Influenza A virus subtype H1N1, also known as
A(H1N1), is a subtype of
influenzavirus A and the most common cause of
influenza (flu) in humans. Some strains of H1N1 are
endemic in humans and cause a small fraction of all
influenza-like illness and a large fraction of all
seasonal influenza. H1N1 strains caused roughly half of all human flu infections in 2006.
[1] Other strains of H1N1 are
endemic in pigs (swine influenza) and
in birds (avian influenza).
In June 2009, WHO declared that flu due to a new strain of swine-origin H1N1 was responsible for the 2009 flu pandemic. This strain is commonly called "swine flu".
nfluenza, commonly referred to as the
flu, is an
infectious disease caused by
RNA viruses of the
family Orthomyxoviridae (the influenza viruses), that affects
birds and
mammals. The name
influenza comes from the
Italian influenza, meaning "influence" (
Latin:
influentia). The most common symptoms of the disease are
chills,
fever,
sore throat,
muscle pains, severe
headache,
coughing,
weakness and
general discomfort.
[1] Fever and coughs are the most frequent symptoms. In more serious cases, influenza
causes pneumonia, which can be fatal, particularly for the young and the elderly. Although it is often confused with other
influenza-like illnesses, especially the
common cold, influenza is a much more severe disease than the common cold and is caused by a different type of virus.
[2] Influenza may produce
nausea and
vomiting, particularly in children,
[1] but these symptoms are more common in the unrelated
gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu".
[3] Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols containing the virus. Influenza can also be transmitted by bird droppings, saliva, nasal secretions, feces and blood. Infection can also occur through contact with these body fluids or through contact with contaminated surfaces. Airborne aerosols have been thought to cause most infections, although which means of transmission is most important is not absolutely clear. Influenza viruses can be inactivated by sunlight, disinfectants and detergents.[4][5] As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection.
Influenza spreads around the world in seasonal epidemics, resulting in the deaths of hundreds of thousands annually — millions in pandemic years. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus in humans. Often, these new strains appear when an existing flu virus spreads to humans from other animal species, or when an existing human strain picks up new genes from a virus that usually infects birds or pigs. An avian strain named H5N1 raised the concern of a new influenza pandemic, after it emerged in Asia in the 1990s, but it has not evolved to a form that spreads easily between people.[6] In April 2009 a novel flu strain evolved that combined genes from human, pig, and bird flu, initially dubbed "swine flu", emerged in Mexico, the United States, and several other nations. WHO officially declared the outbreak to be a "pandemic" on June 11, 2009.
Vaccinations against influenza are usually given to people in developed countries [7] and to farmed poultry.[8] The most common human vaccine is the trivalent influenza vaccine (TIV) that contains purified and inactivated material from three viral strains. Typically, this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain.[9] The TIV carries no risk of transmitting the disease, and it has very low reactivity. A vaccine formulated for one year may be ineffective in the following year, since the influenza virus evolves rapidly, and new strains quickly replace the older ones. Antiviral drugs can be used to treat influenza, with neuraminidase inhibitors being particularly effective.
Treatments for influenza include a range of medications and therapies that are used in response to disease influenza. Treatments may either directly target the influenza virus itself; or instead they may just offer relief to symptoms of the disease, while the body's own immune system works to recover from infection.[1]
The two main classes of antiviral drugs used against influenza are neuraminidase inhibitors, such as zanamivir and oseltamivir, or inhibitors of the viral M2 protein, such as amantadine and rimantadine. These drugs can reduce the severity of symptoms if taken soon after infection and can also be taken to decrease the risk of infection. However, viral stains have emerged that show drug resistance to both classes of drug.
The United States authority on disease prevention, the Centers for Disease Control and Prevention (CDC), recommends that persons suffering from influenza infections:
- Stay at home
- Get plenty of rest
- Drink a lot of liquids
- Do not smoke or drink alcohol
- Consider over-the-counter medications to relieve flu symptoms
- Consult a doctor early on for best possible treatment
- Remain alert for emergency warning signs[2]
Warning signs are symptoms that indicate that the disease is becoming serious and needs immediate medical attention. These include:
- Difficulty breathing or shortness of breath
- Pain or pressure in the chest or abdomen
- Dizziness
- Confusion
- Severe or persistent vomiting
In children other warning signs include irritability, failing to wake up and interact, rapid breathing, and a blueish skin color. Another warning sign in children is if the flu symptoms appear to resolve, but then reappear with fever and a bad cough.[2]
Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures ranging from 38-39 °C (approximately 100-103 °F).[45] Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.[1] Symptoms of influenza may include:
- Body aches, especially joints and throat
- Extreme coldness and fever
- Fatigue
- Headache
- Irritated watering eyes
- Reddened eyes, skin (especially face), mouth, throat and nose
- Abdominal pain (in children with influenza B)[46]
It can be difficult to distinguish between the common cold and influenza in the early stages of these infections,[2] but a flu can be identified by a high fever with a sudden onset and extreme fatigue. Diarrhea is not normally a symptom of influenza in adults,[44] although it has been seen in some human cases of the H5N1 "bird flu"[47] and can be a symptom in children.[48] The symptoms most reliably seen in influenza are shown in the table to the right.[44]
Most sensitive symptoms for diagnosing influenza[44] | Symptom: | sensitivity | specificity |
| Fever | 68-86% | 25-73% |
| Cough | 84-98% | 7-29% |
| Nasal congestion | 68–91% | 19–41% |
| Notes to table: - The ranges given represent different studies that were reviewed.
- Sensitivity is the proportion of people having influenza who exhibit the symptom.
- Specificity is the proportion of people not having influenza who do not exhibit the symptom.
- All three findings, especially fever, were less sensitive in patients over 60 years of age.
|
Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal conjestion can improve diagnostic accuracy.[49] Two decision analysis studies[50][51] suggest that during local outbreaks of influenza, the prevalence will be over 70%,[51] and thus patients with any of these combinations of symptoms may be treated with neuramidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.[51]
The available laboratory tests for influenza continue to improve. The United States Centers for Disease Control and Prevention (CDC) maintains an up-to-date summary of available laboratory tests.[52] According to the CDC, rapid diagnostic tests have a sensitivity of 70–75% and specificity of 90–95% when compared with viral culture. These tests may be especially useful during the influenza season (prevalence=25%) but in the absence of a local outbreak, or peri-influenza season (prevalence=10%[51]).
Influenza
