In?uenza is an acute infectious disease, characterised by a sudden onset, fever and generalised aches and pains. It usually occurs in epidemics and pandemics (see EPIDEMIC; PANDEMIC).
Cause The disease is caused by a VIRUS of the in?uenza group. There are at least three types of in?uenza virus, known respectively as A, B and
C. One of their most characteristic features is that infection with one type provides no protection against another. Equally important is the ease with which the in?uenza virus can change its character. It is these two characteristics which explain why one attack of in?uenza provides little, if any, protection against a subsequent attack, and why it is so di?cult to prepare an e?ective vaccine against the disease.
Epidemics of in?uenza due to virus A occur in Britain at two- to four-year intervals, and outbreaks of virus B in?uenza in less frequent cycles. Virus A in?uenza, for instance, was the prevalent infection in 1949, 1951, 1955 and 1956, whilst virus B in?uenza was epidemic in 1946, 1950, 1954 and, along with virus A, in 1958–59. The pandemic of 1957, which swept most of the world, although fortunately not in a severe form, was due to a new variant of virus A
– the so-called Asian virus – and it has been suggested that it was this variant that was responsible for the pandemics of 1889 and 1918. Since 1957, variants of virus A have been the predominating causes of in?uenza, accompanied on occasions by virus B.
In 1997 and 2004, outbreaks of Chinese avian in?uenza caused alarm. The in?uenza virus had apparently jumped species from birds
– probably chickens – to infect some people. Because no vaccine is available, there was a risk that this might start an epidemic.
Symptoms The incubation period of in?uenza A and B is 2–3 three days, and the disease is characterised by a sudden onset. In most cases this is followed by a short, sharp febrile illness of 2–4 days’ duration, associated with headache, prostration, generalised aching, and respiratory symptoms. In many cases the respiratory symptoms are restricted to the upper respiratory tract, and consist of signs of irritation of the nose, pharynx and larynx. There may be nosebleeds, and a dry, hacking cough is often a prominent and troublesome symptom. The fever is usually remittent and the temperature seldom exceeds 39·4 °C (103 °F), tending to ?uctuate between 38·3 and 39·4 °C (101 and 103 °F).
The most serious complication is infection of the lungs. This infection is usually due to organisms other than the in?uenza virus, and is a complication which can have serious results in elderly people.
The very severe form of ’?u which tends to occur during pandemics – and which was so common during the 1918–19 pandemic – is characterised by the rapid onset of bronchopneumonia and severe prostration. Because of the toxic e?ect on the heart, there is a particularly marked form of CYANOSIS, known as heliotrope cyanosis.
Convalescence following in?uenza tends to be prolonged. Even after an attack of average severity there tends to be a period of weakness and depression.
Treatment Expert opinion is still divided as to the real value of in?uenza vaccine in preventing the disease. Part of the trouble is that there is little value in giving any vaccine until it is known which particular virus is causing the infection. As this varies from winter to winter, and as the protection given by vaccine does not exceed one year, it is obviously not worthwhile attempting to vaccinate the whole community. The general rule therefore is that, unless there is any evidence that a particularly virulent type of virus is responsible, only the most vulnerable should be immunised – such as children in boarding schools, elderly people, and people who suffer from chronic bronchitis or asthma, chronic heart disease, renal failure, diabetes mellitus or immunosuppression (see under separate entries). In the face of an epidemic, people in key positions, such as doctors, nurses and those concerned with public safety, transport and other public utilities, should be vaccinated.
For an uncomplicated attack of in?uenza, treatment is symptomatic: that is, rest in bed, ANALGESICS to relieve the pain, sedatives, and a light diet. A linctus is useful to sooth a troublesome cough. The best analgesics are ASPIRIN or PARACETAMOL. None of the sulphonamides or the known antibiotics has any e?ect on the in?uenza virus; on the other hand, should the lungs become infected, antibiotics should be given immediately, because such an infection is usually due to other organisms. If possible, a sample of sputum should be examined to determine which organisms are responsible for the lung infection. The choice of antibiotic then depends upon which antibiotic the organism is most sensitive to.... influenza
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