Essay Preview: Influenza Case
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Influenza has been traced back to the beginning of the Middle Ages, occurring periodically and varying in severity in countries around the world (Hirsch, 1883). Influenza can occur in two ways: as a “seasonal” epidemic or as a pandemic. Seasonal influenza occurs in the winter months, spreading from the Southern to the Northern Hemispheres and displaying changes in the virus from previous strains (Hope-Simpson, 1981). Pandemic influenza occurs when an influenza outbreak spreads throughout the world and is caused by a novel Influenza A subtype (Monto et al., 2006). Influenza has become a prominent emerging infectious disease through its evolution over the years, by virus mutations and new subtype formations. Its spread is also aided through cross-species transmission, through such species as birds, pigs and humans (Webster, 2002). The similarities and differences between seasonal and pandemic influenza will be examined, as well as the roles that evolution and zoonotic transfer play in their emergence.
The influenza virus is made up of three distinct classes: Influenza A, Influenza B, and Influenza C. Influenza A is subtyped based on the type of surface antigens, the proteins hemagglutinin (HA) and neuraminidase (NA). HA and NA are involved in the attachment and fusion to infected cells and are the target of the immune system (Hunt, 2007). The Influenza A subtype is also classified into strains based on the external antigens (Hunt, 2007). Influenza A is the subtype that can mutate to produce an HA with no pre-existing immunity and is subsequently the focus for pandemics. (Monto et al., 2006) Influenza B and C viruses are predominantly found in humans and are less severe than Influenza A (Hunt, 2007).
Seasonal influenza is responsible for over 365,000 hospitalizations and 51,000 deaths in the United States per year (Thompson et al., 2003; Thompson et al., 2004). When an influenza epidemic evolves into a pandemic, the number of hospitalizations and deaths would subsequently increase. It is estimated that a pandemic could cause up to 314,000 to 734,000 hospitalizations and up to 89,000 to 207,000 deaths (Meltzer et al., 1999). Both seasonal and pandemic influenza are subject to two variations, antigenic drift and shift; these contribute to the evolution of the viruses throughout the years. It is this reason that influenza has been able to occur consistently without the population having immunity. Influenza is a virus that infects multiple species; cross-species transmission allows for this and the evolution of the virus itself is thought to occur within an intermediary species. This is considered to be the swine, since they serve as hosts for infections for avian and human influenza A viruses and are thought to be directly involved in interspecies transmissions (Brown, 2000).
“Seasonal” influenza is a virus that occurs annually in many countries; it can spread easily from person to person contact (WHO Fact Sheet, 2009). These epidemics occur on a seasonal basis during the winter in temperate climates with low temperature and humidity, moving from the Southern to the Northern Hemisphere as their colder seasons change (Hope-Simpson, 1981). Seasonality of the virus can be explained through statistically significant correlations between epidemic cycles and changes in humidity (Makoto, 1999).
Another characteristic of seasonal influenza is that its surface antigens, HA and NA, undergo stepwise mutation, allowing for enough changes in the virus so that the proteins become so different that the existing antibodies can no longer attack the virus (Zambon, 1999). New virus strains are produced as a consequence and the influenza virus can reappear seasonally, with no immune response against these new strains (Gerdil, 2003). The World Health Organization (WHO) has an international surveillance network for influenza, which recommends a seasonal vaccine composition per hemisphere, twice a year, to accommodate for annual adaptation (WHO Recommendations, 2009).
Seasonal influenza can be managed through pharmaceutical and nonpharmaceutical interventions. Inactivated vaccines have demonstrated efficacy against preventing the influenza virus, for subtypes A and B, though it will change year to year due to antigenic drift (Ohmit et al., 2008). Influenza can be treated with antiviral drugs, if necessary. Zanamivir, a neuraminidase inhibitor, was demonstrated to be safe and effective in reducing symptoms if started early (Hayden et al., 1997). Another neuraminidase inhibitor, oseltamivir, was also found to be effective in reducing the duration and severity of influenza (Treanor et al., 2000). Nonpharmaceutical measures to prevent influenza transmission include covering the nose and mouth when sneezing, washing hands, refraining from face touching and avoiding contact with those who are sick (CDC Preventing Flu, 2009).
Pandemic influenza occurs when an influenza outbreak spreads easily person to person throughout the world, is caused by a new Influenza A subtype and brings about serious illness (Monto et al., 2006). This is different from seasonal influenza, since pandemic influenza is more severe and requires a novel subtype, rather than just a mutated form of a previous strain. Pandemic influenza has varied over time by Influenza A subtype; the 1918 “Spanish flu” pandemic was of subtype H1N1, derived from a reassortment of viruses (Reid et al., 1999), the 1957 “Asian flu” pandemic was of subtype H2N2, later evolving to the “Hong Kong flu,” H3N2, in 1968 (Smith et al., 2009), and most recently, the 2009 “swine flu”