Influenza

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Definition

Usually referred to as the flu or grippe, influenza is a highly infectious respiratory disease. The disease is caused by certain strains of the influenza virus. When the virus is inhaled, it attacks cells in the upper respiratory tract, causing such typical flu symptoms as fatigue, fever and chills, a hacking cough, and body aches. Influenza victims are also susceptible to potentially life-threatening secondary infections. Although the stomach or intestinal "flu" is commonly blamed for stomach upsets and diarrhea, the influenza virus rarely causes gastrointestinal symptoms. Such symptoms are most likely due to other organisms such as rotavirus, Salmonella, Shigella, or Escherichia coli.

Description

The flu is considerably more debilitating than the common cold. Influenza outbreaks occur suddenly, and infection spreads rapidly. The annual death toll attributable to influenza and its complications averages 20,000 in the United States alone. In the 1918–1919 Spanish flu pandemic, the death toll reached a staggering 20–40 million worldwide. Approximately 500,000 of these fatalities occurred in America.

Influenza outbreaks occur on a regular basis. The most serious outbreaks are pandemics, which affect millions of people worldwide and last for several months. The 1918-1919 influenza outbreak serves as the primary example of an influenza pandemic. Pandemics also occurred in 1957 and 1968 with the Asian flu and Hong Kong flu, respectively. The Asian flu was responsible for 70,000 deaths in the United States, while the Hong Kong flu killed 34,000.

Epidemics are widespread regional outbreaks that occur every two to three years and affect 5–10% of the population. The Russian flu in the winter of 1977 is an example of an epidemic. A regional epidemic is shorter lived than a pandemic, lasting only several weeks. Finally, there are smaller outbreaks each winter that are confined to specific locales.

The earliest existing descriptions of influenza were written nearly 2,500 years ago by the ancient Greek physician Hippocrates. Historically, influenza was ascribed to a number of different agents, including "bad air" and several different bacteria. It was not until 1933 that the causative agent was identified as a virus.

There are three types of influenza viruses, identified as A, B, and C. Influenza A can infect a range of species, including humans, pigs, horses, and birds, but only humans are infected by types B and C. Influenza A is responsible for most flu cases, while infection with types B and C virus are less common and cause a milder illness.

— Julia Barrett

Dictionary: in·flu·en·za  (ĭn'flū-ĕn'zə)

1. An acute contagious viral infection characterized by inflammation of the respiratory tract and by fever, chills, muscular pain, and prostration. Also called grippe.
2. Any of various viral infections of domestic animals characterized generally by fever and respiratory involvement.

[Italian, from Medieval Latin īnfluentia, influence (so called apparently from the belief that epidemics were due to the influence of the stars). See influence.]

An acute respiratory viral infection characterized by fever, chills, sore throat, headache, body aches, and severe cough. The term flu, which is frequently used incorrectly for various respiratory and even intestinal illnesses (such as stomach flu), should be used only for illness with these classic symptoms. The onset is typically abrupt, in contrast to common colds which begin slowly and progress over a period of days. Influenza is usually epidemic in occurrence. The first documented pandemic, or global epidemic, of influenza is considered to have been in 1580. The influenza pandemic of 1918, the most famous occurrence, was responsible for at least 20 million deaths worldwide.

The three types of influenza viruses, types A, B, and C, are classified in the virus family Orthomyxoviridae, and they are similar, but not identical, in structure and morphology. Types A and B are more similar in physical and biologic characteristics to each other than they are to type C. Influenza viruses may be spherical or filamentous in shape, and they are of medium size among common viruses of humans. See also Animal virus.

When a cell is infected by two similar but different viruses of one type, especially type A, various combinations of the original parental viruses may be packaged or assembled into the new progeny; thus, a progeny virus may be a mixture of gene segments from each parental virus and therefore may gain a new characteristic, for example, a new surface protein. This phenomenon is called genetic reassortment, and the frequency with which it occurs and leads to viruses with new features is a significant cause of the constant appearance of new variants of the virus. In the laboratory, reassortment occurs between animal and human strains as well as between human strains. It probably occurs in nature also, and is thought to contribute to the appearance of new strains that infect humans. Generally, if a new variant is sufficiently different from the vaccine currently in use, the vaccine will provide limited or no protection.

The influenza virus has a short incubation period; that is, there is only a period of 1–3 days between infection and symptoms, and this leads to the abrupt development of symptoms that is a hallmark of influenza infections. The virus is typically shed in the throat for 5–7 days. Complete recovery from uncomplicated influenza usually takes several days, and the individual may feel weak and exhausted for a week or more after the major symptoms disappear. The two main complications of influenza are primary influenza virus pneumonia and secondary bacterial pneumonia. Primary influenza pneumonia is relatively infrequent, occurring in less than 1% of cases during an epidemic, although mortality may be 25–30%. The damage to epithelial cells and subsequent loss of the ability to clear particles from the respiratory tract can lead to secondary bacterial pneumonia. This problem commonly occurs in elderly individuals or those with underlying chronic lung disease or similar problems. Influenza-induced pneumonia may cause as many as 20,000 deaths in a typical influenza season. Another complication, known as Reye's syndrome, may follow influenza, and is more common in children. This disease of the brain develops within 2–12 days of a systemic viral infection, and can result in vomiting, liver damage, coma, and sometimes death.

All three types of influenza viruses can cause disease in humans, but there are significant differences in severity of the disease and the range of hosts. In contrast to the large number of animal species infected by type A virus, types B and C are only rarely isolated from animals and infect predominantly humans.

The presence or absence of antibodies is very important in the epidemiology of influenza. In individuals with no immunity, attack rates may reach 70% and severe illness may result. Even low levels of antibody may provide partial protection in an individual and decrease the severity of the illness to only coldlike symptoms.

During an epidemic, one strain of influenza is predominant, but it is not unusual for two or more other strains to be present as minor infections in a population. Outbreaks of influenza occur during cold-weather months in temperate climates, and typically most cases cluster on a period of 1–2 months, in contrast to broader periods of illness with many other respiratory viruses. An increased death rate due to primary pneumonia and bacterial superinfection is common and is one of the ways that public health authorities monitor an epidemic.

