Influenza

Influenza (commonly known as flu) is a highly contagious illness caused by a group of viruses called the orthomyxoviruses. Infection with these viruses leads to a self-limiting illness usually characterized by fever, muscle aches, fatigue, and upper respiratory infection and inflammation. Children and young adults usually recover from influenza within 3–7 days with no complications; however, in older adults, especially those over 65 with underlying conditions such as heart disease or lung illnesses, influenza can be deadly. Most of the hospitalizations and deaths from influenza occur in this age group. Although an influenza vaccine is available, it does not confer complete protection against all strains of influenza viruses.

Like all viruses, orthomyxoviruses cause illness by entering host cells and replicating within them. The new viruses then burst from the host cell and infect other cells. Orthomyxoviruses are sphere-shaped viruses that contain ribonucleic acid (RNA). The viruses use this RNA as a blue-print for replication within host cells. The outer envelope of an orthomyxovirus is studded with protein spikes that help the virus invade host cells. Two different types of spikes are present on the virus's outer envelope. One type, composed of hemagglutinin protein (HA), fuses with the host cell membrane, allowing the virus particle to enter the cell. The other type of spike, composed of the protein neuraminidase (NA), helps the newly formed virus particles to bud out from the host cell membrane.

The only way a virus can be neutralized and stopped is through the body's immune response. At the present time, no cure or treatment is available that completely destroys viruses within the body. The HA spikes and proteins in the orthomyxovirus envelope stimulate the production of antibodies, immune proteins that mark infected cells for destruction by other immune cells. In a healthy person, it takes about three days for antibodies to be formed against an invading virus. People with impaired immune function (such as people with Acquired Immune Deficiency Syndrome, the elderly, or people with underlying conditions) may not be able to mount an effective immune response to the influenza virus. Therefore, these people may develop serious complications, such as pneumonia, that may lead to hospitalization or death.

Three types of orthomyxoviruses cause illness in humans and animals: types A, B, and C. Type A causes epidemic influenza, in which large numbers of people become infected during a short period of time. Flu epidemics caused by Type A orthomyxoviruses include the worldwide outbreaks of 1918, 1957, 1968, and 1977. Type A viruses infect both humans and animals and usually originate in the Far East, where a large population of ducks and swine incubate the virus and pass it to humans. The Far East also has a very large human population that provides a fertile ground for viral replication. In 1997, a new strain of influenza A jumped from the poultry population in Hong Kong to the human population. H5N1, as the strain was named, was contracted through contact with the feces of chicken. The illness it caused (dubbed avian flu) was severe, and sometimes fatal. Although it was strongly believed that humans could not get the disease from eating properly cooked chicken, the decision was ultimately made to destroy and bury all of the chickens in Hong Kong. This massive effort was carried out in December 1997.

Type B influenza viruses are not as common as type A viruses. Type B viruses cause outbreaks of influenza about every two to four years. Type C viruses are the least common type of influenza virus and cause sporadic and milder infections.

The hallmark of all three kinds of influenza viruses is that they frequently mutate. Due to the small amount of RNA genetic material within a virus, mutation of the genetic material is very common. The result of this frequent mutation is that each flu virus is different, and people who have become immune to one flu virus are not immune to other flu viruses. The ability to mutate frequently therefore allows these viruses to cause frequent outbreaks.

Influenza is characterized by a sudden onset of fever, cough, and malaise. The incubation period of influenza is short, only 1–3 days. The cells that the influenza virus target are the cells of the upper respiratory tract, including the sinuses, bronchi, and alveoli. The targeting of the upper respiratory tract by the viruses accounts for the prominence of respiratory symptoms of flu. In fact, flu viruses are rarely found outside the respiratory tract. Most of the generalized symptoms of flu, such as muscle aches, are probably due to toxin-like substances produced by the virus.

Symptoms last for about 3–6 days; however, lethargy and cough may persist for several days to weeks after a bout with the flu. Children may have more severe symptoms due to a lack of general immunity to influenza viruses. Children also have smaller airways, and thus may not be as able to compensate for respiratory impairment as well as adults.

The most common complication of influenza is pneumonia. Pneumonia may be viral or bacterial. The viral form of pneumonia that occurs with influenza can be very severe. This form of pneumonia has a high mortality rate. Another form of pneumonia that is seen with influenza is a bacterial pneumonia. If the respiratory system becomes severely obstructed during influenza, bacteria may accumulate in the lungs. This type of pneumonia occurs 5–10 days after onset of the flu. Because it is bacterial in origin, it can be treated with antibiotics.

Other complications of influenza include infections of the heart and heart lining, infections of the brain, and Guillain-Barre syndrome (GBS). GBS is a paralytic disease in which the body slowly becomes paralyzed. Paralysis starts in the facial muscles and moves downward. GBS is treated symptomatically and usually resolves by itself. Another complication of influenza is Reye's syndrome. Occurring typically in children, Reye's syndrome is associated with aspirin intake during an attack of influenza. Reye's syndrome is characterized by nausea, vomiting, and progressive neurological dysfunction. Because of the risk of Reye's syndrome, children should not be given aspirin if they have the flu. Non-aspirin pain relievers, such as acetaminophen, should be given instead of aspirin.

Flu is treated with rest and fluids. Maintaining a high fluid intake is important, because fluids increase the flow of respiratory secretions, which may prevent pneumonia. Antiviral medications (amantadine, rimantadine) may be prescribed for people who have initial symptoms of the flu and who are at high risk for complications. This medication does not prevent the illness, but reduces its duration and severity.

A flu vaccine is available that is formulated each year against the current type and strain of flu virus. The virus is grown in chicken eggs, extracted, and then rendered noninfective by chemicals. The vaccine is also updated to the current viral strain by the addition of proteins that match the current strain's composition. The vaccine would be most effective in reducing attack rates if it was effective in preventing influenza in schoolchildren; however, in vaccine trials the vaccine has not been shown to be effective in flu prevention in this age group. In certain populations, particularly the elderly, the vaccine is effective in preventing serious complications of influenza and thus lowers mortality.

Vaccine research is ongoing. One of the more exciting advances in flu vaccines involves research studies examining an influenza vaccine mist, which is sprayed into the nose. This is predicted to be an excellent route of administration, which will confer even stronger immunity against influenza. Because it uses a live virus, it encourages a strong immune response. Furthermore, it is thought to be a more acceptable immunization route for schoolchildren, an important reservoir of the influenza virus.

See also Flu: The great flu epidemic of 1918; Viruses and responses to viral infection