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80-497: Flu is an infectious disease of birds and mammals caused by RNA viruses of the family Orthomyxoviridae, the influenza viruses. Flu or FLU may also refer to: Flu Influenza , commonly known as the 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

160-477: A dry cough , sore or dry throat , hoarse voice , and a stuffy or runny nose . Coughing is the most common symptom. Gastrointestinal symptoms may also occur, including nausea, vomiting, diarrhea, and gastroenteritis, especially in children. The standard influenza symptoms typically last for two to eight days. Some studies suggest influenza can cause long-lasting symptoms in a similar way to long COVID . Symptomatic infections are usually mild and limited to

240-458: A runny nose . The time between exposure to the virus and development of symptoms (the incubation period ) is one to four days, most commonly one to two days. Many infections are asymptomatic. The onset of symptoms is sudden, and initial symptoms are predominately non-specific, including fever, chills, headaches, muscle pain , malaise , loss of appetite , lack of energy, and confusion. These are usually accompanied by respiratory symptoms such as

320-519: A certain strain in childhood still possess antibodies to that strain at a reasonable level later in life, which can provide some protection to related strains. There is, however, an " original antigenic sin ", in which the first HA subtype a person is exposed to influences the antibody-based immune response to future infections and vaccines. Annual vaccination is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups. Vaccines against

400-901: A culture of mammalian cells or embryonated eggs for 3–10 days to monitor cytopathic effect. Final confirmation can then be done via antibody staining, hemadsorption using red blood cells , or immunofluorescence microscopy. Shell vial cultures, which can identify infection via immunostaining before a cytopathic effect appears, are more sensitive than traditional cultures with results in 1–3 days. Cultures can be used to characterize novel viruses, observe sensitivity to antiviral drugs, and monitor antigenic drift, but they are relatively slow and require specialized skills and equipment. Serological assays can be used to detect an antibody response to influenza after natural infection or vaccination. Common serological assays include hemagglutination inhibition assays that detect HA-specific antibodies, virus neutralization assays that check whether antibodies have neutralized

480-461: A distinct "head" and "stalk" structure. M2 proteins form proton channels through the viral envelope that are required for viral entry and exit. Influenza B viruses contain a surface protein named NB that is anchored in the envelope, but its function is unknown. The viral life cycle begins by binding to a target cell. Binding is mediated by the viral HA proteins on the surface of the envelope, which bind to cells that contain sialic acid receptors on

560-425: A hemagglutinin-esterase fusion (HEF) protein on one segment that merges the functions of HA and NA. The final genome segment encodes the viral nucleoprotein (NP). Influenza viruses also encode various accessory proteins, such as PB1-F2 and PA-X, that are expressed through alternative open reading frames and which are important in host defense suppression, virulence, and pathogenicity. The virus particle, called

640-401: A higher risk of developing complications if these individuals are still shedding the virus. Antiviral treatment is also recommended if a person is hospitalized with suspected influenza instead of waiting for test results to return and if symptoms are worsening. Most antiviral drugs against influenza fall into two categories: neuraminidase (NA) inhibitors and M2 inhibitors. Baloxavir marboxil is

720-613: A higher temperature than mammalian influenza viruses. Newly synthesized viral polymerase subunits and NP proteins are imported to the nucleus to further increase the rate of viral replication and form RNPs. HA, NA, and M2 proteins are trafficked with the aid of M1 and NEP proteins to the cell membrane through the Golgi apparatus and inserted into the cell's membrane. Viral non-structural proteins including NS1, PB1-F2, and PA-X regulate host cellular processes to disable antiviral responses. PB1-F2 also interacts with PB1 to keep polymerases in

800-553: A limited number, so it is difficult to predict when the next will happen. The Global Influenza Surveillance and Response System of the World Health Organization (GISRS) tests several millions of specimens annually to monitor the spread and evolution of influenza viruses. 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

880-414: A major focus of research pertaining to antiviral drugs, vaccines, and other measures against influenza. Influenza C virus is subclassified into six genetic/antigenic lineages. Influenza D virus has been isolated from pigs and cattle, the latter being the natural reservoir. Infection has also been observed in humans, horses, dromedary camels, and small ruminants such as goats and sheep. Influenza D virus