Control and prevention of influenza are attempted through the use of drugs and vaccines. Inactivated viral vaccines are used to prevent influenza, although use of attenuated live strains of the virus may better stimulate the cell-mediated immune response and provide higher-quality and longer-lasting immunity. The makeup of the vaccine is modified annually, based upon predictions of the expected prevalent strain for each flu season, but usually contains antigens of two type A viruses and one type B virus. These vaccines take advantage of the natural ability of the viral nucleic acid to reassort and form new strains. The vaccines utilize strains that are not virulent and will replicate at lower temperatures, as found in the nasopharnyx, but not at higher temperatures as found in the lower respiratory tract. The techniques of modern biotechnology are employed to clone copies of parts of the virus or to provide oligonucleotides corresponding to crucial functional areas of the virus, to obtain improved protection and reduced side effects. See also Biotechnology; Vaccination.

A highly contagious infection of the respiratory tract caused by a myxovirus and transmitted by airborne droplet infection. Symptoms include sore throat, cough, fever, muscular pains, and weakness. Fever and constitutional symptoms distinguish influenza from the common cold. Three main strains of influenza virus have been recognized: Type A, Type B, and Type C. New strains of the virus emerge at regular intervals and are named according to geographic origin. Asian flu is a Type A influenza.

Definition

Usually referred to as the flu or grippe, influenza is a highly infectious respiratory disease. Its name comes from the Italian word for "influence," because people in eighteenth-century Europe thought that the disease was caused by the influence of bad weather. We now know that flu is caused by a virus. When the influenza virus is inhaled, it attacks cells in the upper respiratory tract, causing such typical flu symptoms as fatigue, fever and chills, a hacking cough, and body aches. Although the stomach or intestinal "flu" is commonly blamed for stomach upsets and diarrhea, the influenza virus affects humans less often than is commonly believed.

Description

Influenza is considerably more debilitating than the common cold. Influenza outbreaks occur suddenly, and infection spreads rapidly. The annual death toll attributable to influenza and its complications averages 20,000 in the United States alone. In the 1918-1919 Spanish flu pandemic, the death toll reached a staggering 20–40 million worldwide. Approximately 500,000 of these fatalities occurred in North America.

Influenza outbreaks occur on a regular basis. The most serious outbreaks are pandemics, which affect millions of people worldwide and last for several months. The 1918-19 influenza outbreak serves as the primary example of an influenza pandemic. Pandemics also occurred in 1957 and 1968 with the Asian flu and Hong Kong flu, respectively.

Epidemics are widespread regional outbreaks that occur every two to three years and affect 5–10% of the population. A regional epidemic is shorter lived than a pandemic, lasting only several weeks. Finally, there are smaller outbreaks each winter that are confined to specific locales.

There are three types of influenza viruses, identified as A, B, and C. Influenza A can infect a range of animal species, including humans, pigs, horses, and birds, but only humans are infected by types B and C. Influenza A is responsible for most flu cases, while infection with types B and C virus are less common and cause a milder illness.

In the United States, 90% of all deaths from influenza occur among persons older than 65. Flu-related deaths have increased substantially in the United States since the 1970s, largely because of the aging of the American population. In addition, elderly persons are vulnerable because they are often reluctant to be vaccinated against flu.

A new concern regarding influenza is the possibility that hostile groups or governments could use the virus as an agent of bioterrorism. A report published in early 2003 noted that Type A influenza virus has a high potential for use as such an agent because of the virulence of the Type A strain that broke out in Hong Kong in 1997 and the development of laboratory methods for generating large quantities of the virus. The report recommended the stockpiling of present antiviral drugs and speeding up the development of new ones.

Causes & Symptoms

Approximately one to four days after infection with the influenza virus, the victim develops an array of symptoms. Symptoms are usually sudden, although the sequence can be quite variable. They include the onset of headache, sore throat, dry cough, and chills, nasal congestion, fatigue, malaise, overall achiness and a fever that may run as high as 104°F (40°C). Flu victims feel extremely tired and weak, and may not return to their normal energy levels for several days or weeks.

Influenza complications usually arise from bacterial infections of the lower respiratory tract. Signs of a secondary respiratory infection often appear just as the patient seems to be recovering. These signs include high fever, intense chills, chest pains associated with breathing, and a productive cough or sinus discharge with thick yellowish-green sputum. If these symptoms appear, medical treatment is often necessary. Other secondary infections, such as sinus or ear infections, may also require medical intervention. Heart and lung problems and other chronic diseases, can be aggravated by influenza, which is a particular concern with elderly patients.

With children and teenagers, it is advisable to be alert for symptoms of Reye's syndrome, a rare but serious complication that occurs when children are given aspirin. Symptoms of Reye's syndrome are nausea and vomiting, and more seriously, such neurological problems as confusion or delirium. The syndrome is primarily associated with the use of aspirin to relieve flu symptoms in children.

Diagnosis

Although there are specific laboratory tests to identify the flu virus strain from respiratory samples, doctors typically rely on a set of symptoms and the presence of influenza in the community for diagnosis. Specific tests are useful to determine the type of flu in the community, but they do little for individual treatment. Doctors may administer such tests as throat or sinus cultures or blood tests to identify secondary infections.

Since 1999, however, seven rapid diagnostic tests for flu have become commercially available. These tests appear to be especially useful in diagnosing flu in children, allowing doctors to make more accurate treatment decisions in less time.

Treatment

The patient should drink plenty of fluids and eat nutritious foods. Chicken soup with ginger, scallions, and rice noodles is nutritious and has healing powers. Rest, to allow the body to fight infection, is very important. Gargling with salt water (half teaspoon salt in one cup of water) helps to soothe a sore throat. A vaporizer with eucalyptus or Vicks VapoRub will make the patient feel more comfortable by easing breathing and aiding sleep. Applying Vicks ointment over chest and back will assist and speed recovery. Returning to normal activities too quickly invites a possible relapse or complications.

