Influenza

Influenza (often called flu) is infection with one of the influenza viruses. Influenza infection affects the entire body but is most evident in what it does to the airways and lungs.

Influenza virus is categorized as type A or type B. Within each type are many different subtypes (strains). Although some people blame influenza for every cold they develop, real influenza is caused by a very different virus than the common cold and produces much more severe symptoms.

Every year outbreaks of influenza occur during late autumn or winter, although the exact timing of these outbreaks varies throughout the world. Influenza occurs primarily in epidemics, in which many people get sick all at once. In each epidemic, usually only one strain of influenza virus is responsible.

Influenza affects people of all ages. In older people, influenza is particularly serious. Indeed, in a typical year it leads to more than 20,000 deaths in the United States, with older people accounting for over 80% of those deaths.

If influenza were merely an infection in the throat, it might be regarded as little more than an annoyance. However, influenza may lead to other problems, particularly in older people, especially those with chronic obstructive pulmonary disease, heart failure, diabetes, or other chronic diseases. Influenza frequently irritates the trachea and bronchi, leading to tracheitis and bronchitis. In some older people, influenza leads to pneumonia, either by the virus itself or, more commonly, from a bacterial infection. Pneumonia is the main cause of most deaths resulting from influenza infection.

Rarely, inflammation of muscles (myositis) or of the sac surrounding the heart (pericarditis) complicates influenza. Inflammation of the brain (encephalitis), the spinal cord (transverse myelitis), or nerves that branch off from the spinal cord (Guillain-Barré syndrome) may also develop. These complications may be the result of the body's immune system being overstimulated from fighting the influenza virus.

Causes

The influenza virus enters the airways and reaches the lungs when the person inhales tiny droplets containing the virus that have been coughed or sneezed out by an infected person. A person can also develop influenza by touching a surface or object that has been touched, coughed on, or sneezed on by an infected person and then touching his nose or mouth.

Symptoms

The illnesses produced by the two different types and multiple strains of influenza virus are similar in most ways. However, type A influenza tends to produce more severe symptoms than those of type B influenza.

Symptoms usually start within a day or two after the person becomes infected. Symptoms can begin very suddenly. At first the symptoms mimic a cold but quickly become much more severe. Chills are often the first indication of influenza. The person may begin to feel very tired and weak and lose his appetite. Fever is common during the first few days, with body temperature sometimes reaching 102° to 103° F. Muscle aches and pains throughout the body are common, but they tend to be most noticeable in the back and legs. Headaches occur frequently, with aching around and behind the eyes.

The throat very often becomes scratchy and sore. A runny nose, a burning sensation in the chest, and a dry cough may develop. Later, the cough can become severe, and the person may bring up mucus (sputum). The skin may be warm, and the person may appear flushed, especially on the face. The throat may redden, the eyes may water, and the whites of the eyes may become reddened and irritated.

Most symptoms begin to decrease after 2 or 3 days if the person does not develop complications. However, fever sometimes lasts for several more days, and cough may persist even longer, sometimes for a few weeks. Tiredness and weakness may also persist, occasionally for several weeks.

A person with influenza may seem to be getting better but then develop shortness of breath and a worsened cough that may produce bloody mucus if bronchitis or pneumonia develops. Rarely, muscle pain occurs if inflammation of the muscles develops, or chest pain develops with inflammation of the sac surrounding the heart. Confusion and headaches may indicate inflammation of the brain. Loss of sensation and weakness or even paralysis develops if inflammation occurs in the spinal cord or in the nerves that branch off from the spinal cord.

Diagnosis

A doctor usually diagnoses influenza on the basis of the symptoms and on the physical examination findings in the setting of a known influenza outbreak. These findings may include enlarged, tender lymph nodes on one or both sides of the neck and intense redness on the nose and on the back of the throat. The severity of the illness and the presence of a high fever and body aches help distinguish influenza from a cold.

If other cases of influenza have recently been diagnosed in the area, the doctor's diagnosis is reinforced, because influenza occurs mainly in epidemics. However, if there are no other cases of influenza and there is lingering doubt about the diagnosis, the doctor can order tests on samples of blood or mucus from the lungs that can identify the influenza virus.

Prevention

Vaccination is the best way to avoid contracting influenza. The vaccine prevents influenza in most people, although it may be less effective in very old people. Vaccination also significantly decreases the risk of pneumonia, hospitalization, and death. A new vaccine is produced and given each year, because the strain of the influenza virus causing outbreaks continually undergoes changes. Changes in the virus strain render the influenza vaccine used in a previous year ineffective against the newest strain.

The traditional injectable form of influenza vaccine is very safe. Some people receiving the vaccine experience soreness at the injection site. Rarely, the vaccine produces a slight fever for a day or less. People who are allergic to chickens' eggs are usually not vaccinated, because the vaccine contains very tiny amounts of egg protein.

In the United States, the vaccine is usually available in late September or early October. Most assisted living facilities and nursing homes urge every resident to be vaccinated because of the high risk of outbreaks for people living in close quarters. Vaccination in the fall is best, so that people are protected during the winter months, when epidemics of influenza are most likely to peak. However, people can still benefit from receiving the vaccine during the winter months. For most people, about 2 weeks is needed for the vaccination to provide protection against influenza.

A new nasally administered influenza vaccine is now available, but it is not recommended for people 50 or older.

Several antiviral drugs can also be used to prevent infection with influenza virus. Doctors may prescribe these drugs when they know a person has recently been exposed to someone with influenza. In addition, these drugs may be used during an outbreak in a nursing home to protect unvaccinated people who are at high risk of developing influenza, even if they have not yet been exposed to anyone with influenza. Amantadine and rimantadine are older antiviral drugs that offer protection against influenza type A but not influenza type B. These drugs can cause stomach upset, nervousness, sleeplessness, and other side effects, especially in older people. Rimantadine tends to cause fewer side effects than amantadine. Two new drugs, oseltamivir and zanamivir, can prevent infection with both types of influenza virus. These drugs produce minimal side effects.

Treatment and Outlook

The main treatment for influenza is to rest, drink plenty of fluids, and avoid exertion. Fever and aches are sometimes treated with acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs, such as aspirin or ibuprofen). Other measures that are used to treat the common cold, such as decongestants and steam inhalation, may help relieve some symptoms.

Amantadine, rimantadine, oseltamivir, and zanamivir are antiviral drugs helpful in treating influenza. However, these drugs work only if they are taken in the first day or two after the person becomes ill. They help to shorten the duration of symptoms for many people. Most doctors now recommend zanamivir and oseltamivir, which work against both influenza type A and type B. If doctors suspect a complication such as bronchitis or bacterial pneumonia, they prescribe antibiotics.

Most people with uncomplicated influenza recover fully and can remain at home. Some people with influenza begin to resume normal activities a day or two after body temperature returns to normal and muscle aches diminish. However, influenza can be very debilitating in other people, making it difficult to care for themselves. Recovery may take several days or even a few weeks in such people. They may neglect eating properly, which compounds the seriousness of the illness. Older people with influenza who live alone should consider staying with family or friends.

Influenza with a complication such as pneumonia is a more serious illness and is fatal for many older people. Even so, most people with complicated influenza do recover fully with time and treatment. However, treatment requires careful attention and usually hospitalization. Any older person with influenza who is having trouble breathing, has high fevers, or becomes confused should seek emergency medical treatment.