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960-628: A notable exception, which targets the endonuclease activity of the viral RNA polymerase and can be used as an alternative to NA and M2 inhibitors for influenza A virus and influenza B virus. 1977 Russian flu In May 1977, an outbreak of flu took place in northern China including Liaoning , Jilin and Tianjin . The strain was isolated and determined by Chinese researchers to be H1N1 , which mostly affected students in middle and primary schools who lacked immunity to H1N1 virus. Clinical symptoms were relatively mild. Other areas in mainland China and British Hong Kong were also affected in

1040-450: A pandemic. Influenza C virus, like influenza B virus, is primarily found in humans, though it has been detected in pigs, feral dogs, dromedary camels, cattle, and dogs. Influenza C virus infection primarily affects children and is usually asymptomatic or has mild cold-like symptoms, though more severe symptoms such as gastroenteritis and pneumonia can occur. Unlike influenza A virus and influenza B virus, influenza C virus has not been

1120-449: A period of improvement in symptoms for one to three weeks followed by recurrent fever, sputum production, and fluid buildup in the lungs , but can also occur just a few days after influenza symptoms appear. About a third of primary pneumonia cases are followed by secondary pneumonia, which is most frequently caused by the bacteria Streptococcus pneumoniae and Staphylococcus aureus . Influenza viruses comprise four species, each

1200-424: A secondary bacterial infection occurs, then antibiotics may be necessary. Antiviral drugs are primarily used to treat severely ill patients, especially those with compromised immune systems. Antivirals are most effective when started in the first 48 hours after symptoms appear. Later administration may still be beneficial for those who have underlying immune defects, those with more severe symptoms, or those who have

1280-532: A simple way of obtaining assay results, are low cost, and produce results in less than 30 minutes, so they are commonly used, but they can not distinguish between influenza A virus and influenza B virus or between influenza A virus subtypes and are not as sensitive as nucleic-acid based tests. Nucleic acid-based tests (NATs) amplify and detect viral nucleic acid. Most of these tests take a few hours, but rapid molecular assays are as fast as RIDTs. Among NATs, reverse transcription polymerase chain reaction (RT-PCR)

1360-448: A transcriptase, PB2, which recognizes 5' caps , and PA (P3 for influenza C virus and influenza D virus), an endonuclease . The M1 matrix protein and M2 proton channel share a segment, as do the non-structural protein (NS1) and the nuclear export protein (NEP). For influenza A virus and influenza B virus, hemagglutinin (HA) and neuraminidase (NA) are encoded on one segment each, whereas influenza C virus and influenza D virus encode

1440-476: A type of white blood cell, produce antibodies that bind to influenza antigens HA and NA (or HEF ) and other proteins to a lesser degree. Once bound to these proteins, antibodies block virions from binding to cellular receptors, neutralizing the virus. In humans, a sizeable antibody response occurs about one week after viral exposure. This antibody response is typically robust and long-lasting, especially for influenza C virus and influenza D virus. People exposed to

1520-550: A type-1 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein. Almost all possible combinations of H (1 thru 16) and N (1 thru 11) have been isolated from wild birds. In addition H17, H18, N10 and N11 have been found in bats. The influenza A virus subtypes in circulation among humans as of 2018 are H1N1 and H3N2. Influenza B virus mainly infects humans but has been identified in seals, horses, dogs, and pigs. Influenza B virus does not have subtypes like influenza A virus but has two antigenically distinct lineages, termed

1600-425: A virion, is pleomorphic and varies between being filamentous, bacilliform, or spherical in shape. Clinical isolates tend to be pleomorphic, whereas strains adapted to laboratory growth typically produce spherical virions. Filamentous virions are about 250 nanometers (nm) by 80 nm, bacilliform 120–250 by 95 nm, and spherical 120 nm in diameter. The core of the virion comprises one copy of each segment of

1680-400: A year, once for each hemisphere, to discuss which strains should be included based on observation from HA inhibition assays. Other manufacturing methods include an MDCK cell culture-based inactivated vaccine and a recombinant subunit vaccine manufactured from baculovirus overexpression in insect cells. Influenza can be prevented or reduced in severity by post-exposure prophylaxis with

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1760-465: Is a sudden, drastic change in an influenza virus' antigen, usually HA. During antigenic shift, antigenically different strains that infect the same cell can reassort genome segments with each other, producing hybrid progeny. Since all influenza viruses have segmented genomes, all are capable of reassortment. Antigenic shift only occurs among influenza viruses of the same genus and most commonly occurs among influenza A viruses. In particular, reassortment