Herbals

Herbal teas and other preparations can be taken to stimulate the immune system, for antiviral activity, and to relieve symptoms. The following herbs are used to treat influenza:

• Ginger (Zingiber officinalis) reduces fever and pain, has a sedative effect, settles the stomach, and suppresses cough.
• Forsythia (Forsythia suspensa) fruit can be taken as a tea for its anti-inflammatory, fever-reducing, and antimicrobial properties.
• Honeysuckle (Lonicera japonica) flower can be taken as a tea for its anti-inflammatory, fever-reducing, and antimicrobial properties.
• Anise seed (Pimpinella anisum) can be added to tea to expel phlegm, induce sweating, ease nausea, and ease stomach gas.
• Slippery elm (Ulmus rubra) can be taken as a tea or slurry to soothe sore throat and ease cough.
• Echinacea (Echinacea purpurea or angustifolia), in clinical studies, reduced flu symptoms including sore throat, chills, sweating, fatigue, weakness, body aches, and headaches. The usual dosage is 500 mg thrice on the first day, then 250 mg four times daily thereafter.
• Goldenseal (Hydrastis canadensis) has fever reducing, antibacterial, anti-inflammatory, and antitussive properties. The usual dose is 125 mg three to four times daily. Goldenseal shouldn't be taken for more than one week.
• Astragalus (Astragalus membranaceus) boosts the immune system and improves the body's response to stress. The common dose is 250 mg of extract four times daily.
• Cordyceps (Cordyceps sinensis) modulates and boosts the immune system and improves respiration. The usual dose is 500 mg two to three times daily.
• Elder (Sambucus nigra) has antiviral activity, increases sweating, decreases inflammation, and decreases nasal discharge. In a study, elderberry extract reduced flu symptoms within two days whereas placebo took six days. The usual dose is 500 mg of extract thrice daily. Also use 2 tsp of dried flowers in 1 cup of water as a tea.
• Schisandra (Schisandra chinensis) helps the body fight disease and increases endurance.
• Grape (Vitis vinifera) seed extract has antihistamine and anti-inflammatory properties. The usual dose is 50 mg three times daily.
• Eucalyptus (Eucalyptus globulus) or peppermint (Mentha piperita) essential oils added to a steam vaporizer may help clear chest and nasal congestion.
• Boneset infusion (Eupatorium perfoliatum) relieves aches and fever.
• Yarrow (Achillea millefolium) relieves chills.

Other Remedies

Acupuncture and acupressure are said to stimulate natural resistance, relieve nasal congestion and headaches, fight fever, and calm coughs, depending on the points used.

A homeopathic remedy called Oscillococcinum may be taken at the first sign of flu symptoms and repeated for a day or two. This remedy is said to shorten the duration of flu by one or two days. Although oscillococcinum is a popular flu remedy in Europe, however, a research study published in 2003 found it to be ineffective.

Other homeopathic remedies recommended vary according to the specific flu symptoms present. Gelsemium (Gelsemium sempervirens) is recommended to combat weakness accompanied by chills, a headache, and nasal congestion. Bryonia (Bryonia alba) may be used to treat muscle aches, headaches, and a dry cough. For restlessness, chills, hoarseness, and achy joints, poison ivy (Rhus toxicodendron) is recommended. Finally, for achiness and a dry cough or chills, Eupatorium perfoliatum is suggested.

Hydrotherapy can be utilized. A bath to induce a fever will speed recovery from the flu. While supervised, the patient should take a bath as hot as he/she can tolerate and remain in the bath for 20–30 minutes. While in the bath, the patient drinks a cup of yarrow or elder-flower tea to induce sweating. During the bath, a cold cloth is held on the forehead or at the nape of the neck to keep the temperature down. The patient is assisted when getting out of the bath (he/she may feel weak or dizzy) and then gets into bed and covers up with layers of blankets to induce more sweating.

Supplemental vitamins are recommended for treating influenza, and include 500–2000 mg vitamin C, 400 IU to 500 IU of vitamin E, 200 micrograms to 300 micrograms selenium, and 25,000 IU beta-carotene. Zinc lozenges are helpful, as is supplemental zinc at 25 mg per day for two weeks or more.

Traditional Chinese medicine (TCM) uses mixtures of herbs to prevent flu as well as to relieve symptoms once a person has fallen ill. There are several different recipes for these remedies, but most contain ginger and Japanese honeysuckle in addition to other ingredients.

Allopathic Treatment

Because influenza is a viral infection, antibiotics are useless in treating it. However, antibiotics are frequently used to treat secondary infections.

Over-the-counter medications are used to treat flu symptoms. Any medication that is designed to relieve such symptoms as pain and coughing will provide some relief. The best medicine for symptoms is an analgesic, such as aspirin, acetaminophen, or naproxen. Without a doctor's approval, aspirin is generally not recommended for people under 18 owing to its association with Reye's syndrome, a rare aspirin-associated complication seen in children recovering from viral infections. Children should receive acetaminophen or ibuprofen to treat their symptoms.

There are four antiviral drugs marketed for treating influenza as of 2003. To be effective, treatment should begin no later than two days after symptoms appear. Antivirals may be useful in treating patients who have weakened immune systems or who are at risk for developing serious complications. They include amantadine (Symmetrel, Symadine) and rimantadine (Flumandine), which work against Type A influenza; and zanamavir (Relenza) and oseltamavir phosphate (Tamiflu), which work against both Types A and B influenza. Amantadine and rimantadine can cause such side effects as nervousness, anxiety, lightheadedness, and nausea. Severe side effects include seizures, delirium, and hallucination, but are rare and are nearly always limited to people who have kidney problems, seizure disorders, or psychiatric disorders. The new drugs zanamavir and oseltamavir phosphate have few side effects but can cause dizziness, jitters, and insomnia.

Expected Results

Following proper treatment guidelines, healthy people under the age of 65 usually suffer no long-term consequences associated with flu infection. The elderly and the chronically ill are at greater risk for secondary infection and other complications, but they can also enjoy a complete recovery.

