Swine influenza (also swine flu, hog flu, and pig 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). Of the three genera of human flu, two are endemic also in swine: Influenzavirus A is common and Influenzavirus C is 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.Swine flu is common in swine in the midwestern United States (and occasionally in other states), Mexico, Canada, South America, Europe (including the United Kingdom, Sweden, and Italy), Kenya, China, Japan, Taiwan, and other parts of eastern Asia. Swine flu is 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 infrequently circulate between humans as SIV rarely mutates into a form able to pass easily from human to human. 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 2009 flu outbreak in humans that is widely known as "swine flu" apparently is not due to a swine influenza virus. It is due to a new strain of influenza A virus subtype H1N1 that derives from one strain of human influenza virus, one strain of avian influenza virus, and two separate strains of swine influenza virus. The origins of this new strain are unknown, and the World Organization for Animal Health (OIE) reports that this strain has not been isolated in swine. It passes with apparent ease from human to human, an ability attributed to an as-yet unidentified mutation. The strain 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.
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 C
Influenza ASwine 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.
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.
History
The H1N1 form of swine flu is one of the descendants of the Spanish flu that caused a devastating pandemic in humans in 1918–1919. As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza. However, direct transmission from pigs to humans is rare, with only 12 cases in the U.S. since 2005.
The influenza virus constantly changes form, thereby eluding the protective antibodies that people may have developed in response to previous exposures to influenza or to influenza vaccines. Every two or three years the virus undergoes minor changes. But 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 change that enables it to easily infect populations around the world, often infecting hundreds of millions of people whose antibody defenses are unable to resist it. The influenza virus has also been known to change form over a much shorter period of time. For instance, during the Spanish flu pandemic, the initial wave of the disease was relatively mild, while the second wave of the disease a year later was highly lethal.
H1N1 VIRUS TRANSMISSION THROUGH PIG
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In 1957, an Asian flu pandemic infected some 45 million Americans and killed 70,000. It caused about 2 million deaths globally. 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." In the U.S. during an average year, there are approximately 50 million cases of "normal" flu leading to around 36,000 deaths, mostly to extremely young, old, or frail persons, with a large percentage of those due to complications such as pneumonia.
Medical researchers worldwide, recognizing that the swine flu virus might again mutate into something as deadly as the Spanish flu, are carefully watching the latest 2009 outbreak of swine flu and making contingency plans for a possible global pandemic. Several countries took precautionary measures to reduce the chances for a global pandemic of the disease.
Signs and symptoms
In humans
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).
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
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.
Interaction with H5N1Avian 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 a survey of swine infections with H5N1 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.
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.
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
Methods of preventing the spread of influenza among swine include facility management, herd management, and vaccination. Because much of the illness and death associated with swine flu involves secondary infection by other pathogens, control strategies that rely on vaccination may be insufficient.
Control of swine influenza by vaccination has become more difficult 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. The United States Department of Agriculture researchers say that while pig vaccination keeps pigs from getting sick, it does not block infection or shedding of the virus.
Facility management includes using disinfectants and ambient temperature to control virus in the environment. The virus is unlikely to survive outside living cells for >2 wk except in cold (but above freezing) conditions, and it is readily inactivated by disinfectants.
Control of swine influenza by vaccination has become more difficult 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. The United States Department of Agriculture researchers say that while pig vaccination keeps pigs from getting sick, it does not block infection or shedding of the virus.
Facility management includes using disinfectants and ambient temperature to control virus in the environment. The virus is unlikely to survive outside living cells for >2 wk except in cold (but above freezing) conditions, and it is readily inactivated by disinfectants.
Herd management includes not adding pigs carrying influenza to herds that have not been exposed to the virus. The virus survives in healthy carrier pigs for up to 3 months and can be recovered from them between outbreaks. Carrier pigs are usually responsible for the introduction of SIV into previously uninfected herds and countries. After an outbreak, as immunity in exposed pigs wanes, new outbreaks of the same strain can occur.
Prevention of transmission to humans
The transmission from swine to human is believe to occur mainly in swine farms where farmers are in close contact with live pigs. Although strains of swine influenza are usually not able to infect humans this may occasionally happen, so farmers and veterinarians are encouraged to use a face mask when dealing with infected animals. The use of vaccines on swine to prevent their infection is a major method of limiting swine to human transmission.
Prevention of spread in humans
Influenza spreads between humans through coughing or sneezing and people 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 for analysis.
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. Although the current trivalent influenza vaccine is unlikely to provide protection against the new 2009 H1N1 strain, vaccines against the new strain are being developed and could be ready as early as June 2009.
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. Although the current trivalent influenza vaccine is unlikely to provide protection against the new 2009 H1N1 strain, vaccines against the new strain are being developed and could be ready as early as June 2009.
Experts agree that hand-washing can help prevent viral infections, including ordinary influenza and the swine flu virus. Influenza can spread in coughs or sneezes, but an increasing body of evidence shows small droplets containing the 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. Public health and other responsible authorites have action plans which social distancing actions to request or require depending on the severity of the outbreak.
Treatment In humans
If a person becomes sick with swine flu, 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). Beside antivirals, palliative care, at home or in the hospitals, focuses on controlling fevers and maintaining fluid balance. The U.S. Centers for Disease Control and Prevention 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 in the 2009 outbreak have been found resistant to amantadine and rimantadine.
In the U.S., on April 27, 2009, the FDA issued Emergency Use Authorizations to make available Relenza and Tamiflu antiviral drugs to treat the swine influenza virus in cases for which they are currently unapproved. The agency issued these EUAs to allow treatment of patients younger than the current approval allows and to allow the widespread distribution of the drugs, including by non-licensed volunteers.
