Influenza

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Influenza is one of the most important viral diseases and has plagued humans throughout recorded history. Despite vigorous and ongoing efforts to reduce the burden of influenza through vaccination, the disease causes about 35,000 deaths yearly in the United States and between 250,000 and 500,000 deaths worldwide. Most of the mortality attributed to the virus is actually due to bacterial pneumonia that attacks the lungs after the influenza virus damages the lungs.

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Colorized electron micrograph of influenza viruses (seen in gold) Source:CDC

Contents

Other Names

Influenza is commonly known as the flu.

Types

The influenza viruses are complex and change from year to year in response to mutations and evolutionary pressure.[1] Three major types of virus (A, B, and C) are known, only two of which (A and B) cause significant disease in humans; other types of influenza virus are known to infect domestic animals such as birds and pigs. Individual strains of the virus are named according to where they first cause outbreaks of the disease, such as New Jersey (A), Bangkok (A), and so on.

Both influenza A and B cause seasonal epidemics around the world. Due to the extremely unstable genome (DNA structure) of the virus, hundreds of different variations of the virus may be infecting humans at any time, which greatly complicates efforts to develop a vaccine or predict especially dangerous outbreaks. Of the two types, influenza B changes more slowly and causes disease which is milder than that caused by influenza A. Influenza B is much more dangerous in people with weakened immune systems or with ongoing significant illness. Influenza A is usually the virus behind yearly epidemics of the flu.

Signs and Symptoms

A typical case of influenza starts with high fever, chills, muscle aches, headaches, a nonproductive cough, and/or a distinct feeling of unwellness. These symptoms are soon followed by worsening upper respiratory symptoms [nasal congestion, runny nose (rhinorrhea)], sore throat, and/or drenching sweats. Patients may feel profoundly ill and weak. In most individuals, the illness lasts for about a week and is followed by complete recovery, with most people thinking that they have suffered from an especially bad "cold" (a different type of viral syndrome). Due to variations in patients' response to the disease and because of the large number of influenza subtypes, the course of the disease is unpredictable and, therefore, potentially lethal, even in otherwise healthy adults and children.

In this video, Dr. Joe Bresee of the CDC describes the main symptoms of flu, including the new H1N1 flu, and when it is serious enough to seek medical help.

Video at YouTube

Causes

Influenza is caused by infection with one or more types of influenza virus.

Diagnosis

Diagnosis is most often made by rinsing the nasopharynx and analyzing it to detect the presence of the influenza virus. Influenza can also be detected in an individual with blood tests, though this is less common.

Treatment

No cure is currently available once influenza has been contracted. However, there are several antiviral drugs that are useful for minimizing the symptoms and duration of the influenza infection.[2]

In this video, Dr. Joe Bresee of the CDC explains the nature of antiviral drugs and how they are used.

Video at YouTube


One group of antivirals is the neuraminidase inhibitors such as zanamivir and oseltamivir (Tamiflu). They are effective against both influenza A and B.

The second group of antivirals consists of amantadine and rimantadine. These are only effective against influenza A.

Both classes of antivirals must be given within 48 hours of onset of symptoms to be effective. It is also possible for the particular strain of influenza contracted to be resistant to the antivirals.

Other supportive measures such as fever reduction, hydration, and rest are very helpful in reducing symptom severity and duration.

Prevention

Routine vaccination and diligent surveillance efforts are the most important way to control the disease and limit the spread of outbreaks. Since the viruses are constantly evolving, decisions must be made every year by public health authorities regarding the viral strains that vaccines will be targeted against. Even though vaccines are specific for individual strains of the virus, there is evidence that people in high-risk groups who get yearly flu shots tend to have milder illnesses when new strains emerge.

The virus can reside in domesic animals and cause disease in humans, and animal-to-human transmission is facilitated by close interactions with animals. Prevention efforts should therefore include good animal husbandry practices. Extensive exposure to domestic birds was one of the factors that contributed to the avian influenza H5N1 outbreak in Asia in 2006. The H5N1 strain of influenza A is especially dangerous, with mortality rates in excess of 50%.[3]

How Influenza is Spread

The disease is highly contagious and primarily spreads from person to person in droplets of liquid (saliva or sputum) expelled through coughing or sneezing. The virus can also contaminate surfaces that are later touched by uninfected individuals, who may then touch their own mouth or nose to initiate another infection. Since people become contagious before they begin to feel ill, the virus can spread very effectively and affect high proportions of people who visit shared areas such as classrooms or office buildings.