Most people recover fully from an influenza infection, but it should not be viewed complacently. Influenza is a serious disease, and approximately 1 in 1,000 cases proves fatal.

Prevention

The Centers for Disease Control and Prevention recommend that people get an influenza vaccine injection each year before flu season starts. In the United States, flu season typically runs from late December to early March. Vaccines should be received two to six weeks prior to the onset of flu season to allow the body enough time to establish immunity.

Each season's flu vaccine contains three virus strains that are the most likely to be encountered in the coming flu season. The virus strains used to make the vaccine are inactivated and will not cause illness. When there is a good match between the anticipated flu strains and the strains used in the vaccine, the vaccine is 70-90% effective in people under 65. Because immune response diminishes somewhat with age, people over 65 may not receive the same level of protection from the vaccine, but even if they do contract the flu, the vaccine diminishes the severity and helps prevent complications.

It should be noted that certain people should not receive an influenza vaccine. Infants six months and younger have immature immune systems and will not benefit from the vaccine. Because the vaccines are prepared using hen eggs, people who have severe allergies to eggs or other vaccine components should not receive the influenza vaccine. Some persons may receive a course of amantadine or rimantadine, which are 70-90% effective in preventing influenza.

Certain groups are strongly advised to be vaccinated because they are at greater risk for influenza-related complications:

• All people 65 years and older.
• Residents of nursing homes and chronic-care facilities.
• Adults and children who have chronic heart or lung problems.
• Adults and children who have chronic metabolic diseases, such as diabetes and renal dysfunction, as well as severe anemia or inherited hemoglobin disorders.
• Children and teenagers who are on long-term aspirin therapy.
• Anyone who is immunocompromised, including HIV-infected persons, cancer patients, organ transplant recipients, and patients receiving steroids, chemotherapy, or radiation therapy.
• Anyone in contact with the above groups, such as teachers, care givers, health-care personnel, and family members.
• Travelers to foreign countries.

As of early 2003, researchers are working on developing an intranasal flu vaccine in aerosol form. An aerosol vaccine using a weakened form of Type A influenza virus has been tested in pilot studies and awaits further clinical trials.

The following dietary supplements may be taken to help prevent influenza:

• Elderberry prevents influenza virus from infecting cells.
• Astragalus: 250–500 mg daily.
• Multivitamins with zinc.
• Vitamin C; 500 mg.
• Echinacea; at the first sign of malaise or infection, take 3–5 ml of tincture or 2 tablets three or four times daily for three to 10 days.

Resources

Books

Pelletier, Kenneth R., MD. The Best Alternative Medicine, Part II, "CAM Therapies for Specific Conditions: Colds/Flu." New York: Simon & Schuster, 2002.

"Respiratory Viral Diseases: Influenza." Section 13, Chapter 162 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Periodicals

Elkins, Rita. "Combat Colds and Flu." Let's Live. 68 (January 2000): 81+.

Jonas, W. B., T. J. Kaptchuk, and K. Linde. "A Critical Overview of Homeopathy." Annals of Internal Medicine 138 (March 4, 2003): 393–399.

Krug, R. M. "The Potential Use of Influenza Virus as an Agent for Bioterrorism." Antiviral Research 57 (January 2003): 147–150.

La Valle, James B., and Ernie Hawkins. "Colds and Flu: A Natural Approach." Drug Store News. 20 (12/14/98): CP17+.

Oxford, J. S., S. Bossuyt, S. Balasingam, et al. "Treatment of Epidemic and Pandemic Influenza with Neuraminidase and M2 Proton Channel Inhibitors." Clinical Microbiology and Infection 9 (January 2003): 1–14.

Roth, Y., J. S. Chapnik, and P. Cole. "Feasibility of Aerosol Vaccination in Humans." Annals of Otology, Rhinology, and Laryngology 112 (March 2003): 264–270.

Shortridge, K. F., J. S. Peiris, and Y. Guan. "The Next Influenza Pandemic: Lessons from Hong Kong." Journal of Applied Microbiology 94 (2003 Supplement): 70S–79S.

Storch, G. A. "Rapid Diagnostic Tests for Influenza." Current Opinion in Pediatrics 15 (February 2003): 77–84.

Thompson, W. W., D. K. Shay, E. Weintraub, et al. "Mortality Associated with Influenza and Respiratory Syncytial Virus in the United States." Journal of the American Medical Association 289 (January 8, 2003): 179–186.

Organizations

Centers for Disease Control and Prevention. 1600 Clifton Road, NE, Atlanta, Georgia 30333. (888) CDC-FACTS (888-232-3228). .

National Institute of Allergy and Infectious Diseases (NIAID). 31 Center Drive, MSC 2520, Bethesda, MD 20892-2520. .

Other

NIAID Fact Sheet: Flu. Bethesda, MD: NIAID, January 2003. .

[Article by: Belinda Rowland; Rebecca J. Frey, PhD]

Definition

Usually referred to as the flu or grippe, influenza is a highly infectious respiratory disease. The disease is caused by certain strains of the influenza virus. When the virus is inhaled, it attacks cells in the upper respiratory tract, causing typical flu symptoms such as fatigue, fever and chills, a hacking cough, and body aches. Influenza victims are also susceptible to potentially life-threatening secondary infections. Although the stomach or intestinal "flu" is commonly blamed for stomach upsets and diarrhea, the influenza virus rarely causes gastrointestinal symptoms. Such symptoms are most likely due to other organisms such as rotavirus, Salmonella, Shigella, or Escherichia coli.

Description

The flu is considerably more debilitating than the common cold. Influenza outbreaks occur suddenly, and infection spreads rapidly. In the 1918–19 Spanish flu pandemic, the death toll reached a staggering 20 to 40 million worldwide. Approximately 500,000 of these fatalities occurred in the United States.

Influenza outbreaks occur on a regular basis. The most serious outbreaks are pandemics, which affect millions of people worldwide and last for several months. The 1918–19 influenza outbreak serves as the primary example of an influenza pandemic. Pandemics also occurred in 1957 and 1968 with the Asian flu and Hong Kong flu, respectively. The Asian flu was responsible for 70,000 deaths in the United States, while the Hong Kong flu killed 34,000.