1840-468: Is acidified by cellular vATPase to have lower pH, which triggers a conformational change in HA that allows fusion of the viral envelope with the endosomal membrane. At the same time, hydrogen ions diffuse into the virion through M2 ion channels, disrupting internal protein-protein interactions to release RNPs into the host cell's cytosol . The M1 protein shell surrounding RNPs is degraded, fully uncoating RNPs in

1920-763: Is characterized by high levels of viral replication in the lower respiratory tract, accompanied by a strong pro-inflammatory response called a cytokine storm . Infection with H5N1 or H7N9 especially produces high levels of pro-inflammatory cytokines. In bacterial infections, early depletion of macrophages during influenza creates a favorable environment in the lungs for bacterial growth since these white blood cells are important in responding to bacterial infection. Host mechanisms to encourage tissue repair may inadvertently allow bacterial infection. Infection also induces production of systemic glucocorticoids that can reduce inflammation to preserve tissue integrity but allow increased bacterial growth. The pathophysiology of influenza

2000-419: Is dependent on vaccination with biosecurity. Diagnosis based on symptoms is fairly accurate in otherwise healthy people during seasonal epidemics and should be suspected in cases of pneumonia, acute respiratory distress syndrome (ARDS), sepsis , or if encephalitis, myocarditis , or breakdown of muscle tissue occur. Because influenza is similar to other viral respiratory tract illnesses, laboratory diagnosis

2080-436: 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

2160-477: Is distantly related to influenza C virus. While cattle workers have occasionally tested positive to prior influenza D virus infection, it is not known to cause disease in humans. Influenza C virus and influenza D virus experience a slower rate of antigenic evolution than influenza A virus and influenza B virus. Because of this antigenic stability, relatively few novel lineages emerge. Every year, millions of influenza virus samples are analysed to monitor changes in

2240-425: Is exported out of the nucleus and translated by host ribosomes in a cap-dependent manner to synthesize viral proteins. RdRp also synthesizes complementary positive-sense strands of the viral genome in a complementary RNP complex which are then used as templates by viral polymerases to synthesize copies of the negative-sense genome. During these processes, RdRps of avian influenza viruses (AIVs) function optimally at

2320-427: Is in the area about two meters around an infected person via respiratory droplets that come into contact with mucosa of the upper respiratory tract. Transmission through contact with a person, bodily fluids, or intermediate objects ( fomites ) can also occur, since influenza viruses can survive for hours on non-porous surfaces. If one's hands are contaminated, then touching one's face can cause infection. Influenza

2400-506: Is intracellular and performed by ubiquitous proteases, which allows for infection of a greater variety of cells, thereby contributing to more severe disease. Cells possess sensors to detect viral RNA, which can then induce interferon production. Interferons mediate expression of antiviral proteins and proteins that recruit immune cells to the infection site, and they notify nearby uninfected cells of infection. Some infected cells release pro-inflammatory cytokines that recruit immune cells to

2480-614: Is necessary for confirmation. Common sample collection methods for testing include nasal and throat swabs. Samples may be taken from the lower respiratory tract if infection has cleared the upper but not lower respiratory tract. Influenza testing is recommended for anyone hospitalized with symptoms resembling influenza during flu season or who is connected to an influenza case. For severe cases, earlier diagnosis improves patient outcome. Diagnostic methods that can identify influenza include viral cultures , antibody- and antigen-detecting tests, and nucleic acid-based tests. Viruses can be grown in

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2560-734: 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, as does wearing a mask. Annual vaccination can help to provide protection against influenza. Influenza viruses, particularly influenza A virus, evolve quickly, so flu vaccines are updated regularly to match which influenza strains are in circulation. Vaccines provide protection against influenza A virus subtypes H1N1 and H3N2 and one or two influenza B virus subtypes. Influenza infection

2640-544: Is recommended for people who have yet to receive a vaccine for the current flu season, who have been vaccinated less than two week since contact, if there is a significant mismatch between vaccine and circulating strains, or during an outbreak in a closed setting regardless of vaccination history. These are the main ways that influenza spreads When vaccines and antiviral medications are limited, non-pharmaceutical interventions are essential to reduce transmission and spread. The lack of controlled studies and rigorous evidence of

