Molecular Expressions Cell Biology: The Influenza (Flu) Virus

Table of Contents
The Influenza (Flu) Virus

Next to the common cold, influenza or "the flu" is perhaps the most familiar respiratory infection in the world. In the United States alone, approximately 25 to 50 million people contract influenza each year. The symptoms of the flu are similar to those of the common cold, but tend to be more severe. Fever, headache, fatigue, muscle weakness and pain, sore throat, dry cough, and a runny or stuffy nose are common and may develop rapidly. Gastrointestinal symptoms associated with influenza are sometimes experienced by children, but for most adults, illnesses that manifest in diarrhea, nausea, and vomiting are not caused by the influenza virus though they are often inaccurately referred to as the "stomach flu." A number of complications, such as the onset of bronchitis and pneumonia, can also occur in association with influenza and are especially common among the elderly, young children, and anyone with a suppressed immune system.

Influenza is highly contagious and is more common during the colder months of the year. Contrary to traditional belief, however, the climate itself is not directly to blame for the increase in incidence, but rather is attributable to the greater amount of time spent indoors in close proximity to other individuals during inclement weather. The influenza virus is chiefly transmitted through airborne respiratory secretions released when an infected individual coughs or sneezes. Incubation typically is from one to two days from the time of infection, and most people begin to naturally recover from symptoms within a week. The vast majority of influenza-related deaths are caused by complications of the flu rather than the actual influenza virus.

Three distinct types of influenza virus, dubbed

The structure of the influenza virus (see Figure 1) is somewhat variable, but the virion particles are usually spherical or ovoid in shape and 80 to 120 nanometers in diameter. Sometimes filamentous forms of the virus occur as well, and are more common among some influenza strains than others. The influenza virion is an enveloped virus that derives its lipid bilayer from the plasma membrane of a host cell. Two different varieties of glycoprotein spike are embedded in the envelope. Approximately 80 percent of the spikes are

Mutations in the antigenic structure of the influenza virus have resulted in a number of different influenza subtypes and strains. Specific varieties of the virus are generally named according to the particular antigenic determinants of hemagglutinin (13 major types) and neuraminidase (9 major types) surface proteins they possess, as in influenza

Influenza A also experiences another type of mutation called

In addition to vaccines, a few other weapons have been designed to combat the flu. The antiviral medications amantadine and rimantadine can help reduce severity of illness in individuals with influenza that begin utilizing the drugs within two days of the onset of symptoms. These drugs work by hindering the change in pH that is necessary for the flu virion to release its contents into the cytosol of a host cell. Two additional antiviral drugs, zanamavir and oseltamivir, are effective against both A and B types of influenza. Instead of interfering with pH shifts, zanamavir and oseltamivir block the glycoprotein neuraminidase so that the release of new virus particles is inhibited and their spread is thwarted. It is important to note that antibiotics are not capable of fighting the influenza virus itself, but are sometimes given to patients with the flu to stem attacks of opportunistic microorganisms that are responsible for many influenza complications.

Though widespread familiarity with the flu makes it seem relatively benign to much of the general population, the virus can be devastating. In 1918 and 1919, more than 20 million people died from a strain of the virus commonly known as the Spanish flu that circulated through almost all inhabited regions of the globe. Many other outbreaks have occurred since that time, though none have been as deadly. Nevertheless, influenza together with complications of the virus is consistently among the top ten common causes of death in the United States, ranking higher than some other much more widely publicized killers, such as the HIV virus that causes AIDS.

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