influenza

—influenzal, adj. —influenzalike, adj.

/in'flooh en"zeuh/, n.

1. Pathol. an acute, commonly epidemic disease, occurring in several forms, caused by numerous rapidly mutating viral strains and characterized by respiratory symptoms and general prostration. Cf. flu.

2. Vet. Pathol. an acute, contagious disease occurring in horses and swine, characterized by fever, depression, and catarrhal inflammations of the eyes, nasal passages, and bronchi, and caused by a virus.

[1735-45; < It < ML influentia INFLUENCE]

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or flu or grippe

Acute viral infection of the upper or lower respiratory tract.

Influenza viruses A (the most common), B, and C produce similar symptoms, but infection with or vaccination against one does not give immunity against the others. Chills, fatigue, and muscle aches begin abruptly. The temperature soon reaches 38–40 °C (101–104 °F). Head, muscle, abdominal, and joint aches may be accompanied by sore throat. Recovery starts in three to four days, and respiratory symptoms become more prominent. Bed rest, high fluid intake, and aspirin or other antifever drugs are standard treatment. Influenza A tends to occur in wavelike annual pandemics. Mortality is usually low, but in rare outbreaks (see influenza epidemic of 1918–19) it reaches immense proportions. Most deaths result from pneumonia or bronchitis.

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▪ disease

also called  flu  or  grippe 

      an acute viral infection of the upper or lower respiratory tract that is marked by fever, chills, and a generalized feeling of weakness and pain in the muscles, together with varying degrees of soreness in the head and abdomen.

      Influenza is caused by any of several strains of orthomyxoviruses, categorized as types A, B, and C. The three major types generally produce similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against the others. The A viruses cause the great influenza epidemics, and the B viruses cause smaller localized outbreaks; the C viruses are not important causes of disease in humans. Between pandemics, the viruses undergo constant, rapid evolution (a process called antigenic drift) in response to the pressures of human population immunity. Periodically, they undergo major evolutionary change by acquiring a new genome segment from another influenza virus (antigenic shift), effectively becoming a new subtype to which none, or very few, of the population is immune.

      Influenza type A virus is the most frequent cause of influenza; this type occurs in numerous strains or subtypes that are differentiated mainly on the basis of two surface proteins, hemagglutinin (H) and neuraminidase (N). An antigenic shift in the influenza A virus can produce a pandemic affecting most of the world within a matter of months. For example, the influenza epidemic of 1918–19, the most destructive influenza outbreak in history and one of the most severe disease epidemics ever encountered, was caused by a subtype of influenza A known as H1N1. During this epidemic an estimated 25 million persons throughout the world died of the so-called Spanish flu, which was first widely reported in Spain but seems to have originated in the United States. Subsequent outbreaks of influenza have been much less severe. Influenza A subtype H2N2, or Asian flu, for instance, apparently began in East Asia early in 1957, and by midyear it had circled the globe. After 10 years of evolution that produced annual epidemics, the Asian flu disappeared in 1968, only to be replaced by a new influenza A subtype, H3N2. This virus, also known as Hong Kong flu, is still in circulation. In 1997 a type of avian influenza (bird flu), or bird flu, virus broke out among domesticated poultry in Hong Kong and then infected a small number of people, killing some of them. This same virus, H5N1, reappeared among chicken flocks in Southeast Asia during the winter of 2003–04, again infecting some people fatally.

      The flu may affect individuals of all ages, though the highest incidence of the disease is among children and young adults. Influenza is generally more frequent during the colder months of the year. Infection is transmitted from person to person through the respiratory tract, by such means as inhalation of infected droplets resulting from coughing and sneezing. As the virus particles gain entrance to the body, they selectively attack and destroy the ciliated epithelial cells that line the upper respiratory tract, bronchial tubes, and trachea. The incubation period of the disease is one to two days, after which the onset of symptoms is abrupt, with sudden and distinct chills, fatigue, and muscular aches. The temperature rises rapidly to 38 to 40 °C (101 to 104 °F). A diffuse headache and severe muscular aches throughout the body are experienced, often accompanied by irritation or a sense of rawness in the throat. In three to four days the temperature begins to fall and the person begins to recover. Symptoms associated with respiratory tract infection, such as coughing and nasal discharge, become more prominent and may be accompanied by lingering feelings of weakness. Mortality from the flu is often high because so many people are infected. Death is caused, in most cases, by complications such as pneumonia or bronchitis, usually among older people who are weakened by other debilitating disorders.

      The antiviral drug amantadine has a beneficial effect on cases of influenza involving the type A virus. A newer category of drugs, the neuraminidase inhibitors, was introduced in the late 1990s; these drugs inhibit both the influenza A and B viruses. Other than this, bed rest, ingestion of fluids, and the use of analgesics to control fever remain the standard treatment. It is recommended that children and teenagers with the flu not be given aspirin, as treatment of viral infections with aspirin is associated with Reye syndrome, a very serious illness. Individual protection against the flu may be bolstered by injection of a vaccine containing two or more circulating influenza viruses. These viruses are produced in chick embryos and rendered noninfective; standard commercial preparations ordinarily include the type B influenza virus and several of the A subtypes. Protection from one vaccination seldom lasts more than a year, and yearly vaccination may be recommended, particularly for those individuals who are unusually susceptible to influenza or whose weak condition could lead to serious complications in case of infection. Routine immunization in healthy people, however, is also recommended. In order to prevent human-infecting bird flu viruses from mutating into more dangerous subtypes, public health authorities try to limit the viral “reservoir” where antigenic shift may take place by ordering the destruction of infected poultry flocks.

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