2720-413: Is recommended to avoid alcohol and tobacco use while ill. Aspirin is not recommended to treat influenza in children due to an elevated risk of developing Reye syndrome . Corticosteroids are not recommended except when treating septic shock or an underlying medical condition, such as chronic obstructive pulmonary disease or asthma exacerbation, since they are associated with increased mortality. If

2800-764: Is removed and facilities are disinfected and "no carry-over" policies to eliminate infectious material before new poultry arrive can be used to reduce the spread of influenza viruses. If a novel influenza viruses has breached the aforementioned biosecurity measures, then rapid detection to stamp it out via quarantining, decontamination, and culling may be necessary to prevent the virus from becoming endemic. Vaccines exist for avian H5, H7, and H9 subtypes that are used in some countries. In China, for example, vaccination of domestic birds against H7N9 successfully limited its spread, indicating that vaccination may be an effective strategy if used in combination with other measures to limit transmission. In pigs and horses, management of influenza

2880-464: Is significantly influenced by which receptors influenza viruses bind to during entry into cells. Mammalian influenza viruses preferentially bind to sialic acids connected to the rest of the oligosaccharide by an α-2,6 link, most commonly found in various respiratory cells, such as respiratory and retinal epithelial cells. AIVs prefer sialic acids with an α-2,3 linkage, which are most common in birds in gastrointestinal epithelial cells and in humans in

2960-566: Is simpler. Examples are B/Santiago/29615/2020 and C/Minnesota/10/2015. Influenza viruses have a negative-sense , single-stranded RNA genome that is segmented. The negative sense of the genome means it can be used as a template to synthesize messenger RNA (mRNA). Influenza A virus and influenza B virus have eight genome segments that encode 10 major proteins. Influenza C virus and influenza D virus have seven genome segments that encode nine major proteins. Three segments encode three subunits of an RNA-dependent RNA polymerase (RdRp) complex: PB1,

3040-516: Is slower in B than A and slowest in C and D. Antigenic drift is a major cause of seasonal influenza, and requires that flu vaccines be updated annually. HA is the main component of inactivated vaccines, so surveillance monitors antigenic drift of this antigen among circulating strains. Antigenic evolution of influenza viruses of humans appears to be faster than in swine and equines. In wild birds, within-subtype antigenic variation appears to be limited but has been observed in poultry. Antigenic shift

3120-461: Is the most traditional and considered the gold standard for diagnosing influenza because it is fast and can subtype influenza A virus, but it is relatively expensive and more prone to false-positives than cultures. Other NATs that have been used include loop-mediated isothermal amplification -based assays, simple amplification-based assays, and nucleic acid sequence-based amplification. Nucleic acid sequencing methods can identify infection by obtaining

3200-407: Is usually transmissible from one day before the onset of symptoms to 5–7 days after. 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. Children ages 2–17 are considered to be the primary and most efficient spreaders of influenza. Children who have not had multiple prior exposures to influenza viruses shed

3280-501: Is very common in AIVs, creating a large diversity of influenza viruses in birds, but is uncommon in human, equine, and canine lineages. Pigs, bats, and quails have receptors for both mammalian and avian influenza A viruses, so they are potential "mixing vessels" for reassortment. If an animal strain reassorts with a human strain, then a novel strain can emerge that is capable of human-to-human transmission. This has caused pandemics, but only

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3360-519: Is when an influenza virus' antigens change due to the gradual accumulation of mutations in the antigen's (HA or NA) gene. This can occur in response to evolutionary pressure exerted by the host immune response. Antigenic drift is especially common for the HA protein, in which just a few amino acid changes in the head region can constitute antigenic drift. The result is the production of novel strains that can evade pre-existing antibody-mediated immunity. Antigenic drift occurs in all influenza species but

3440-661: The United Kingdom . The virus reached the United States in January 1978. The first outbreak in the U.S. was reported in a high school in Cheyenne , where the clinical attack rate was more than 70% but involved solely students. Even though infections were seen in schools and military bases throughout the U.S., there were few reports of infection in people older than 26, and the death rate in affected individuals

