Influenza Virus AKA The Flu
Influenza is an acute infectious virus disease of the respiratory tract. Influenza, which is commonly known as flu or grippe, is extremely contagious and often occurs in epidemics. Generally, influenza epidemics occur in the winter or early spring. Depending on the type of virus and the population's state of immunity, the disease may affect entire cities, states, countries, continents, or the entire world (pandemic) in a relatively short time.
Although the highest incidence of influenza occurs among school children, all age groups are affected. The impact of influenza epidemics is demonstrated by high rates of absenteeism in schools and industries. The effect of this disease ;an also be shown by the excessive mortality rates that result from each epidemic. Most of he severe illnesses and deaths are among the elderly and people with chronic diseases, but some young healthy persons have also died from complications attributed to influenza. Deaths resulting from the pandemic of 1918-1919 numbered in the millions, and in subsequent epidemics, including the one in 1957, death figures reached the thousands.
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Cause of Influenza
There are two distinct but similar types of viruses that cause influenza; both are members of a group of viruses known as the myxoviruses. This group also includes the viruses that cause mumps and measles.
The two basic influenza viruses are designated as type A and type B. They are medium-sized viruses about 100 millimicrons in diameter and contain ribonucleic acid (RNA) and an enzyme, neuraminidase, in their protein coat. Although they are closely related, an infection with one type does not confer immunity to the other.
Both viruses can be readily inactivated by a variety of disinfectants, by solutions that are acidic or alkaline, or by exposure to a temperature of 56C (about 133F) for a few minutes. They can be propagated in living chick embryos, mice, ferrets, and certain types of tissue cultures. In addition, both types have the ability to agglutinate (clump together) red blood cells, a property that provides a practical laboratory method of diagnosing the disease.
Another characteristic of influenza viruses is their ability to alter their structure slightly whenever their host population becomes immune to them. Since 1933, when type A virus was first isolated, two new strains have appeared. One of these variants, called type Al, was discovered in 1947, and the second variant, called A2, was found ten years later. Type B virus was first isolated in 1940, and variants were found in 1954 and 1962.
Type A influenza viruses differ from type B viruses in that they cause larger epidemics, including pandemics, and produce more severe illnesses and higher death rates. Another major difference is the frequency of the epidemics caused by the two virus types. Epidemics caused by type A viruses occur at intervals of two or three years while epidemics caused by type B viruses occur at intervals of three to six years.
Transmission of Influenza
Once an outbreak has started, the disease spreads rapidly. A healthy person acquires the viruses in droplets of respiratory secretions that are dispersed when an infected person coughs or sneezes. Relatively large amounts of virus are expelled this way, and even people with asymptomatic or mild infections spread the disease in this manner.
Once the viruses are inhaled, they become implanted on the cells lining the respiratory tract and begin to multiply. If the person is immune to the virus, some of the antibody in his bloodstream enters the area and neutralizes the invading virus particles. If the person is not immune to the virus, the particles destroy the cells lining the upper respiratory tract, the trachea (windpipe), and the bronchial tubes. The cells primarily involved are the ciliated epithelial cells, and it is the destruction of these cells that produces the respiratory symptoms of the disease. The factors causing the systemic, or generalized, symptoms of influenza are not so evident. Since the viruses themselves are not found in the bloodstream of infected people, they cannot affect other parts of the body as they do the respiratory tract. However, it is believed that the breakdown products of the epithelial cells and virus particles are absorbed into the blood and may produce the fever and aching associated with influenza.
Symptoms of Influenza
Following an incubation period of one to two days, there is a sudden onset of symptoms, with chills or chilliness, fever ranging from 100F to 105F (38C to 41C), headache, backache, muscular aching, loss of appetite, and a general feeling of illness. Other generalized symptoms frequently occur, including weakness, prostration, nausea, and eye pain or burning sensations hi the eyes, especially when exposed to light. Symptoms involving the respiratory tract are usually present at the onset but they may not be prominent. Among the early respiratory symptoms are a feeling of dryness in the throat with a sensation of irritation or fullness, a dry cough, and a discomfort or slight burning sensation in the trachea. Sometimes there is also a watery nasal discharge, severe sore throat, vomiting, or mental confusion.
The patient's fever continues from one to five days. Then, as his temperature begins to decline, the respiratory symptoms become more prominent. The nasal discharge increases, and the cough becomes deep and produces mucus. General weakness and coughing may continue for several days.
Diagnosis of Influenza
The definitive diagnosis of influenza often entails isolating the virus from throat washings, cultivating it in the laboratory, usually in chick embryos or rhesus monkey kidney tissue cultures, and finally identifying it. A more rapid diagnostic test can be done with material obtained from the patient's throat with a throat swab. This material is streaked on a microscope slide and treated with anti-serum. If the virus is present, it will appear when the slide is viewed through a microscope with ultraviolet light.
Treatment of Influenza
There is no specific treatment for influenza, but bed rest and supportive measures to control fever and reduce pain are necessary. Unless complications arise, antibiotics, such as penicillin, are not given. The most important aspect of treatment concerns the early detection of complications. Although influenza itself is not considered a serious disease, it sometimes leads to serious complications, especially in the elderly, pregnant women, infants, and those suffering from chronic diseases. The most common complications of influenza are bronchitis and bacterial pneumonia. Less frequent complications include encephalitis, virus pneumonia, and myocarditis (inflammation of the heart muscle). Early danger signs include increased rate of respiration, sudden weakness, blueness of the lips and tongue, chest pain, and an increase in sputum containing pus or blood.
Prevention of Influenza
There are no suitable methods of avoiding contact with the virus when an influenza epidemic occurs. The closing of schools is not helpful in reducing the spread of the disease, but avoiding crowded areas is a reasonable attempt at avoiding infection.
The most effective method of preventing influenza in all age groups is vaccination, and most influenza vaccines are prepared with both type A and type B viruses. The viruses are propagated in chick embryos and are then purified, concentrated, and inactivated. Thus, when a person is vaccinated, he develops antibodies against the viruses without developing the acute form of the disease. Sometimes, however, mild influenza symptoms, such as muscular aching and fever, may appear after vaccination. Researchers have therefore developed an experimental vaccine that produces no symptoms. Another experimental vaccine contains a substance that prolongs the duration of immunity, and in the Soviet Union a vaccine containing attenuated (weakened) live viruses has been used.
Because a person's immunity to influenza falls rather rapidly a few months after he has been vaccinated or has recovered from the disease, it is necessary to be vaccinated each year, preferably in the fall. Although vaccination is the best method for preventing influenza, prophylactic drugs have been developed for at least one virus strain— type A2.
Another aspect of influenza prevention concerns a continuous worldwide watch for influenza outbreaks and the immediate identification of any new virus strains. Once a new strain is discovered, it is extremely important that it be incorporated into influenza vaccines to prevent a spreading epidemic. The two major organizations concerned with locating influenza outbreaks and classifying new strains are the World Health Organization (WHO) and the U.S. Public Health Service.