Typically, the symptoms of influenza appear quite suddenly about one or two days after infection. The first symptoms are usually chills, often accompanied by a fever, with body temperatures ranging from 38–39°C. In many cases, the patient may be so ill that they re confied to bed for several days with pains throughout their bodies.
Typical symptoms of influenza infection include:
During the early stages, colds and flu often present with similar symptoms, however, a flu can typically be identified by a high fever with a sudden onset and extreme fatigue. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal congestion can improve diagnostic accuracy.
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The period that a person is infective with influenza (known as virus shedding) begins the day before symptoms appear and virus is then released for between 5 to 7 days, although some people may shed virus for longer periods. People who contract influenza are most infective between the second and third days after infection. The amount of virus shed appears to correlate with fever, with higher amounts of virus shed when temperatures are highest. Children are much more infectious than adults and shed virus from just before they develop symptoms until two weeks after infection. The influenza virus is spread in three main ways: By direct transmission — this is whan an infected person sneezes mucus directly into the eyes, nose or moth of another person. By the airbourne route — when someone inhales the aerosol particles produced by an infected person coughing, sneezing or spitting. Though han to mucous membrane traismission — this is when a hand contaminated with flu firus directly touches the eye, nos or mouth. This can be through coming into contact with contaminated surfaces or with personal contact, such as a handshake. How long influenza survives in airborne droplets seems to be influenced by the levels of humidity and UV radiation: with low humidity and a lack of sunlight in winter aiding its survival. As the influenza virus can persist outside of the body, it can also be transmitted by contaminated surfaces such as banknotes, doorknobs, light switches and other household items. The length of time the virus will persist on a surface varies, with the virus surviving for one to two days on hard, non-porous surfaces such as plastic or metal, for about fifteen minutes from dry paper tissues, and only five minutes on skin. How Influenza Viruses Cause DiseaseOne part of the process by which influenza viruses invades human cells is the cleavage of the viral hemagglutinin protein by any one of several human proteases.[44] In mild and avirulent viruses, the structure of the hemagglutinin means that it can only be cleaved by proteases found in the throat and lungs, so these viruses cannot infect other tissues. However, in highly virulent strains, such as H5N1, the hemagglutinin can be cleaved by a wide variety of proteases, allowing the virus to spread throughout the body. The viral hemagglutinin protein is responsible for determining both which species a strain can infect and where in the human respiratory tract a strain of influenza will bind. However, many of the symptoms of influenza are actually the direct result of the immune system's efforts at eliminating the virus. With the common symptoms of the flu such as fever, headaches, and fatigue are the result of the huge amounts of proinflammatory cytokines and chemokines (such as interferon or tumor necrosis factor) produced from influenza-infected cells. It should be noted though, that in contrast to rhinoviruses that cuase the common cold, influenza does cause tissue damage, so symptoms are not entirely due to the inflammatory response. This massive immune response might produce a life-threatening cytokine storm and this effect has been proposed by the mechanism by which certain flu starins can become lethal (such as happened in the 1918 flu pandemic). |
Vaccination against various influenza strains is common for high risk groups (children and the elderly, or in people who have asthma, diabetes, heart disease, or are immuno-compromised) in the developed world.
The most common human vaccine is the trivalent influenza vaccine (TIV) that contains purified and inactivated antigens against three viral strains. Typically, this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain. The TIV carries no risk of transmitting the disease, and it has very low reactivity.
Every year, the World Health Organization predicts which strains of the virus are most likely to be circulating in the next year, allowing pharmaceutical companies to develop vaccines that will provide the best immunity against these strains. Vaccines have also been developed to protect poultry from avian influenza. These vaccines can be effective against multiple strains and are used either as part of a preventative strategy, or combined with culling in attempts to eradicate outbreaks
However, as the vaccine is based on a prediction of which strains are likely to be prevalent in a given year (and only some serotypes are included) it is possible to be vaccinated against influenza, but still get the disease. Indeed, though the vaccine itself is reformulated each season for a few specific flu strains but cannot possibly include all the strains actively infecting people in the world for that season. It takes about six months for the manufacturers to formulate and produce the millions of doses required to deal with the seasonal epidemics; occasionally, a new or overlooked strain becomes prominent during that time and infects people although they have been vaccinated.