Epidemics are widespread regional outbreaks that occur every two to three years and affect 5–10 percent of the population. The Russian flu in the winter of 1977 is an example of an epidemic. A regional epidemic is shorter lived than a pandemic, lasting only several weeks. Finally, there are smaller outbreaks each winter that are confined to specific locales.

The earliest existing descriptions of influenza were written nearly 2,500 years ago by the ancient Greek physician Hippocrates. Historically, influenza was ascribed to a number of different agents, including "bad air" and several different bacteria. In fact, its name comes from the Italian word for "influence," because people in eighteenth-century Europe thought that the disease was caused by the influence of bad weather. It was not until 1933 that the causative agent was identified as a virus.

There are three types of influenza viruses, identified as A, B, and C. Influenza A can infect a range of animal species, including humans, pigs, horses, and birds, but only humans are infected by types B and C. Influenza A is responsible for most flu cases, while infection with types B and C virus are less common and cause a milder illness.

Demographics

The annual death toll attributable to influenza and its complications averages 20,000 in the United States alone. In the United States, 90 percent of all deaths from influenza occur among persons older than 65. Flu-related deaths have increased substantially in the United States since the 1970s, largely because of the aging of the American population. In addition, elderly persons are vulnerable because they are often reluctant to be vaccinated against flu.

Hospitalization due to complications of influenza are common in children. Among children with chronic illnesses, about 500 children per every 100,000 between the ages of birth and age four are hospitalized annually due to influenza, while about 100 children per 100,000 without chronic illnesses are hospitalized annually. Among those with underlying high-risk conditions, infants younger than six months have the highest hospitalization rates (approximately 10–40 per 100,000 population).

Causes and Symptoms

Approximately one to four days after infection with the influenza virus, the victim is hit with an array of symptoms. "Hit" is an appropriate term, because symptoms are sudden, harsh, and unmistakable. Typical influenza symptoms include the abrupt onset of a headache, dry cough, and chills, rapidly followed by overall achiness and a fever that may run as high as 104°F (40°C). As the fever subsides, nasal congestion and a sore throat become noticeable. Flu victims feel extremely tired and weak and may not return to their normal energy levels for several days or even a couple of weeks.

Influenza complications usually arise from bacterial infections of the lower respiratory tract. Signs of a secondary respiratory infection often appear just as the victim seems to be recovering. These signs include high fever, intense chills, chest pains associated with breathing, and a productive cough with thick yellowish green sputum. If these symptoms appear, medical treatment is necessary. Other secondary infections, such as sinus or ear infections may also require medical intervention. Children with heart and lung problems, as well as other chronic diseases, are at higher risk for complications from influenza.

With children and teenagers, it is advisable to be alert for symptoms of Reye's syndrome, a rare, but serious complication. Symptoms of Reye's syndrome are nausea and vomiting, and more seriously, neurological problems such as confusion or delirium. The syndrome has been associated with the use of aspirin to relieve flu symptoms.

Diagnosis

Although there are specific viral culture tests available to identify the flu virus strain from respiratory samples, results can take several days. Therefore, doctors typically rely on a set of symptoms and the presence of influenza in the community for diagnosis. Specific tests are useful to determine the type of flu in the community, but they do little for individual treatment. Doctors may administer tests, such as throat cultures, to identify secondary infections.

Several rapid (30-minute) diagnostic tests for flu have become commercially available. These tests appear to be especially useful in diagnosing flu in children, allowing doctors to make more accurate treatment decisions in less time.

Treatment

Essentially, a bout of influenza must be allowed to run its course. Symptoms can be relieved with bed rest and by keeping well hydrated. A steam vaporizer may make breathing easier, and pain relievers can mask the aches and pain. Food may not seem very appetizing, but an effort should be made to consume nourishing food. Recovery should not be pushed. Returning to normal activities too quickly invites a possible relapse or complications.

Drugs

Since influenza is a viral infection, antibiotics are useless in treating it. However, antibiotics are frequently used to treat secondary infections.

Over-the-counter medications are used to treat flu symptoms, but it is not necessary to purchase a medication marketed specifically for flu symptoms. Any medication that is designed to relieve symptoms, such as pain and coughing, will provide some relief. Medications containing alcohol, however, should be avoided because of the dehydrating effects of alcohol. The best medicine for symptoms is simply an analgesic, such as acetaminophen or naproxen. (Without a doctor's approval, aspirin is generally not recommended for people under 18 owing to its association with Reye's syndrome, a rare aspirin-associated complication seen in children recovering from the flu. To be on the safe side, children should receive acetaminophen or ibuprofen to treat their symptoms.)

As of 2004, there were a number of antiviral drugs marketed for treating influenza. To be effective, treatment should begin no later than two days after symptoms appear. These medications are useful for decreasing the severity and duration of symptoms. Antivirals may be useful in treating patients who have weakened immune systems or who are at risk for developing serious complications. They include amantadine (Symmetrel, Symadine) and rimantadine (Flumandine), which work against Type A influenza, and zanamavir (Relenza) and oseltamavir phosphate (Tamiflu), which work against both Types A and B influenza. Amantadine and rimantadine can cause side effects such as nervousness, anxiety, lightheadedness, and nausea. Severe side effects include seizures, delirium, and hallucination, but are rare and are nearly always limited to people who have kidney problems, seizure disorders, or psychiatric disorders. Zanamavir and oseltamavir phosphate can cause dizziness, jitters, and insomnia.

Prognosis

Following proper treatment guidelines, healthy people under the age of 65 usually suffer no long-term consequences associated with flu infection. The elderly and the chronically ill are at greater risk for secondary infection and other complications, but they can also enjoy a complete recovery.

Most people recover fully from an influenza infection, but it should not be viewed complacently. Influenza is a serious disease, and approximately one in 1,000 cases proves fatal.

