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Influenza (flu)
Types Avian A/H5N1 subtype Canine Equine Swine A/H1N1 subtype
Vaccines 2009 pandemic Pandemrix Live attenuated Seasonal flu vaccine brands
Treatment Amantadine Baloxavir marboxil Laninamivir Oseltamivir Peramivir Rimantadine Umifenovir Zanamivir
Pandemics 1889-1890 Russian flu 1918 Spanish flu 1957-1958 Asian flu 1968 Hong Kong flu 1977 Russian flu 2009 swine flu
Outbreaks 1976 swine flu 2006 H5N1 India 2007 Australian equine 2007 Bernard Matthews H5N1 2008 West Bengal 2015 United States H5N2 outbreak 2020–2022 H5N8 outbreak
See also Flu season Influenza evolution Influenza research Influenza-like illness Vaccine reformulations
v t e

Swine influenza (also swine flu) refers to influenza caused by any strain of the influenza virus endemic in pigs (swine). Strains endemic in swine are called swine influenza virus (SIV).

Swine flu is common in swine and rare in humans. People who work with swine, especially people with intense exposures, are at risk of catching swine influenza if the swine carry a strain able to infect humans. However, these strains rarely are able to pass from human to human. Rarely, SIV mutates into a form able to pass easily from human to human. The strain responsible for the 2009 swine flu outbreak is believed to have undergone such a mutation. This virus is named swine flu because one of its surface proteins is similar to viruses that usually infects pigs, but this strain is spreading in people and it is unknown if it infects pigs.

In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort. The strain responsible for the 2009 swine flu outbreak in most cases causes only mild symptoms and the infected person makes a full recovery without requiring medical attention and without the use of antiviral medicines.

Of the three genera of human flu, two are endemic also in swine: Influenzavirus A (common) and Influenzavirus C (rare). Influenzavirus B has not been reported in swine. Within Influenzavirus A and Influenzavirus C, the strains endemic to swine and humans are largely distinct.

Background

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The swine flu is likely a descendant of the infamous "Spanish flu" that caused a devastating pandemic in humans in 1918–1919. In less than a year, that pandemic killed more an estimated 50 million people worldwide. Descendants of this virus have persisted in pigs; they probably circulated in humans until the appearance of the Asian flu in 1957, and reemerged in 1977. Direct transmission from pigs to humans is rare, with 12 cases in the U.S. since 2005.

The flu virus is perhaps the trickiest known to medical science; it constantly changes form to elude the protective antibodies that the body has developed in response to previous exposures to influenza or to influenza vaccines. Every two or three years the virus undergoes minor changes. Then, at intervals of roughly a decade, after the bulk of the world's population has developed some level of resistance to these minor changes, it undergoes a major shift that enables it to tear off on yet another pandemic sweep around the world, infecting hundreds of millions of people who suddenly find their antibody defenses outflanked. Even during the Spanish flu pandemic, the initial wave of the disease was relatively mild and the second wave was highly lethal.

In 1957, an Asian flu pandemic infected some 45 million Americans and killed 70,000. Eleven years later, lasting from 1968 to 1969, the Hong Kong flu pandemic afflicted 50 million Americans and caused 33,000 deaths, costing approximately $3.9 billion. In 1976, about 500 soldiers became infected with swine flu over a period of a few weeks. However, by the end of the month investigators found that the virus had "mysteriously disappeared" and there were no more signs of swine flu anywhere on the post. There were isolated cases around the U.S. but those cases were supposedly to individuals who caught the virus from pigs.

Medical researchers worldwide, recognizing that the swine flu virus might again mutate into something as deadly as the Spanish flu, were carefully watching the latest 2009 outbreak of swine flu and making contingency plans for a possible global pandemic.

Classification

SIV strains isolated to date have been classified either as Influenzavirus C or one of the various subtypes of the genus Influenzavirus A.

Influenza A

Swine influenza is known to be caused by influenza A subtypes H1N1, H1N2, H3N1, H3N2, and H2N3.

