccinated.  Common side effects from the vaccine include local tenderness, and less commonly fever and malaise for a day.  Efficacy is highest in the young and lowest in the old due to the idea of original antigenic sin.  The vaccine is grown in  embryonated hen’s eggs so those with egg allergies have contraindications to the vaccine; therefore a search for another effective vaccine is being researched, such as a live attenuated vaccine.

Game action:  Oh no!  An influenza epidemic…. If you got your annual influenza vaccine, then you win. If not, then you’ll be sick in bed this week!!!

Medical Virolgy by White and Fenner (1994) p 494-499
Principles of Virology by Flint, Enquist, Krug, Racaniello, and Skalka (2000) p 760