Prevention

The Centers for Disease Control and Prevention recommends that people get an influenza vaccine injection each year before flu season starts. In the United States, flu season typically runs from late December to early March. Vaccines should be received two to six weeks prior to the onset of flu season to allow the body enough time to establish immunity. Adults only need one dose of the yearly vaccine, but children under nine years of age who have not previously been immunized should receive two doses with a month between each dose.

Each season's flu vaccine contains three virus strains that are the most likely to be encountered in the coming flu season. When there is a good match between the anticipated flu strains and the strains used in the vaccine, the vaccine is 70–90 percent effective in people under 65. Because immune response diminishes somewhat with age, people over 65 may not receive the same level of protection from the vaccine, but even if they do contract the flu, the vaccine diminishes the severity and helps prevent complications.

The virus strains used to make the vaccine are inactivated and will not cause the flu. In the second half of the twentieth century, flu symptoms were associated with vaccine preparations that were not as highly purified as modern vaccines, not to the virus itself. In 1976, there was a slightly increased risk of developing Guillain-Barré syndrome, a very rare disorder, associated with the swine flu vaccine. This association occurred only with the 1976 swine flu vaccine preparation and as of 2004 had not recurred.

Serious side effects with modern vaccines are extremely unusual. Some people experience a slight soreness at the point of injection, which resolves within a day or two. People who have never been exposed to influenza, particularly children, may experience one to two days of a slight fever, tiredness, and muscle aches. These symptoms start within six to 12 hours after the vaccination.

It should be noted that certain people should not receive an influenza vaccine. Infants six months and younger have immature immune systems and will not benefit from the vaccine. Since the vaccines are prepared using hen eggs, people who have severe allergies to eggs or other vaccine components should not receive the influenza vaccine. As an alternative, they may receive a course of amantadine or rimantadine, which are also used as a protective measure against influenza. Other people who might receive these drugs are those that have been immunized after the flu season has started or who are immunocompromised, such as people with advanced HIV disease. Amantadine and rimantadine are 70–90 percent effective in preventing influenza.

Certain groups are strongly advised to be vaccinated because they are at increased risk for influenza-related complications. These groups are:

• children under age two
• all people 65 years and older
• residents of nursing homes and chronic-care facilities, regardless of age
• adults and children who have chronic heart or lung problems, such as asthma
• adults and children who have chronic metabolic diseases, such as diabetes and renal dysfunction, as well as severe anemia or inherited hemoglobin disorders
• children and teenagers who are on long-term aspirin therapy
• women who will be in their second or third trimester during flu season or women who are nursing
• anyone who is immunocompromised, including HIV-infected persons, cancer patients, organ transplant recipients, and patients receiving steroids, and those receiving chemotherapy or radiation therapy
• anyone in contact with the above groups, such as teachers, care givers, healthcare personnel, and family members
• travelers to foreign countries

A person need not be in one of the at-risk categories listed above, however, to receive a flu vaccination. Anyone who wants to forego the discomfort and inconvenience of an influenza attack may receive the vaccine.

Parental Concerns

Parents should make sure that their children who fall into any of the risk categories should be vaccinated against the flu. Pregnant women in the second or third trimesters should also be vaccinated. Flu vaccines are available through pediatricians or local public health departments. Parents should also make sure kids follow good hygiene practices, including regular hand washing, and covering the mouth when sneezing or coughing. Children may acquire secondary infections, such as ear infections or sinus infections, so parents should call the pediatrician if a child develops a high fever, sudden pain in the ears or sinuses, or develops a productive cough with thick yellow-green phlegm.

Resources

Books

Subbarao, Kanta. "Influenza Viruses." In Principles and Practice of Pediatric Infectious Diseases, 2nd ed. Edited by Sarah S. Long et al. St. Louis, MO: Elsevier, 2003.

Wright, Peter. "Influenza Viruses." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.

Periodicals

Larkin, M. "Will influenza be the next bioweapon?" Lancet Infectious Disease 138 (January 7, 2003): 53.

Neff, M. J. "ACIP Releases 2004 Guidelines on the Prevention and Control of Influenza." Annals of Internal Medicine 70) (July 2004): 199–204.

Stiver, G. "The treatment of influenza with antiviral drugs." Canadian Medical Association Journal 138 (January 2003): 49–56.

Organizations

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. Web site: www.cdc.gov.

National Institute of Allergy and Infectious Diseases (NIAID). 31 Center Drive, MSC 2520, Bethesda, MD 20892–2520. Web site: www.niaid.nih.gov

Web Sites

"Flu." Health Matters, November 2004. Available online at www.niaid.nih.gov/factsheets/flu.htm (accessed December 28, 2004).

[Article by: Julia Barrett Rebecca J. Frey, PhD Rosalyn Carson-DeWitt, MD]

Influenza is a potentially severe acute respiratory illness caused by various strains of the influenza virus. The different strains all produce characteristic symptoms, and because major outbreaks are associated with increased mortality, occurrences can be identified in history. Outbreaks consistent with influenza can be traced back at least to the court of Elizabeth I. Some have speculated that the Plague of Athens described by Thucydides was influenza complicated by bacterial superinfection. The influenza syndrome, commonly known as the flu, with its fever, cough, rapid onset and body aches, is not only typical enough to be recognized in the past, but it also allows physicians to recognize it, especially when it is known that the virus is circulating. Unfortunately, death is the other consistent phenomena associated with influenza. Mortality statistics are the principal way the intensity of an influenza outbreak is quantified, and are so characteristic that viral identification of etiology is not required.

The Virus and Its Antigens

The influenza viruses contain RNA (ribonucleic acid) and are somewhat unusual in that they have a segmented genome, which means that there are eight distinct segments to the single-stranded RNA. Influenza types A and B are the only strains with epidemic potential; type C viruses are difficult to work with in the laboratory and are one of the multiple agents able to cause the common cold. While the viruses are classified into type A and B on the basis of their internal components, it is the surface antigens that are important in eliciting antibodies that will protect against future infection. These surface antigens and their changes make influenza challenging to control. Two types of changes are recognized.