In swine, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the world. In the United States, the H1N1 subtype was exclusively prevalent among swine populations before 1998; however, since late August 1998, H3N2 subtypes have been isolated from pigs. As of 2004, H3N2 virus isolates in US swine and turkey stocks were triple reassortants, containing genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages.

Interaction with H5N1

Avian influenza virus H3N2 is endemic in pigs in China and has been detected in pigs in Vietnam, increasing fears of the emergence of new variant strains. Health experts say pigs can carry human influenza viruses, which can combine (i.e. exchange homologous genome sub-units by genetic reassortment) with H5N1, passing genes and mutating into a form which can pass easily among humans. H3N2 evolved from H2N2 by antigenic shift. In August 2004, researchers in China found H5N1 in pigs.

Nature magazine reported that Chairul Nidom, a virologist at Airlangga University's tropical disease center in Surabaya, East Java, conducted an independent research study in 2005. He tested the blood of 10 apparently healthy pigs housed near poultry farms in West Java where avian flu had broken out. Five of the pig samples contained the H5N1 virus. The Indonesian government has since found similar results in the same region. Additional tests of 150 pigs outside the area were negative.

Signs and symptoms

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According to the Centers for Disease Control and Prevention (CDC), in humans the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general. Symptoms include fever, cough, sore throat, body aches, headache, chills and fatigue. The 2009 outbreak has shown an increased percentage of patients reporting diarrhea and vomiting.

Because these symptoms are not specific to swine flu, a differential diagnosis of probable swine flu requires not only symptoms but also a high likelihood of swine flu due to the person's recent history. For example, during the 2009 swine flu outbreak in the United States, CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the 7 days preceding their illness onset." A diagnosis of confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).

Pathophysiology

Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells; typically in the nose, throat and lungs of mammals and intestines of birds (Stage 1 in infection figure).

Swine flu in humans

People who work with poultry and swine, especially people with intense exposures, are at increased risk of zoonotic infection with influenza virus endemic in these animals, and constitute a population of human hosts in which zoonosis and reassortment can co-occur. Transmission of influenza from swine to humans who work with swine was documented in a small surveillance study performed in 2004 at the University of Iowa. This study among others forms the basis of a recommendation that people whose jobs involve handling poultry and swine be the focus of increased public health surveillance. The 2009 swine flu outbreak is an apparent reassortment of several strains of influenza A virus subtype H1N1, including a strain endemic in humans and two strains endemic in pigs, as well as an avian influenza.

The CDC reports that the symptoms and transmission of the swine flu from human to human is much like that of seasonal flu. Common symptoms include fever, lethargy, lack of appetite and coughing, while runny nose, sore throat, nausea, vomiting and diarrhea have also been reported. It is believed to be spread between humans through coughing or sneezing of infected people and touching something with the virus on it and then touching their own nose or mouth. Swine flu cannot be spread by pork products, since the virus is not transmitted through food. The swine flu in humans is most contagious during the first five days of the illness although some people, most commonly children, can remain contagious for up to ten days. Diagnosis can be made by sending a specimen, collected during the first five days, to the CDC for analysis.

The swine flu is susceptible to four drugs licensed in the United States, amantadine, rimantadine, oseltamivir and zanamivir; however, for the 2009 outbreak it is recommended it be treated under medical advice only with oseltamivir and zanamivir to avoid drug resistance. The vaccine for the human seasonal H1N1 flu does not protect against the swine H1N1 flu, as they are antigenically very different.

Prevention

Prevention of swine influenza has three components: prevention in swine, prevention of transmission to humans, and prevention of its spread among humans.

Prevention in swine

Swine influenza has become a greater problem in recent decades as the evolution of the virus has resulted in inconsistent responses to traditional vaccines. Standard commercial swine flu vaccines are effective in controlling the infection when the virus strains match enough to have significant cross-protection, and custom (autogenous) vaccines made from the specific viruses isolated are created and used in the more difficult cases.