One of the most effective ways of preventing infection in the first place is to change personal habits. This includes good personal health and hygiene habits such as: not touching your eyes, nose or mouth; frequent hand washing (with soap and water, or with alcohol-based hand rubs); covering coughs and sneezes; avoiding close contact with sick people; and staying home yourself if you are sick. Avoiding spitting is also recommended. Smoking increases the risk of contracting influenza, as well as producing more severe disease symptoms.
Since influenza spreads through both aerosols and contact with contaminated surfaces, surface sanitizing may help prevent some infections. Alcohol is an effective disinfectant and when used in conjunction with quaternary ammonium compouds, the sanitizing effects can be prolonged.
Typically, those suffering from influenza are advised to get plenty of rest, drink plenty of liquids, avoid using alcohol and tobacco and, if necessary, take medications such as acetaminophen (paracetamol) to relieve the fever and muscle aches associated with the flu. Children and teenagers with flu symptoms (particularly fever) should avoid taking aspirin during an influenza infection (especially influenza type B), because doing so can lead to Reye's syndrome, a rare but potentially fatal disease of the liver.
Because influenza is a viral disease (just like the common cold), antibiotics HAVE NO EFFECT on the influenza viruses and should not be prescribed. They are only useful to treat secondary bacterial infections (such as bacterial penumonia).
It should be noted, though, that antiviral medications can be effective against influenza (particularly if taken early), though some strains of influenza are beginning to show resistence to the standard antivirals).
Typical antivirals include:
Neuraminidase inhibitors — drugs such as oseltamivir (Tamiflu) and zanamivir (Relenza) are neuraminidase inhibitors that are designed to halt the spread of the viruses in the body and are typically effective against influenza A and B strains.
M2 Inhibitors — Drugs such as amantadine and rimantidine are effective as they block a viral ion channel (M2) and thus prevent the viruses from infecting cells in the host in the first palce. These drugs are partially effective against influenza A, if given early, but have no effect against influenza B (influenza B viruses do not posess M2 molecules). But many strains are resistant to this class of medicatons.
It is well known that influenza is a seasonal disease, with the peak prevalence in winter. Of course, as the Northern and Southern Hemispheres have opposing winters this means that any given year actually has two different flu seasons (one in the Southern Hemisphere and one in the Northern Hemisphere).
One possible explanation for the seasonality of influenza infection is that people tend to be indoors more often during the winter and are thus in closer proximity, thus increasing the risk of transmission from person to person. Another factor is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus also survives longer on surfaces at colder temperatures and aerosol transmission of the virus is highest in cold environments (less than 5 °C) with low relative humidity (which may well be the main cause of seasonal influenza transmission in temperate regions of the world).
There may also be a link between infection and vitamin D levels. The proposal being that the main cause of influenza epidemics during the winter is connected to the seasonal fluctuations in the levels of vitamin D in the human body. This vitamin is produced in the skin under the influence of solar (or artificial) UV radiation. This could explain why influenza occurs mostly in winter and during the tropical rainy season, when people stay indoors, away from the sun, and their vitamin D levels fall.
In a normal year's flu seasons there are between three and five million cases of severe illness and up to 500,000 deaths worldwide. Roughly three times per century, a pandemic occurs, which infects a large proportion of the world's population and can kill tens of millions of people (see history section). Indeed, one study estimated that if a strain with similar virulence to the 1918 influenza emerged today, it could kill between 50 and 80 million people.
Flu pandemics seem to have occured sporadically throughout human history, though the first convincing record of an influenza pandemic was of an outbreak in 1580, which began in Russia and spread to Europe via Africa. In Rome, over 8,000 people were killed, and several Spanish cities were almost wiped out. Pandemics continued sporadically throughout the 17th and 18th centuries, with the pandemic of 1830–1833 being particularly widespread; it infected approximately a quarter of the people exposed.