One change occurs in both type A and B viruses and is a result of point mutations in the segments of the genome coding for two specific surface antigens (the neuraminidase [N] and the hemagglutinin [H] segments). These mutations are the reason that both type A and B viruses change regularly from year to year, though type A changes somewhat more rapidly than type B. Such changes are referred to as "antigen drift." Another change is more dramatic, only occurring with type A viruses, and is an example of "antigen shift." It takes place when one or two gene segments are replaced in a circulating virus. The same two antigens, or proteins, are involved in both types of change. The various influenza A viruses are categorized into subtypes by the differences in those two antigens, such as A (H₁N₁) or A (H₃N₂).

The most widely accepted theory explaining this antigen shift is that the segments come from animal influenza viruses. Type B influenza is confined to humans, while type A exists in numerous species of birds and domestic animals. There are fifteen types of hemagglutinin in the influenza virus of birds, but only three in human viruses, which gives an ample opportunity for the segment coding for the hemagglutinin to move from avian viruses to human. This has apparently happened in the past, and is likely to occur in the future, either directly or through pigs. In 1997, in Hong Kong, an avian virus infected humans directly, but did not become adapted to humans by exchange of gene segments. If it had, a pandemic undoubtedly would have resulted.

Pandemics: History and Impact

While some trace influenza pandemics back to ancient Greece, the first documented occurrence was in 1889 (see Table 1). In that and subsequent years, outbreaks of influenza were reported in many areas of the world, and in the United States, deaths reported in the state of Massachusetts for the first time demonstrated the U- or J-shaped mortality curve—an elevated mortality in young children, low mortality until age forty-five, followed by gradually increasing mortality with a relatively sharp inflection upward at age sixty-five. By testing blood specimens of persons who lived through this period, researchers have been able to hypothesize about the strain of virus that caused this pandemic. In 1899, there was an apparent antigen shift, but this was determined serologically, not on the basis of an observed pandemic.

It is now certain that a virus resembling one isolated from pigs in the 1930s caused the devastating 1918 pandemic. No influenza viruses were isolated until the 1930s, so that any identification of viruses responsible for events occurring before that time has traditionally been done by testing the blood of people living through the period of an outbreak. Confirmation of this approach has recently taken place using modern molecular technique involving tissue of individuals who died during the 1918 pandemic. The virus is now termed A(H₁N₁). The estimated death toll from this pandemic has been revised upwards from 20 million to 40 million, since large segments of the world— mainly the current developing countries—were originally omitted from the counts. The lethality of this pandemic was related in large part to the death of an unexpectedly large number of healthy

Table 1

young adults. This resulted in a W-shaped agespecific mortality curve. It is hoped that genetic research with lung tissue, either stored or recovered from bodies, will enable epidemiologists to predict the potential behavior of future pandemic strains of influenza when they are identified. However, this has not as yet been possible, so it is only by observing the epidemiology of infection that the age-specific pattern of illness can be determined.

The first influenza viruses were isolated from humans in the early 1930s. However, the next pandemic did not occur until 1957, when the A(H₂N₂) virus appeared in South China. The pandemic that resulted was the most severe since 1918, but again exhibited the more typical U-shaped mortality curve, concentrated in very young children and older individuals. A little more than ten years later, in 1968, the hemagglutinin changed and the resulting pandemic was similar to 1957 in age distribution, but more moderate in overall impact.

Two more episodes have occurred since 1968 that had the potential to be full pandemics. In 1977, the A(H₁N₁) virus returned, with outbreaks occurring first in China and then in the former Soviet Union. Since the virus had circulated twenty or more years before, when worldwide outbreaks occurred, these epidemics were confined to younger individuals. This virus has continued to circulate, along with the A(H₃N₂) and B viruses. Finally, in 1997, A(H₅N₁) moved from chickens to humans in Hong Kong. There were eighteen confirmed cases, with six deaths that were not restricted to older individuals. Fortunately, this avian virus did not become fully adapted to humans. No human-to-human transmission was observed, but this episode showed how a catastrophic pandemic might have occurred had such adaptation taken place.

Prevention and Control of Influenza

A vaccine for the prevention of influenza was developed during World War II in order to maintain military readiness. This was done in recognition of the high morbidity that could result among troops exposed to the virus. A similar inactivated vaccine is still in use, improved in both potency and lack of side effects. It is known to be 70 to 90 percent efficacious in healthy young adults as long as the vaccine viruses resemble those circulating. This necessitates updating the viruses in the vaccine each year. For this and other reasons, the vaccine must be given annually. Since vaccination programs must be sustained, the goal in most countries has been to reduce influenza mortality by vaccinating older individuals and those with chronic underlying diseases. An exception to this has been Japan, where, for a time, school-age children were vaccinated in an effort to control influenza morbidity. It has been repeatedly demonstrated that the inactivated vaccine is effective in preventing hospitalization and death in older individuals and, as such, is also cost effective. The inactivated vaccine is cost effective in healthy adults only when the attack rates are above 12 percent. A live attenuated influenza vaccine has been used in the former Soviet Union for many years, and another is in development in the United States. Because of its delivery—intranasally rather than by injection—it may prove to be particularly useful in children and younger adults.

Antiviral drugs have been available both for treatment and prophylaxis. Two of these are active only against type A viruses. A new group of drugs, acting as neuraminidase inhibitors, is active against both type A and B viruses. These drugs have been shown to have a prevention efficacy similar to vaccines. They start protecting more quickly than the vaccine, but have to be taken daily to continue protection. Therefore, vaccination will continue to be the usual means of prophylaxis. The neuraminidase inhibitors also significantly shorten the duration of illness, reducing severity and preventing complications. Influenza can be debilitating, even in the absence of complications, so that the drugs will be used for treatment during defined influenza outbreaks. They are likely also to be useful prophylactically, especially for outbreak control in nursing homes.

Bibliography

Belshe, R. B.; Mendelman, P. M.; Treanor, J. et al. (1998). "The Efficacy of Live Attenuated, Cold-adapted, Trivalent, Intranasal Influenza Virus Vaccine in Children." New England Journal of Medicine 149:1113–1117.

