Influenza
Influenza, commonly known as "the flu" or just "flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin one to four (typically two) days after exposure to the virus and last for about two to eight days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia from the virus or a subsequent bacterial infection. Other complications include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease.
For other uses, see Influenza (disambiguation), Flu (disambiguation), and Grippe (disambiguation).Influenza
flu, the flu, grippe (French for flu)
1–4 days after exposure
2–8 days
Influenza viruses
Antiviral drugs such as oseltamivir
There are four types of influenza virus: A, B, C, and D. Aquatic birds are the primary source of Influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs. Influenza B virus (IBV) and influenza C virus (ICV) primarily infect humans, and influenza D virus (IDV) is found in cattle and pigs. IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. IDV can infect humans but is not known to cause illness. In humans, influenza viruses are primarily transmitted through respiratory droplets from coughing and sneezing. Transmission through aerosols and surfaces contaminated by the virus also occur.
Frequent hand washing and covering one's mouth and nose when coughing and sneezing reduce transmission. Annual vaccination can help to provide protection against influenza. Influenza viruses, particularly IAV, evolve quickly, so flu vaccines are updated regularly to match which influenza strains are in circulation. Vaccines provide protection against IAV subtypes H1N1 and H3N2 and one or two IBV subtypes. Influenza infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction (PCR) to identify viral nucleic acid. The disease can be treated with supportive measures and, in severe cases, with antiviral drugs such as oseltamivir. In healthy individuals, influenza is typically self-limiting and rarely fatal, but it can be deadly in high-risk groups.
In a typical year, five to 15 percent of the population contracts influenza. There are three to five million severe cases annually, with up to 650,000 respiratory-related deaths globally each year. Deaths most commonly occur in high-risk groups, including young children, the elderly, and people with chronic health conditions. In temperate regions of the world, the number of influenza cases peaks during winter, whereas in the tropics, influenza can occur year-round. Since the late 1800s, large outbreaks of novel influenza strains that spread globally, called pandemics, have occurred every 10 to 50 years. Five flu pandemics have occurred since 1900: the Spanish flu from 1918 to 1920, which was the most severe; the Asian flu in 1957; the Hong Kong flu in 1968; the Russian flu in 1977; and the swine flu pandemic in 2009.
Mechanism
Transmission
People who are infected can transmit influenza viruses through breathing, talking, coughing, and sneezing, which spread respiratory droplets and aerosols that contain virus particles into the air. A person susceptible to infection can contract influenza by coming into contact with these particles.[15][28] Respiratory droplets are relatively large and travel less than two meters before falling onto nearby surfaces. Aerosols are smaller and remain suspended in the air longer, so they take longer to settle and can travel further than respiratory droplets.[28][4] Inhalation of aerosols can lead to infection,[29] but most transmission is in the area about two meters around an infected person via respiratory droplets[10] that come into contact with mucosa of the upper respiratory tract.[29] Transmission through contact with a person, bodily fluids, or intermediate objects (fomites) can also occur,[10][28] since influenza viruses can survive for hours on non-porous surfaces.[4] If one's hands are contaminated, then touching one's face can cause infection.[30]
Influenza is usually transmissible from one day before the onset of symptoms to 5–7 days after.[11] In healthy adults, the virus is shed for up to 3–5 days. In children and the immunocompromised, the virus may be transmissible for several weeks.[10] Children ages 2–17 are considered to be the primary and most efficient spreaders of influenza.[1][11] Children who have not had multiple prior exposures to influenza viruses shed the virus at greater quantities and for a longer duration than other children.[1] People who are at risk of exposure to influenza include health care workers, social care workers, and those who live with or care for people vulnerable to influenza. In long-term care facilities, the flu can spread rapidly after it is introduced.[11] A variety of factors likely encourage influenza transmission, including lower temperature, lower absolute and relative humidity, less ultraviolet radiation from the Sun,[29][31] and crowding.[28] Influenza viruses that infect the upper respiratory tract like H1N1 tend to be more mild but more transmissible, whereas those that infect the lower respiratory tract like H5N1 tend to cause more severe illness but are less contagious.[10]
Prognosis
In healthy individuals, influenza infection is usually self-limiting and rarely fatal.[10][11] Symptoms usually last for 2–8 days.[13] Influenza can cause people to miss work or school, and it is associated with decreased job performance and, in older adults, reduced independence. Fatigue and malaise may last for several weeks after recovery, and healthy adults may experience pulmonary abnormalities that can take several weeks to resolve. Complications and mortality primarily occur in high-risk populations and those who are hospitalized. Severe disease and mortality are usually attributable to pneumonia from the primary viral infection or a secondary bacterial infection,[1][11] which can progress to ARDS.[13]
