Influenza is a syndrome that is caused by infection either influenza type A or B virus. Other viral illnesses may mimic influenza, but only influenza occurs in epidemics that affect all age groups.

Influenza virus strikes in yearly epidemics. Worldwide epidemics are referred to as pandemics. These epidemics begin each year in the orient, each caused by a new strain of virus. The virus strain is named for the location it was first isolated. It is believed that these new strains arise in poultry, then jump to human hosts and spread rapidly because of international travel. Because influenza viruses have such high variability, previous exposure to them provides little if any immunity from season to season. This is both the reason for the yearly epidemics, and the reason a flu shot against the new strain must be taken every year for protection.

Influenza, the illness

After an incubation period of 1-2 days, the beginning of influenza is usually sudden. Symptoms patients report include

  • fever, sometimes with chills or even frank shaking chills
  • headache
  • myalgias or muscle aches
  • malaise or tiredness
  • loss of appetite
  • arthralgia, or aching joints
  • respiratory symptoms
    • cough
    • nasal discharge
    • stuffy nose
    • hoarseness
    • dry or sore throat
  • eye symptoms
    • photophobia (light hurts the eyes)
    • tearing
    • burning
    • pain on moving the eyes

Physical mainifestations or signs of the flu are the same for young and old alike, and include

  • fever rises rapidly to a peak of up to 106°F (41°C) within 12 hours
    • tends to drift down about a degree F a day
    • lasts for about 3 days (with a range of 1 to 5 days)
    • fever is more common and runs higher in children
  • respiratory tract signs
    • clear nasal discharge or nasal stuffiness
    • red throat, without tonsillar pus
    • small, tender lymph nodes are found in the neck area
    • there may be sounds on chest examination (rales or rhonchi) that suggest pneumonia or bronchitis.
    • cough
      • characteristically becomes worse as the illness progresses
      • often with discomfort or burning under the breastbone
      • cough may remain from 3-4 days to 1-2 weeks after fever subsides
    • fatigue and malaise may last 1-2 weeks after other signs and symptoms are gone


Viral cultures are sometimes used to verify the presence of influenza virus and to determine the type. In practice, these are usually reserved for hospitalized patients. In epidemics, this is rarely done or necessary after the first cases are diagnosed. If the patient appears to have the flu in a flu epidemic, you can pretty well bet that he does. Direct antigen or card tests are now more widely used in office medical practice. These tests are used to determine whether to start patients on anti-viral therapy, discussed below.

Treatment of influenza

 Symptom relief is the same as for common colds and sore throats. Pain and fever medicine (acetaminophen, Tylenol® and others) can be used for fever control, and for the headache, muscle aches, and cough. Aspirin and other salicylate-containing medications such as Pepto-Bismol® are not advised for children with influenza. The use of salicylates in children has been associated with Reye syndrome.

Antiviral agents specific for the influenza viruses:

  • the newer drugs
    • oseltamivir (Tamiflu®) oral drug for those 18 and older
    • zanamivir (Relenza®) inhaled drug for those 12 and older
  • older oral drugs
    • amantadine (Symmetrel®) available as capsule and liquid, and usable in children from age 1 and older; may cause dizziness, anxiety, depression, mental status change, rash, nausea, dizzy spells (orthostatic hypotension), swelling, or urinary retention.
    • rimantadine (Flumadine®) also available as capsule or liquid; may cause GI disturbance, dizziness, headache, urinary retention; nervous system disturbances are less than with amantadine.

Amantadine and rimantadine are both active only against influenza A virus. They are effective in the treatment of influenza A when given within 48 hours of the first symptoms of influenza. They are given during the period of acute illness only and for no longer than 5-7 days.

 Antibiotics should not be used for uncomplicated flu. They are of no benefit and create the risk of infection with resistant bacteria. Thus, antibiotics are reserved for treatment of bacterial complications of the flu.

Complications of the flu

  • pneumonia
    • influenza viral pneumonia
      • predominantly in persons with heart disease (especially rheumatic heart disease) or in pregnant women
      • after typical onset of influenza, there is usually rapid progression of fever, cough, shortness of breath, with falling blood oxygen concentrations
      • there is no specific treatment
      • the disease is often rapidly fatal in adults
      • in young children, a much milder form of influenza pneumonia can occur; quick recovery is the rule
    • secondary bacterial pneumonia
      • usually in the elderly, or those with pre-existing lung disease
      • classic flu symptoms, usually with some improvement, and then an apparent relapse
      • the pneumonia responds to standard antibiotic treatment

Reye syndrome is a rare and often fatal complication, most often after influenza B viral infection, but also after varicella and influenza A.

Other rare complications of the flu include

  • Myositis and myoglobinuria - tender leg muscles, sometimes sufficiently severe enough to prevent walking; this happens mostly in children, after influenza A and influenza B infections
  • Guillain-Barre syndrome and other very rare nervous system disorders have been reported after flu infections
  • myocarditis and pericarditis have been associated rarely with influenza A and B viral infection

An ounce of prevention

The very best prevention for influenza remains annual vaccination. Flu shots are about 70-90% effective at preventing the illness if there is a good match between the vaccine and the flu strain. It makes sense to immunize just about everybody over the age of 6 months against influenza each year. High risk groups including

  • the elderly
  • the chronically ill of all ages
  • asthmatics
  • diabietics
as well as those persons who are most likely to transmit the disease to them are all good candidates for the vaccine. Because children spread influenza the most readily in the community, they are prime candidates for the flu shot. Their parents and any adults who do not relish a bout of very uncomfortable illness are likewise well advised to get an annual flu shot.

The annual flu shot consists ordinarily of a single dose of vaccine. Two doses of vaccine are required for the first year it is given to children who have never been vaccinated or had the flu, or to adults in the event of a major shift of the flu strain. The shot should be given in late October or November. It takes about two weeks to build sufficient immunity to ward off the flu.

High risk individuals caught unprotected by the shot in an epidemic can receive the immunization and take amantidine daily for two weeks to protect from infection while immunity builds.

Side effects of the flu shot are minimal, with mild soreness at the injection site in a minority of patients. Systemic symptoms following vaccination are rare and are usually coincidental rather than caused by the vaccine. (The same rate of systemic side effects is seen in vaccine and placebo recipients in controlled trials.) Gillain-Barre syndrome has not been associated with influenza vaccine since the 1976 swine influenza program.

A new vaccine, Flumist® should be on the market next year. It is a nasally administered vaccine that gives excellent immunity and does not require an injection. It should be a big hit.

Source: Reese: Practical Approach to Infectious Diseases, 4th ed., Copyright 1996 Richard E. Reese and Robert F. Betts

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