In this video, Dr. Joe Bresee of the CDC describes how to keep from getting the flu and spreading it to others by taking these three steps.

Video at YouTube

Research

Expected Outcome

History

Chart showing excess mortality caused by the 1918 pandemic. Source: National Museum of Health and Medicine (NMHM)
Emergency hospital during influenza epidemic, Camp Funston, Kansas Source: NMHM
Influenza surely predates recorded history. One of the earliest recorded plagues (430 BC in Athens) may have been caused by a particularly virulent strain, and the virus may have also had a role in destroying the army of Charlemagne in AD 876. Major pandemics occurred in 1627, 1729, 1788, 1830, 1847, 1872, 1890, 1918 (see below), 1957, and 1968. An epidemic in the American colonies in 1732 was reported by the English doctor John Huxham, who linked the disease with an old Italian folk word that associated colds, coughs, and fever to the "influence" of the stars—hence the name "influenza".

Within the space of a few months during 1918 and 1919, a dramatic influenza pandemic swept the world. It is commonly referred to as the "Spanish" flu pandemic, although the key mutations leading to the especially virulent responsible strain may have originated elsewhere.[4] The pandemic killed about 700,000 people in the US and about 40 million people worldwide; this total exceeded the death toll of World War I by about 10 million and amounted to about 2% of the world's population at the time. Most of the deaths were due to bacterial pneumonia that took advantage of the sublethal damage caused by the virus.[5]

Pandemics in 1957 and 1968 also caused tens of thousands of deaths in the US. In both instances, the responsible virus strains probably originated as reassortants, in which one or both human-adapted viral surface proteins were replaced by proteins from avian influenza virus strains.[6]

Epidemiology

Related Videos

Preventing Pandemics Through Healthcare Reform

Jeffrey Koplan, former head of the CDC and Vice President for Global Health at Emory University, discusses the vital importance of a healthcare reform that provides some level of universal coverage to people and encourages them to come in when theyre sick at early stages in preventing future pandemics:
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The Origins of Swine Flu

Michael Worobey, Professor of Evolutionary Biology at the University of Arizona, uncovers the origins of the current H1N1 virus and how it rested latent within pigs for up to ten years prior to 2009, and how it transfers between species:


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Preparing for Swine Flu

According to the World Health Organization, The H1N1 Virus is estimated to affect 2 billion people (1/3 of the worlds population) over the next two years. Former head of the CDC, Jeff Koplan discusses the difficulties in preparing for this.


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Personal Stories: Why Flu Vaccination Matters

In this video, moving personal stories help inform parents about the dangers of flu to children and the benefits of vaccination.

Video at YouTube

References

  1. Hampson AW, Mackenzie JS. The influenza viruses. Med J Aust. 2006 Nov 20;185(10 Suppl):S39-43. Abstract | Full Text | PDF
  2. Wright P. Influenza viruses. In: Kliegman RM, Behrman RE, Jenson HB. Nelson Textbook of Pediatrics. 18th ed. Philadelphia PA: W.B. Saunders; 2007:1384-1387.
  3. Influenza team. Highly pathogenic avian influenza A/H5N1-update and overview of 2006. Euro Surveill. 2006 Dec 21;11(12):E061221.1. Citation | Full Text
  4. Oxford JS. The so-called Great Spanish Influenza Pandemic of 1918 may have originated in France in 1916. Philos Trans R Soc Lond B Biol Sci. 2001 Dec 29;356(1416):1857-9. Abstract
  5. Brundage JF, Shanks GD. Deaths from bacterial pneumonia during 1918-19 influenza pandemic. Emerg Infect Dis. 2008 Aug;14(8):1193-9. Abstract | Full Text | PDF
  6. Reid AH, Taubenberger JK. The origin of the 1918 pandemic influenza virus: a continuing enigma. J Gen Virol. 2003 Sep;84(Pt 9):2285-92. Abstract | Full Text | PDF

External Links

  • A free, 225-page downloadable textbook "Influenza Report 2006" is available through FlyingPublisher.
Medpedia-logo.gif The basis of this article is contributed from Medpedia.com These articles are licensed under the GNU Free Documentation License It may have since been edited beyond all recognition. But we thank Medpedia for allowing its use.
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