Medicine for Africa - Medical Information Service
Influenza, often also just called ‘the flu’, is a viral illness affecting primarily the respiratory tract, and thus, not to be confused with a normal cold, which is caused by various bacteria. The influenza viruses are members of the orthomyxoviridiae family of viruses.
Every year during the winter months, there is an explosive increase of influenza cases; one in ten adults and almost every third child become infected with the influenza virus. The incubation period of influenza (time between infection and appearance of symptoms) is usually about 1 to 3 days.
Especially affected are children in schools or crèches, people with frequent interpersonal contacts, airline travelers, but also older people and people with chronic diseases (e.g. asthma), as well as people with cardiovascular diseases or with a compromised immune system.
The influenza virus has originally been an ‘avian’ virus, meaning that it was present in birds only. However, at some point in time, it mutated and crossed over into mammals. Now, the virus also affects pigs, horses and humans.
The virus usually spreads from host to host by way of droplets derived from sneezing, coughing and the likes, but also via direct contact (e.g. doorknobs that have been touched by a sick person after s/he sneezed into the hand).
The influenza viruses are divided into three major types – type A, B and C. Influenza type A or type B are the major culprits for seasonal influenza outbreaks, as well as the cause of occasional epidemics of various seriousness; type C usually causes only a very mild respiratory illness or even no symptoms at all.
The influenza virus is an enveloped single-stranded RNA (ribonucleic acid) virus, which is encased (’enveloped’) by two types of surface proteins, which are called hemagglutinin (H) and neuraminidase (N). The ‘H’ protein enables the virus to attach itself to the membranes of cells, such as respiratory mucosa and red cells, while the box-shaped ‘N’ protein makes up the enzymatic properties of the virus.
At this point in time, 16 known H subtypes and nine known N subtypes have been identified to be present in the influenza virus A, the major type of most influenza outbreaks.
These surface proteins can occur in different combinations, resulting in a large variety of different viral subtypes. Since the influenza viruses have the ability to continuously mutate (change) in their viral RNA make-up, they are able to constantly evade the immune system of their hosts (birds, humans and other animals), which makes any host susceptible to a new variant of influenza virus throughout the host’s life.
The viral changes (mutations) are the cause of regional epidemics that occur from time to time, in spite of influenza vaccines. Major antigenic shifts of both the ‘H’ and ‘N’ proteins can often result in worldwide epidemics, called a pandemic.
As the virus enters the cell by ‘endocytosis’ (engulfing), viral RNA derived proteins are released into the cytoplasm and transported into the cell’s nucleus. Then, new viral proteins are translated from messenger RNA (mRNA), and the new viral RNA, encased in a capsid protein, is transported to the cell surface and released by ‘budding’ (‘exocytosis’, expulsion) from the cell, releasing a new virus into the body’s circulation. During this process, the host’s cell does not die, at least not for a while.
The influenza virus has its genome ‘packaged’ in eight different segments, allowing it to interchange one or more gene segments with other influenza viruses that infected the same cell. These ‘recombinants’ contribute to the rapid development of new influenza strains over a relatively short period of time (months to a few years). This characteristic also makes it difficult to assure that last year’s influenza vaccine will be 100% effective for the coming year’s influenza type, although it usually provides some protection because of the genome overlap (see Prevention).
However, from time to time, it is likely that the virus mutates into an almost completely new combination of genomes, and then, it has the potential of creating an epidemic or, rarely a pandemic.
Since the viral envelope is a lipoprotein, it makes the virion (the viral ‘nucleus’) susceptible to heat, detergents, solvents and drying. Thus, general hygiene and hygienic handling of all communal surfaces can help to prevent the spread of the influenza virus considerably.
Epidemic influenza remains the biggest and unconquered acute threat to human health, inflicting damage and death far beyond familiar notification data. The impact of influenza A is particularly severe during periodic pandemics owing to novel antigenic variants which override immunity from experience of earlier subtypes.
Mild cases of influenza, the ‘flu’, often present itself like a common cold. However, most cases of the flu can be distinguished from a cold, because its symptoms are more severe, usually occur suddenly and include high fever – temperatures of 39°C – 40°C or 102°F – 104°F, or more (even more in children), and other symptoms, such as:
The incubation period (time between infection and appearance of symptoms) is short, usually only one to three days; rapid spread of the virus may lead to epidemics.
In contrast to the common cold, where a runny nose (coryza) is a typical symptom, coryza is not present in cases of influenza.
In simple cases of influenza, those acute symptoms usually subside within about one week (three to five days); however, a feeling of exhaustion, sometimes accompanied by personal performance decrease, can persist for several weeks thereafter.
Complications associated with influenza can occur at any age, but typically surface in elderly people and people with chronic cardio-pulmonary diseases or other immune suppressing illnesses, and can include:
These complications usually appear only after the acute infection period has passed (reconvalescence), can be independent of the severity of the acute infection, and can become terminal (end in death).
The influenza virus can be diagnosed by various laboratory tests:
There is no treatment for viral influenza. At times, antibiotics are given to prevent or cure bacterial superinfections (see Symptoms).
Antibiotics should not be given automatically, as they can also kill other bacteria, thus ‘paving’ the way for the virus to multiply itself easier and even faster (less ‘competition’ by bacteria).
There are vaccines available against the influenza virus. Since the virus changes itself constantly, the latest vaccine is developed on the basis of the antigenic composition of the last year’s influenza virus. However, since this vaccine is based on last year’s version of the virus, it does not provide 100% effective protection against the current version, because of the above mentioned constant antigenic changes of the virus. Nevertheless, the vaccination is recommended, especially for older people or people with chronic and/or immune suppressing diseases, because there is usually some cross-over of antigenicity and thus, at least a partial protection (usually about 70%) to the newest type of viral influenza.
There are several types of vaccines available:
If there is a large antigenic shift (mutation) between last year’s influenza virus and the latest viral type, widespread infection can lead to epidemics and even pandemics (worldwide epidemics) with severe consequences.
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