Epidemiology : outbreaks in swine
2007 Philippine outbreakOn 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.
Epidemiology: 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 October 1, 1976, the immunization program began and by October 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 the lack of 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 modern 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. Still, as observed by a participant in the immunization program, the vaccine killed more Americans than the disease did.
1988 swine flu outbreak
In September 1988, a swine flu virus killed one woman in Wisconsin, and infected at least hundreds of others. 32-year old Barbara Ann Wieners was eight months pregnant when she and her husband, Ed, became ill after visiting the hog barn at the Walworth County Fair. Barbara died eight days later, though doctors were able to induce labor and deliver a healthy daughter before she passed away. Her husband recovered from his symptoms.
Influenza-like illnesses were reportedly widespread among the pigs at the fair they had visited, and 76% of the swine exhibitors there tested positive for the swine flu antibody but no serious illnesses were detected among this group. Additional studies suggested between one and three health care personnel who had contact with the patient developed mild influenza-like illnesses with antibody evidence of swine flu infection.
Influenza-like illnesses were reportedly widespread among the pigs at the fair they had visited, and 76% of the swine exhibitors there tested positive for the swine flu antibody but no serious illnesses were detected among this group. Additional studies suggested between one and three health care personnel who had contact with the patient developed mild influenza-like illnesses with antibody evidence of swine flu infection.
2009 flu outbreak
The new strain of swine influenza A (H1N1) involved in the 2009 flu outbreak in humans is a reassortment of several strains of influenza A virus subtype H1N1 that are, separately, endemic in humans, endemic in birds, and endemic 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 confirmed case in Mexico occurred in La Gloria, Veracruz. Residents of La Gloria have long complained about clouds of flies drawn to manure cesspools created by Virgia-based Smithfield farm. While a large percentage of residents fell ill before the test was developed for the novel flu virus, only one of the available samples when rechecked proved to be positive.
The current strain of swine flu can spread more efficiently human-to-human than previously known swine H1N1 strains. After its initial detection in the U.S. and Mexico, it was soon detected in a number of patients in several countries who had travelled to Mexico. By April 30, except in the southern U.S., transmission to individuals who had not travelled to Mexico had occurred in only a small number of individuals in close contact with someone who had. The new strain of swine influenza A (H1N1) is currently listed by the United States and World Health Organization as a Phase 5 pandemic virus.
The earliest confirmed case in Mexico occurred in La Gloria, Veracruz. Residents of La Gloria have long complained about clouds of flies drawn to manure cesspools created by Virgia-based Smithfield farm. While a large percentage of residents fell ill before the test was developed for the novel flu virus, only one of the available samples when rechecked proved to be positive.
The current strain of swine flu can spread more efficiently human-to-human than previously known swine H1N1 strains. After its initial detection in the U.S. and Mexico, it was soon detected in a number of patients in several countries who had travelled to Mexico. By April 30, except in the southern U.S., transmission to individuals who had not travelled to Mexico had occurred in only a small number of individuals in close contact with someone who had. The new strain of swine influenza A (H1N1) is currently listed by the United States and World Health Organization as a Phase 5 pandemic virus.
Spread within Mexico
The outbreak was first detected in Mexico City, where surveillance began picking up a surge in cases of influenza-like illness (ILI) starting March 18. The surge was assumed by Mexican authorities to be "late-season flu" (which usually coincides with a mild Influenzavirus B peak) until April 21, when a U.S. Centers for Disease Control and Prevention alert concerning two isolated cases of a novel swine flu was reported in the media. This new strain was promptly confirmed in Mexico, connecting the new strain to the ongoing outbreak of ILI. The first death from swine flu occurred on April 13, when a diabetic woman from Oaxaca died from respiratory complications. Some samples were sent to the U.S.-based CDC on April 18.
In March and April 2009, over 3000 cases of suspected swine flu in humans were detected in Mexico and the southwestern United States. The disease was detected in several countries on multiple continents within weeks of its initial discovery. The strain appears to be unusually lethal in Mexico but not in other countries. Although there have been reports of 152 "probable deaths" in Mexico City and "more than 100 dead from swine flu", the WHO had received reports of only 7 confirmed deaths total and explicitly denied the larger figure as of April 29.
Mexico's schools, universities, and all public events will be closed from April 24, 2009 to May 6, 2009. On April 27, 2009, a few schools in the U.S. closed due to confirmed cases in students.
In March and April 2009, over 3000 cases of suspected swine flu in humans were detected in Mexico and the southwestern United States. The disease was detected in several countries on multiple continents within weeks of its initial discovery. The strain appears to be unusually lethal in Mexico but not in other countries. Although there have been reports of 152 "probable deaths" in Mexico City and "more than 100 dead from swine flu", the WHO had received reports of only 7 confirmed deaths total and explicitly denied the larger figure as of April 29.
Mexico's schools, universities, and all public events will be closed from April 24, 2009 to May 6, 2009. On April 27, 2009, a few schools in the U.S. closed due to confirmed cases in students.
There have also been cases reported in the states of San Luis Potosí, Hidalgo, Querétaro and Mexico State. Some cases in Mexico and the United States have been confirmed by the World Health Organization to be a new strain of H1N1. The Mexican fatalities are mainly young adults of 25 to 45, a common trait of pandemic flu. Mexican Health Minister José Ángel Córdova on April 24, said "We’re dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable."