3520-517: The United States , some researchers estimate the influenza mortality rate (not the infection fatality rate or the case fatality rate ) around 5 in every 100,000 population, less than that of the typical seasonal influenza (~6 in every 100,000 population). Most of the infected people were under the age of 26 or 25, presumably because older people retained immunity from exposure to previous H1N1 strains. Contradicting these descriptions, one review article proposed that 700,000 people died due to

3600-471: The upper respiratory tract , but progression to pneumonia is relatively common. Pneumonia may be caused by the primary viral infection or a secondary bacterial infection . Primary pneumonia is characterized by rapid progression of fever, cough, labored breathing , and low oxygen levels that cause bluish skin . It is especially common among those who have an underlying cardiovascular disease such as rheumatic heart disease . Secondary pneumonia typically has

3680-569: The B/Victoria/2/1987-like and B/Yamagata/16/1988-like lineages, or simply (B/)Victoria(-like) and (B/)Yamagata(-like). Both lineages are in circulation in humans, disproportionately affecting children. However, the B/Yamagata lineage might have become extinct in 2020/2021 due to COVID-19 pandemic measures. Influenza B viruses contribute to seasonal epidemics alongside influenza A viruses but have never been associated with

3760-409: The air. A person susceptible to infection can contract influenza by coming into contact with these particles. 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. Inhalation of aerosols can lead to infection, but most transmission

3840-465: The airways, loss of alveolar structure, loss of lung epithelial integrity due to epithelial cell infection and death, and degradation of the extracellular matrix that maintains lung structure. In particular, alveolar cell infection appears to drive severe symptoms since this results in impaired gas exchange and enables viruses to infect endothelial cells, which produce large quantities of pro-inflammatory cytokines . Pneumonia caused by influenza viruses

3920-463: The antiviral drugs oseltamivir , which can be taken orally by those at least three months old, and zanamivir , which can be inhaled by those above seven years. Chemoprophylaxis is most useful for individuals at high risk for complications and those who cannot receive the flu vaccine. Post-exposure chemoprophylaxis is only recommended if oseltamivir is taken within 48 hours of contact with a confirmed or suspected case and zanamivir within 36 hours. It

4000-427: The cytoplasmic side of the membrane. The viral genome is incorporated inside a viral envelope derived from portions of the cell membrane that have HA, NA, and M2 proteins. At the end of budding, HA proteins remain attached to cellular sialic acid until they are cleaved by the sialidase activity of NA proteins. The virion is then released from the cell. The sialidase activity of NA also cleaves any sialic acid residues from

4080-404: The cytosol. RNPs are then imported into the nucleus with the help of viral localization signals. There, the viral RNA polymerase transcribes mRNA using the genomic negative-sense strand as a template. The polymerase snatches 5' caps for viral mRNA from cellular RNA to prime mRNA synthesis and the 3'-end of mRNA is polyadenylated at the end of transcription. Once viral mRNA is transcribed, it

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4160-530: The effectiveness of some measures has hampered planning decisions and recommendations. Nevertheless, strategies endorsed by experts for all phases of flu outbreaks include hand and respiratory hygiene, self-isolation by symptomatic individuals and the use of face masks by them and their caregivers, surface disinfection, rapid testing and diagnosis, and contact tracing . In some cases, other forms of social distancing including school closures and travel restrictions are recommended. Reasonably effective ways to reduce

4240-462: The flu are trivalent or quadrivalent, providing protection against an H1N1 strain, an H3N2 strain, and one or two influenza B virus strains corresponding to the two influenza B virus lineages. Two types of vaccines are in use: inactivated vaccines that contain "killed" (i.e. inactivated) viruses and live attenuated influenza vaccines (LAIVs) that contain weakened viruses. There are three types of inactivated vaccines: whole virus, split virus, in which

4320-685: The following months. In the same year, the H1N1 strain was detected in Siberia shortly after the outbreak in China, and then spread rapidly across the Soviet Union , which was the first country to report the outbreak to the World Health Organization (the People's Republic of China was not a member of WHO until 1981 ). Therefore, the pandemic was named "Russian flu". In 1977, Russian flu reached

4400-541: The genome bound to NP nucleoproteins in separate ribonucleoprotein (RNP) complexes for each segment. There is a copy of the RdRp, all subunits included, bound to each RNP. The genetic material is encapsulated by a layer of M1 matrix protein which provides structural reinforcement to the outer layer, the viral envelope . The envelope comprises a lipid bilayer membrane incorporating HA and NA (or HEF ) proteins extending outward from its exterior surface. HA and HEF proteins have