Present vaccination strategies for SIV control and prevention in swine farms, typically include the use of one of several bivalent SIV vaccines commercially available in the United States. Of the 97 recent H3N2 isolates examined, only 41 isolates had strong serologic cross-reactions with antiserum to three commercial SIV vaccines. Since the protective ability of influenza vaccines depends primarily on the closeness of the match between the vaccine virus and the epidemic virus, the presence of nonreactive H3N2 SIV variants suggests that current commercial vaccines might not effectively protect pigs from infection with a majority of H3N2 viruses.

Prevention of transmission to humans

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Prevention of spread in humans

Recommendations to prevent spread of the virus among humans include using standard infection control against influenza. This includes frequent washing of hands with soap and water or with alcohol-based hand sanitizers, especially after being out in public. Vaccines against the H1N1 strain in the 2009 human outbreak are being developed and could be ready as early as June 2009.

Experts agree that hand-washing can help prevent viral infections, a surprisingly effective way to prevent all sorts of diseases, including ordinary influenza and the new swine flu virus. Influenza can spread in coughs or sneezes, but an increasing body of evidence shows little particles of virus can linger on tabletops, telephones and other surfaces and be transferred via the fingers to the mouth, nose or eyes. Alcohol-based gel or foam hand sanitizers work well to destroy viruses and bacteria. Anyone with flu-like symptoms such as a sudden fever, cough or muscle aches should stay away from work or public transportation and should see a doctor to be tested.

Social distancing is another tactic. It means staying away from other people who might be infected and can include avoiding large gatherings, spreading out a little at work, or perhaps staying home and lying low if an infection is spreading in a community.

Treatment

In response to requests from the U.S. Centers for Disease Control and Prevention, on April 27, 2009 the FDA issued Emergency Use Authorizations to make available diagnostic and therapeutic tools to identify and respond to the swine influenza virus under certain circumstances. The agency issued these EUAs for the use of certain Relenza and Tamiflu antiviral drugs, and for the rRT-PCR Swine Flu Panel diagnostic test.

The CDC recommends the use of Tamiflu (oseltamivir) or Relenza (zanamivir) for the treatment and/or prevention of infection with swine influenza viruses, however, the majority of people infected with the virus make a full recovery without requiring medical attention or antiviral drugs. The virus isolates that have been tested from the US and Mexico are however resistant to amantadine and rimantadine. If a person gets sick, antiviral drugs can make the illness milder and make the patient feel better faster. They may also prevent serious flu complications. For treatment, antiviral drugs work best if started soon after getting sick (within 2 days of symptoms).

Antiviral Stockpiles

Some countries have issued orders to stockpile antivirals . These typically have an expiry date of five years after manufacturing.

Preparedness

To maintain a secure household during a pandemic flu, the Water Quality & Health Council recommends keeping as supplies food and bottled water, portable power sources and chlorine bleach as an emergency water purifier and surface sanitizer.

Epidemiology

Outbreaks in swine

2007 Philippine outbreak

On August 20, 2007 Department of Agriculture officers investigated the outbreak (epizootic) of swine flu in Nueva Ecija and Central Luzon, Philippines. The mortality rate is less than 10% for swine flu, unless there are complications like hog cholera. On July 27, 2007, the Philippine National Meat Inspection Service (NMIS) raised a hog cholera "red alert" warning over Metro Manila and 5 regions of Luzon after the disease spread to backyard pig farms in Bulacan and Pampanga, even if these tested negative for the swine flu virus.

Outbreaks in humans

Swine flu has been reported numerous times as a zoonosis in humans, usually with limited distribution, rarely with a widespread distribution. The 1918 flu pandemic in humans was associated with H1N1, thus may reflect a zoonosis either from swine to humans or from humans to swine. Evidence available from that time is not sufficient to resolve this question. The "Spanish" influenza pandemic of 1918–19 infected one third of the world's population (or around 500 million persons at that time) and caused around 50 million deaths.

