The first thing in the need to know about influenza is the terminology… and we've come to recognize quite a bit of terminology surrounding the flu. Seasonal flu (sometimes called the common flu) is exactly what it sounds like; that strain of flu that circulates a given area every year. Avian flu (highly pathologic avian influenza) is the name given to a strain of flu that mainly circulates in Asia impacting various bird species with limited transmission to humans. Swine flu on the other hand, is the name given to a strain of influenza that emerged from South America–Mexico–in late 2008. This strain of influenza was particularly troublesome because it seemed to impact otherwise healthy people in a very dramatic way. And lastly, the term pandemic. A pandemic has been seen by the media as a term that indicates large numbers of deaths from disease. Although throughout history this is often the case, a pandemic is not an automatic term for mass fatalities. The term pandemic simply means the disease has spread around the globe and impacted many areas of population.
2. Types of Influenza.types of influenza viruses… so more concerned about, others, not so much. Influenza virus belongs to the category of diseases known as Orthomyxoviruses. The three types of flu are Type A, Type B, and Type C. Type A influenza is known as a multi-host pathogen infecting both humans, swine, and birds. This is the most virulent group and is classified by its surface antigens into subtypes. It is these subtypes that make up the H and N that we hear so much about on the news. H stands for hemagglutinin and N indicates neurominidase. Both of these are surface proteins on the virus that allow the virus to get into a host cell, reproduce, and then escape. Remember, viruses are parasites and need to have a host to survive. There are 15 different types of H's and nine types of N's giving us a total of 135 potential combinations of type A influenza. Type B influenza is seen mostly in humans and although it's very common it is much less severe than Type A influenza. Epidemics involving type B influenza occur much less often than those involving Type A. It's important to note here that human seasonal flu vaccine includes two strains of Type a and one strain of Type B protection. Given that there are 135 potential type a influenza combinations and only two are included in the seasonal flu vaccine, indicates why we have years when the seasonal flu vaccine is less effective than others… that is, scientists have to guess which two strains of influenza should be included in the vaccine. Type C influenza infects humans and swine and has a completely different pattern of surface proteins. Normally Type C presents with rare occurrences and has mild or no symptoms. In fact, by age 15 most people have antibodies against Type C influenza.
Prevention of transmission of flu sometimes takes on a life of its own. We need to remember that the flu virus is one of the most infectious pathogens we know of and that type a influenza is prone to subtle changes in its structure that make it a challenge to our immune systems year after year. It's also important to remember that droplets aerosols and direct contact can spread influenza. The flu virus can remain active on a contaminated surface or item for up to 48 hours.
We'll discuss prevention strategies, P. P. E., and pharmacology versus non-pharmacology strategies in our Medical/Biological posting next week.