Other respiratory complications that may occur include sinusitis, bronchitis, bronchiolitis, excess fluid buildup in the lungs, and exacerbation of chronic bronchitis and asthma. Middle ear infection and croup may occur, most commonly in children.[10][1] Secondary S. aureus infection has been observed, primarily in children, to cause toxic shock syndrome after influenza, with hypotension, fever, and reddening and peeling of the skin.[1] Complications affecting the cardiovascular system are rare and include pericarditis, fulminant myocarditis with a fast, slow, or irregular heartbeat, and exacerbation of pre-existing cardiovascular disease.[10][11] Inflammation or swelling of muscles accompanied by muscle tissue breaking down occurs rarely, usually in children, which presents as extreme tenderness and muscle pain in the legs and a reluctance to walk for 2–3 days.[1][11][15]
Influenza can affect pregnancy, including causing smaller neonatal size, increased risk of premature birth, and an increased risk of child death shortly before or after birth.[11] Neurological complications have been associated with influenza on rare occasions, including aseptic meningitis, encephalitis, disseminated encephalomyelitis, transverse myelitis, and Guillain–Barré syndrome.[15] Additionally, febrile seizures and Reye syndrome can occur, most commonly in children.[1][11] Influenza-associated encephalopathy can occur directly from central nervous system infection from the presence of the virus in blood and presents as sudden onset of fever with convulsions, followed by rapid progression to coma.[10] An atypical form of encephalitis called encephalitis lethargica, characterized by headache, drowsiness, and coma, may rarely occur sometime after infection.[1] In survivors of influenza-associated encephalopathy, neurological defects may occur.[10] Primarily in children, in severe cases the immune system may rarely dramatically overproduce white blood cells that release cytokines, causing severe inflammation.[10]
People who are at least 65 years of age,[11] due to a weakened immune system from aging or a chronic illness, are a high-risk group for developing complications, as are children less than one year of age and children who have not been previously exposed to influenza viruses multiple times. Pregnant women are at an elevated risk, which increases by trimester[1] and lasts up to two weeks after childbirth.[11][35] Obesity, in particular a body mass index greater than 35–40, is associated with greater amounts of viral replication, increased severity of secondary bacterial infection, and reduced vaccination efficacy. People who have underlying health conditions are also considered at-risk, including those who have congenital or chronic heart problems or lung (e.g. asthma), kidney, liver, blood, neurological, or metabolic (e.g. diabetes) disorders,[10][1][11] as are people who are immunocompromised from chemotherapy, asplenia, prolonged steroid treatment, splenic dysfunction, or HIV infection.[11] Tobacco use, including past use, places a person at risk.[35] The role of genetics in influenza is not well researched,[1] but it may be a factor in influenza mortality.[13]
In animals
Birds
Aquatic birds such as ducks, geese, shorebirds, and gulls are the primary reservoir of IAVs.[17][18] In birds, AIVs may be either low pathogenic avian influenza (LPAI) viruses that produce little to no symptoms or highly pathogenic avian influenza (HPAI) viruses that cause severe illness. Symptoms of HPAI infection include lack of energy and appetite, decreased egg production, soft-shelled or misshapen eggs, swelling of the head, comb, wattles, and hocks, purple discoloration of wattles, combs, and legs, nasal discharge, coughing, sneezing, incoordination, and diarrhea. Birds infected with an HPAI virus may also die suddenly without any signs of infection.[41]
The distinction between LPAI and HPAI can generally be made based on how lethal an AIV is to chickens. At the genetic level, an AIV can be usually be identified as an HPAI virus if it has a multibasic cleavage site in the HA protein, which contains additional residues in the HA gene.[18][27] Most AIVs are LPAI. Notable HPAI viruses include HPAI H5N1 and HPAI H7N9. HPAI viruses have been a major disease burden in the 21st century, resulting in the death of large numbers of birds. In H7N9's case, some circulating strains were originally LPAI but became HPAI by acquiring the HA multibasic cleavage site. Avian H9N2 is also of concern because although it is LPAI, it is a common donor of genes to H5N1 and H7N9 during reassortment.[1]
Migratory birds can spread influenza across long distances. An example of this was when an H5N1 strain in 2005 infected birds at Qinghai Lake, China, which is a stopover and breeding site for many migratory birds, subsequently spreading the virus to more than 20 countries across Asia, Europe, and the Middle East.[17][27] AIVs can be transmitted from wild birds to domestic free-range ducks and in turn to poultry through contaminated water, aerosols, and fomites.[1] Ducks therefore act as key intermediates between wild and domestic birds.[27] Transmission to poultry typically occurs in backyard farming and live animal markets where multiple species interact with each other. From there, AIVs can spread to poultry farms in the absence of adequate biosecurity. Among poultry, HPAI transmission occurs through aerosols and contaminated feces,[1] cages, feed, and dead animals.[17] Back-transmission of HPAI viruses from poultry to wild birds has occurred and is implicated in mass die-offs and intercontinental spread.[18]
AIVs have occasionally infected humans through aerosols, fomites, and contaminated water.[1] Direction transmission from wild birds is rare.[27] Instead, most transmission involves domestic poultry, mainly chickens, ducks, and geese but also a variety of other birds such as guinea fowl, partridge, pheasants, and quails.[18] The primary risk factor for infection with AIVs is exposure to birds in farms and live poultry markets.[17] Typically, infection with an AIV has an incubation period of 3–5 days but can be up to 9 days. H5N1 and H7N9 cause severe lower respiratory tract illness, whereas other AIVs such as H9N2 cause a more mild upper respiratory tract illness, commonly with conjunctivitis.[1] Limited transmission of avian H2, H5-7, H9, and H10 subtypes from one person to another through respiratory droplets, aerosols, and fomites has occurred,[1][2] but sustained human-to-human transmission of AIVs has not occurred. Before 2013, H5N1 was the most common AIV to infect humans. Since then, H7N9 has been responsible for most human cases.[17]