4480-660: The late 1800s, pandemic outbreaks of novel influenza strains 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. The symptoms of influenza are similar to those of a cold, although usually more severe and less likely to include

4560-400: The lower respiratory tract. Cleavage of the HA protein into HA 1 , the binding subunit, and HA 2 , the fusion subunit, is performed by different proteases, affecting which cells can be infected. For mammalian influenza viruses and low pathogenic AIVs, cleavage is extracellular, which limits infection to cells that have the appropriate proteases, whereas for highly pathogenic AIVs, cleavage

4640-520: The nucleic acid sequence of viral samples to identify the virus and antiviral drug resistance. The traditional method is Sanger sequencing , but it has been largely replaced by next-generation methods that have greater sequencing speed and throughput. Treatment in cases of mild or moderate illness is supportive and includes anti-fever medications such as acetaminophen and ibuprofen , adequate fluid intake to avoid dehydration, and rest. Cough drops and throat sprays may be beneficial for sore throat. It

4720-414: The nucleus longer. M1 and NEP proteins localize to the nucleus during the later stages of infection, bind to viral RNPs and mediate their export to the cytoplasm where they migrate to the cell membrane with the aid of recycled endosomes and are bundled into the segments of the genome. Progeny viruses leave the cell by budding from the cell membrane, which is initiated by the accumulation of M1 proteins at

4800-615: The one in the early 1960s. ) H1N1 reappeared in 1977 and the strain of the Russian flu was almost identical to one that had been isolated in 1950. This feature of the 1977 strain has been interpreted as pointing towards an anthropogenic origin of the virus, and the pandemic is the only documented human epidemic believed to result from research activity. The Russian flu was relatively benign. In 1977, Chinese researchers found uneven attack rates among different groups of students, as well as many mild and asymptomatic infections. In

4880-418: The population contracts influenza. There are 3 to 5 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 , the number of influenza cases peaks during winter, whereas in the tropics , influenza can occur year-round. Since

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4960-695: The primary measures used. Placing poultry houses and piggeries on high ground away from high-density farms, backyard farms, live poultry markets, and bodies of water helps to minimize contact with wild birds. Closure of live poultry markets appears to the most effective measure and has shown to be effective at controlling the spread of H5N1, H7N9, and H9N2 . Other biosecurity measures include cleaning and disinfecting facilities and vehicles, banning visits to poultry farms, not bringing birds intended for slaughter back to farms, changing clothes, disinfecting foot baths, and treating food and water. If live poultry markets are not closed, then "clean days" when unsold poultry

5040-420: The primary reservoir of influenza A virus, especially aquatic birds such as ducks, geese, shorebirds, and gulls, but the virus also circulates among mammals, including pigs, horses, and marine mammals. Subtypes of Influenza A are defined by the combination of the antigenic viral proteins haemagglutinin (H) and neuraminidase (N) in the viral envelope ; for example, " H1N1 " designates an IAV subtype that has

5120-462: 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. Influenza A virus and influenza B virus circulate in humans and cause seasonal epidemics , and influenza C virus causes a mild infection, primarily in children. Influenza D virus can infect humans but

5200-449: The probability of reassortment. In general, influenza vaccines are only effective if there is an antigenic match between vaccine strains and circulating strains. Most commercially available flu vaccines are manufactured by propagation of influenza viruses in embryonated chicken eggs, taking 6–8 months. Flu seasons are different in the northern and southern hemisphere, so the WHO meets twice

5280-402: The result of lung inflammation and compromise caused by epithelial cell infection and death, combined with inflammation caused by the immune system's response to infection. Non-respiratory organs can become involved, but the mechanisms by which influenza is involved in these cases are unknown. Severe respiratory illness can be caused by multiple, non-exclusive mechanisms, including obstruction of

5360-507: The sanitizing effect lasts for longer. In hospitals, quaternary ammonium compounds and bleach are used to sanitize rooms or equipment that have been occupied by people with influenza symptoms. At home, this can be done effectively with a diluted chlorine bleach. Since influenza viruses circulate in animals such as birds and pigs, prevention of transmission from these animals is important. Water treatment , indoor raising of animals, quarantining sick animals, vaccination, and biosecurity are