1976 U.S. outbreak

On February 5, 1976, an army recruit at Fort Dix said he felt tired and weak. He died the next day and four of his fellow soldiers were later hospitalized. Two weeks after his death, health officials announced that swine flu was the cause of death and that this strain of flu appeared to be closely related to the strain involved in the 1918 flu pandemic. Alarmed public-health officials decided that action must be taken to head off another major pandemic, and they urged President Gerald Ford that every person in the U.S. be vaccinated for the disease.

However, the vaccination program was plagued by delays and public relations problems. But on Oct. 1, 1976, the immunization program began and by Oct. 11, approximately 40 million people, or about 24% of the population, had received swine flu immunizations. That same day, three senior citizens died soon after receiving their swine flu shots and there was a media outcry linking the deaths to the immunizations, despite not having any positive proof. According to science writer Patrick Di Justo, however, by the time the truth was known — that the deaths were not proven to be related to the vaccine — it was too late. "The government had long feared mass panic about swine flu — now they feared mass panic about the swine flu vaccinations." This became a strong setback to the program.

There were reports of Guillain-Barré syndrome, a paralyzing neuromuscular disorder, affecting some people who had received swine flu immunizations. This syndrome is a rare side-effect of influenza vaccines, with an incidence of about one case per million vaccinations. As a result, Di Justo writes that "the public refused to trust a government-operated health program that killed old people and crippled young people." In total, less than 33 percent of the population had been immunized by the end of 1976. The National Influenza Immunization Program was effectively halted on Dec. 16.

Overall, about 500 cases of Guillain-Barré syndrome (GBS), resulting in death from severe pulmonary complications for 25 people, which, according to Dr. P. Haber, were probably caused by an immunopathological reaction to the 1976 vaccine. Other influenza vaccines have not been linked to GBS, though caution is advised for certain individuals, particularly those with a history of GBS.

2009 swine flu outbreak

The new strain of influenza involved in the 2009 swine flu outbreak is a reassortment of several strains of influenza A virus subtype H1N1 that are, separately, endemic in humans and in swine. Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in United States pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. Viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, but there is no formal national surveillance system to determine what viruses are circulating in pigs in the United States. The origins of this new strain remain unknown.

The earliest known human case, 5 year old Edgar Hernandez, was near a Virginia-based Smithfield Foods pig farm in La Gloria, Veracruz state, Mexico, that raises almost 1 million pigs a year. Residents of La Gloria have long complained about the clouds of flies that are drawn to the so-called 'manure lagoons' created by such mega-farms. Edgar Hernandez was thought to be suffering from ordinary influenza but laboratory testing revealed he had contracted swine flu. The boy went on to make a full recovery.

Dr. Anne Schuchat, interim Deputy Director for CDC Science and Public Health, said that the American cases were found to be made up of genetic elements from four different flu viruses – North American swine influenza, North American avian influenza, human influenza, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences." Pigs have been shown to act as a potential "mixing vessel" in which reassortment can occur between flu viruses of several species. This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, presumably due to superinfection in an individual human. Influenza viruses readily undergo reassortment due to antigenic shift because their genome is split between eight pieces of RNA (see Orthomyxoviridae).

The current strain of swine flu can adapt to humans and spread more efficiently than previously known swine H1N1 strains. Moreover, co-infection of H1N1 swine flu and Oseltamivir resistant H1N1 season flu can lead to acquisition of H274Y by the swine flu via recombination or reassortment. Swine H1N1 with human H1 and N1 have been reported.

The 1918 pandemic strain has polymorphism from swine and human H1N1 in all eight pieces of RNA gene segments. Similar swapping of gene segments in humans co-infected with seasonal human influenza and swine H1N1 can lead to rapid evolution.

See also

References

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External links

• Centers for Disease Control and Prevention (CDC) - Swine Flu
• World Health Organization (WHO): Swine influenza
• Medical Encyclopedia Medline Plus: Swine Flu

• Current events from April 2009
• Articles with specifically marked weasel-worded phrases from April 2009
• Pandemics
• Influenza
• Medical disasters
• 1976 disasters
• Swine diseases
• Animal virology