The most famous (and most lethal) outbreak to date was the 1918 flu pandemic (also known as the Spanish flue pandemic), caused by type A influenza, H1N1 subtype. This lasted over the winter of 1918/1919. It is not known exactly how many it killed, but estimates range from 20 to 100 million people. The 1918 flu pandemic was truly global, with some of the first cases described in West Africa, and it spreadeven to the Arctic and remote Pacific islands. The unusually severe disease killed between 2 and 20% of those infected, as opposed to the more usual flu epidemic mortality rate of 0.1%.
This huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms.[164] Indeed, symptoms in 1918 were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, 'One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred'. Though it should be noted that the majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive haemorrhages and oedema in the lung.
Another unusual feature of this pandemic was that it mostly killed young adults, with 99% of pandemic influenza deaths occurring in people under 65, and more than half in young adults 20 to 40 years old. This is unusual since influenza is normally most deadly to the very young (under age 2) and the very old (over age 70).
Influenza infects mammals and brids and the transfer of viral strains between species can occur. Indeed, birds are thought to be the main animal reservoir of the disease. Variants of flu virus are sometimes named according to the species the strain is endemic in or adapted to. The main variants named using this convention are: Bird Flu, Human Flu, Swine Flu, Horse Flu and Dog Flu.
Direct transmission of an influenza virus from pigs to humans is occasionally possible (this is called zoonotic swine flu). In all, 50 human cases are known to have occurred since the virus was identified in the mid-20th century, which have resulted in six deaths. The 1918 flu is now thought to have originated in swine herds kept close to army bases.
In 2009, a swine-origin H1N1 virus strain commonly referred to as "swine flu" caused the 2009 flu pandemic, but there is no evidence that it is endemic to pigs (i.e. actually a swine flu) or of transmission from pigs to people, instead the virus is spreading from person to person. This strain is a reassortment of several strains of H1N1 that are usually found separately, in humans, birds, and pigs.
The influenza viruses are RNA viruses belonging to the Orthomyxoviridae family. There are three genera of influenza viruses that affect humans: Influenza A, Influenza B and Influenza C.
This genus has a single species, influenza A virus. Wild aquatic birds are the natural hosts for thhe majority of influenza A types. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics. The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
H1N1, which caused Spanish Flu in 1918, and Swine Flu in 2009
H2N2, which caused Asian Flu in 1957
H3N2, which caused Hong Kong Flu in 1968
H5N1, which caused Bird Flu in 2004
H7N7, which has unusual zoonotic potential
H1N2, endemic in humans, pigs and birds
H9N2
H7N2
H7N3
H10N7
This genus has a single species, influenza B virus. Influenza B almost exclusively infects humans and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the seal and the ferret. This type of influenza mutates at a rate 2–3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.
This genus has one species, influenza C virus, which infects humans, dogs and pigs, sometimes causing both severe illness and local epidemics. However, influenza C is less common than the other types and typically only causes mild disease in children.
Influenzaviruses A, B and C are very similar in overall structure. The virus particle is 80–120 nanometers in diameter and usually roughly spherical, although filamentous forms can occur. These filamentous forms are more common in influenza C, which can form cordlike structures up to 500 micrometers long on the surfaces of infected cells. However, despite these varied shapes, the viral particles of all influenza viruses are similar in composition. These are made of a viral envelope containing two main types of glycoproteins, wrapped around a central core. The central core contains the viral RNA genome and other viral proteins that package and protect this RNA. RNA tends to be single stranded but in special cases it is double. Unusually for a virus, its genome is not a single piece of nucleic acid; instead, it contains seven or eight pieces of segmented negative-sense RNA, each piece of RNA containing either one or two genes, which code for a gene product (protein).
Viruses are essentially too simple to replicate (copy) themselves on their own. To reproduce they need to hijack the cellular machinery of a more complex cell. This is why viruses infect other organisms, so that they can use the cells of those organisms to make copies of themselves.
This is why viruses cause disease.
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