5440-458: The sick, there is mixed evidence on beneficial effects in the community. Smoking raises 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 sanitizer against influenza viruses, while quaternary ammonium compounds can be used with alcohol so that

5520-492: The site of infection. Immune cells control viral infection by killing infected cells and phagocytizing viral particles and apoptotic cells. An exacerbated immune response can harm the host organism through a cytokine storm. To counter the immune response, influenza viruses encode various non-structural proteins, including NS1, NEP, PB1-F2, and PA-X, that are involved in curtailing the host immune response by suppressing interferon production and host gene expression. B cells ,

5600-428: The sole member of its own genus. The four influenza genera comprise four of the seven genera in the family Orthomyxoviridae . They are: Influenza A virus is responsible for most cases of severe illness as well as seasonal epidemics and occasional pandemics. It infects people of all ages but tends to disproportionately cause severe illness in the elderly, the very young, and those with chronic health issues. Birds are

5680-411: The sun, and crowding. 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. In humans, influenza viruses first cause infection by infecting epithelial cells in the respiratory tract. Illness during infection is primarily

5760-475: The surface of the cell membrane. For N1 subtypes with the "G147R" mutation and N2 subtypes, the NA protein can initiate entry. Prior to binding, NA proteins promote access to target cells by degrading mucus, which helps to remove extracellular decoy receptors that would impede access to target cells. After binding, the virus is internalized into the cell by an endosome that contains the virion inside it. The endosome

5840-438: The transmission of influenza include good personal health and hygiene habits such as: not touching the eyes, nose or mouth; frequent hand washing (with soap and water, or with alcohol-based hand rubs); covering coughs and sneezes with a tissue or sleeve; avoiding close contact with sick people; and staying home when sick. Avoiding spitting is also recommended. Although face masks might help prevent transmission when caring for

5920-536: The viral surface, which helps prevent newly assembled viruses from aggregating near the cell surface and improving infectivity. Similar to other aspects of influenza replication, optimal NA activity is temperature- and pH-dependent. Ultimately, presence of large quantities of viral RNA in the cell triggers apoptosis (programmed cell death), which is initiated by cellular factors to restrict viral replication. Two key processes that influenza viruses evolve through are antigenic drift and antigenic shift . Antigenic drift

6000-477: 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 . There are four types of influenza virus: types A, B, C, and D. Aquatic birds are

6080-461: The virus at greater quantities and for a longer duration than other children. People 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. A variety of factors likely encourage influenza transmission, including lower temperature, lower absolute and relative humidity , less ultraviolet radiation from

6160-613: The virus is disrupted by a detergent, and subunit, which only contains the viral antigens HA and NA. Most flu vaccines are inactivated and administered via intramuscular injection. LAIVs are sprayed into the nasal cavity. Vaccination recommendations vary by country. Some recommend vaccination for all people above a certain age, such as 6 months, whereas other countries limit recommendations to high-risk groups. Young infants cannot receive flu vaccines for safety reasons, but they can inherit passive immunity from their mother if vaccinated during pregnancy. Influenza vaccination helps to reduce

6240-481: The virus' antigenic properties, and to inform the development of vaccines. To unambiguously describe a specific isolate of virus, researchers use the internationally accepted influenza virus nomenclature, which describes, among other things, the species of animal from which the virus was isolated, and the place and year of collection. As an example – A/chicken/Nakorn-Patom/Thailand/CU-K2/04(H5N1) : The nomenclature for influenza B, C and D, which are less variable,

6320-560: The virus, and enzyme-linked immunoabsorbant assays. These methods tend to be relatively inexpensive and fast but are less reliable than nucleic-acid based tests. Direct fluorescent or immunofluorescent antibody (DFA/IFA) tests involve staining respiratory epithelial cells in samples with fluorescently-labeled influenza-specific antibodies, followed by examination under a fluorescent microscope. They can differentiate between influenza A virus and influenza B virus but can not subtype influenza A virus. Rapid influenza diagnostic tests (RIDTs) are

6400-452: Was low. Since late 1977, the H1N1 strain has begun to co-circulate with the H3N2 strain in humans, as seasonal flu . There have been various H1N1 strains . The 1918 Spanish flu was caused by an H1N1 strain, and H1N1 strains afterwards became endemic and circulated around the world until 1957, when they all but vanished. (There were some isolated reports of other H1N1 strains such as

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