Dolin, R.; Reichman, R. C.; Madore, H. P.; Maynard, R.; Linton, P. N.; and Webber-Jones, J. (1982). "A Controlled Trial of Amantadine and Rimantadine in the Prophylaxis of Influenza A Infection." New England Journal of Medicine 307:580–584.

Douglas, R. B., Jr. (1990). "Drug Therapy: Prophylaxis and Treatment of Influenza." New England Journal of Medicine 322:443–450.

Eickhoff, T. C.; Sherman, I. L.; and Serfling, R. E. (1961). "Observations on Excess Mortality Associated with Epidemic Influenza." Journal of the American Medical Association 176:776–782.

Gubareva, L. B.; Kaiser, L.; and Hayde, F. G. (2000). "Influenza Virus Neuraminidase Inhibitors." New England Journal of Medicine 355:827–835.

Lui, K. J., and Kendal, A. P. (1987). "Impact of Influenza Epidemics on Mortality in the United States from October 1972 to May 1985." American Journal of Public Health 77:712–716.

Monto, A. S., and Arden, N. H. (1992). "Implications of Viral Resistance to Amantadine in Control of Influenza A." Clinical Infectious Diseases 15:362–367.

Monto, A. S., and Kioumehr, F. (1975). "The Tecumseh Study of Respiratory Illness. IX. Occurrence of Influenza in the Community, 1966–1971." American Journal of Epidemiology 102:553–563.

Patriarca, P. A.; Weber, J. A.; Parker, R. A. et al. (1985). "Efficacy of Influenza Vaccine in Nursing Homes: Reduction in Illness and Complications During an Influenza A (H₃N₂) Epidemic." Journal of the American Medical Association 253:1136–1139.

Sullivan, F. M.; Monto, A. S.; and Longini, I. M., Jr. (1993). "Estimates of the U.S. Health Impact of Influenza." American Journal of Public Health 83: 1712–1716.

Wilde, J. A.; McMillan, J. A.; Serwint, J.; Butta, J; O'Riordan, M. A.; and Steinhoff, M. C. (1999). "Effectiveness of Influenza Vaccine in Health Care Professionals: A Randomized Trial." Journal of the American Medical Association 281:908–913.

— ARNOLD MONTO

For more information on influenza, visit Britannica.com.

Influenza, commonly called "the flu," reached America early in colonial history, and its periodic visitations have continued since then. John Eliot described the first epidemic, which struck in 1647, as "a very depe cold, with some tincture of a feaver and full of malignity.…" In the succeeding years a series of outbreaks, described in such terms as "a general catarrh," "winter feavers," "epidemical colds," and "putrid pleurisies," swept through the colonies, bringing death on a large scale. The cause of these epidemics remains unknown, but from accounts of the symptoms and the pandemic nature of the outbreaks, some strain of influenza is a logical suspect. Colonial records show many local outbreaks, with some form of respiratory disease reaching major epidemic proportions in 1675, 1688, 1732–1733, 1737, 1747–1750, 1761, and 1789–1791.

The nineteenth century saw a similar pattern of influenza epidemics—major pandemics interspersed with local or regional outbreaks. The disease spread through Europe and America in 1830, 1837, and 1847, eased up for a long period, and then broke out on a worldwide scale from 1889 to 1893.There were two minor outbreaks involving an unusual number of pneumonic complications in 1916 and 1917.In the summer of 1918, a deceptively mild wave of influenza swept through army camps in Europe and America, immediately followed by the second and third waves of the greatest recorded pandemic of influenza in history. In America the heaviest toll was exacted by a major wave lasting from September to November of 1918; the pandemic killed an estimated 15 million individuals worldwide. In the United States, the disease infected approximately 28 percent of the population, killing 450,000, with half of the deaths occurring among young adults between the ages of twenty and forty.

Several outbreaks struck in the 1920s, but the morbidity and mortality from influenza gradually declined in the succeeding years, although a Metropolitan Life Insurance Company study showed that influenza combined with pneumonia consistently remained the third-ranking cause of death among its policyholders as late as 1935.

Various forms of influenza have persisted; rarely do as many as three years go by without a fairly serious outbreak. Most occurrences are minor, but once or twice every decade the disease flares up. The introduction of new therapeutics in the 1940s led to a steady drop in the overall influenza mortality rate until the outbreaks of Asiatic influenza in 1957, 1958, and 1960.The influenza death rate per 100,000 reached 4.4 in the latter year, the last time this figure exceeded 4 per 100,000.

In 1933 the influenza virus now known as influenza virus A was identified, and other strains were later discovered. Although the impact of influenza vaccines has been limited, the introduction of sulfonamides, penicillin, and antibiotics in the World War II era greatly improved the treatment for pneumonia associated with influenza and thus helped reduce the fatality rate from influenza. Improved sanitary standards have also most likely helped reduce the number and virulence of influenza outbreaks.

Bibliography

Bett, Walter R. The History and Conquest of Common Diseases. Norman: University of Oklahoma Press, 1954.

Crosby, Alfred W. America's Forgotten Pandemic: The Influenza of 1918. New York: Cambridge University Press, 1989.

Duffy, John. Epidemics in Colonial America. Baton Rouge: Louisiana State University Press, 1953.

Frost, W. H. "The Epidemiology of Influenza." Journal of the American Medical Association, 73 (1919): 313–318.

Kolata, Gina Bari. Flu: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus that Caused It. New York: Farrar, Straus and Giroux, 1999.

Commonly called the flu; an acute and infectious disease of the respiratory system caused by a virus and characterized by fever, muscle pain, headache, and inflammation of the mucous membranes in the respiratory tract.

An acute viral infection of the respiratory tract, occurring usually in epidemics, and pandemics. Influenza viruses are single-stranded RNA viruses belonging to the family Orthomyxoviridae, which contains three genera termed A, B and C. All of the viruses of interest to veterinarians are influenza type A viruses and include those causing equine influenza, swine influenza and avian influenza (fowl plague).
Common usage is to diagnose influenza or ‘flu’ in many nonspecific respiratory infections in animals or those caused by other viruses, but this is etiologically incorrect. Typical examples are cat flu and goose influenza.