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Swine influenza (also called swine flu, hog flu, and pig flu) refers to influenza caused by those strains of influenza virus that usually infect pigs and are called swine influenza virus (SIV).[1] Swine influenza is common in pigs in the midwestern United States (and occasionally in other states), Mexico, Canada, South America, Europe (including the United Kingdom, Sweden, and Italy), Kenya, Mainland China, Taiwan, Japan and other parts of eastern Asia.[1]
Transmission of SIV from pigs to humans is not common. When it results in human influenza, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at risk of catching swine flu. However, only about fifty such transmissions have been recorded since the mid-20th Century, when identification of influenza subtypes became possible. (Importantly, eating pork does not pose a risk of infection.) Rarely, these strains of swine flu can pass 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" is due to a new strain of influenza A virus subtype H1N1 that was produced by reassortment from one strain of human influenza virus, one strain of avian influenza virus, and two separate strains of SIV. The origin of this new strain is unknown, and the World Organization for Animal Health (OIE) reports that this strain has not been isolated in pigs.[2] It passes with apparent ease from human to human, an ability attributed to an as-yet unidentified mutation.[3] This 2009 H1N1 strain causes the normal symptoms of influenza, such as fever, coughing and headache.[4]
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Transmission between pigs
The main route of transmission is through direct contact between infected and uninfected animals.[14] These close contacts are particularly common during animal transport. The direct transfer of the virus probably occurs either by pigs touching noses, or through dried mucus. Airborne transmission through the aerosols produced by pigs coughing or sneezing are also an important means of infection.[14] The virus usually spreads quickly through a herd, infecting all the pigs within just a few days.[1]
Transmission to 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.[25] 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.[26] 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.[25] 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.[27]
Interaction with avian H5N1 in pigs
Pigs are unusual as they can be infected with influenza strains that usually infect three different species: pigs, birds and humans.[28] This makes pigs a host where influenza viruses might exchange genes, producing new and dangerous strains.[28] 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.[29] H3N2 evolved from H2N2 by antigenic shift.[30] In August 2004, researchers in China found H5N1 in pigs.[31] These H5N1 infections may be quite common, in a survey 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.[32][33]
Signs and symptoms
In swine
In pigs influenza infection produces fever, lethargy, sneezing, coughing, difficulty breathing and decreased appetite.[14] In some cases the infection can cause abortion. Although mortality is usually low (around 1-4%),[1] the virus can produce weight loss and poor growth, causing economic loss to farmers.[14] Infected pigs can loose up to 12 pounds of body weight over a 3 to 4 week period.[14]
In humans
See also: The Centers for Disease Control and Prevention (CDC): Symptoms of Swine Flu in YouTube
Direct transmission of a swine flu virus from pigs to humans is occasionally possible (this is called zoonotic swine flu). In all, 50 cases are known to have occurred since the first report in the medical literature in 1958, which have resulted in a total of six deaths.[35] Of these six people, one was pregnant, one had leukemia, one had Hodgkin disease and two were known to be previously healthy.[35] Despite these apparently low numbers of infections, the true rate of infection may be higher, since most cases only cause a very mild disease, and will probably never be reported or diagnosed.[35]
According to the Centers for Disease Control and Prevention (CDC), in humans the symptoms of the 2009 "swine flu" H1N1 virus 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.[36] The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from pigs to humans, but from person to person.
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."[37] A diagnosis of confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).[37]
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.[38][39] 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.[40][41] 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.[42]
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.[1] 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.[1]
Prevention in humans
- Prevention of pig to human transmission
The transmission from swine to human is believed 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. Risk factors that may contribute to swine-to-human transmission include smoking and not wearing gloves when working with sick animals.[43]
- Prevention of human to human transmission
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.[44] Swine flu cannot be spread by pork products, since the virus is not transmitted through food.[44] 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.[45]
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.[46] Although the current trivalent influenza vaccine is unlikely to provide protection against the new 2009 H1N1 strain,[47] vaccines against the new strain are being developed and could be ready as early as June 2009.[48]
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 contact 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 authorities have action plans which social distancing actions to request or require depending on the severity of the outbreak.
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Classification
Of the three genera of influenza viruses that cause human flu, two also cause influenza in pigs, with Influenzavirus A being common in pigs and Influenzavirus C being rare.[5] Influenzavirus B has not been reported in pigs. Within Influenzavirus A and Influenzavirus C, the strains found in pigs and humans are largely distinct, although due to reassortment there have been transfers of genes among strains crossing swine, avian, and human species boundaries.
Influenza C
Influenza C viruses infect both humans and pigs, but do not infect birds.[6] Transmission between pigs and humans have occurred in the past.[7] For example, influenza C caused a small outbreaks of a mild form of influenza amongst children in Japan,[8] and California.[9] Due to its limited host range and the lack of genetic diversity in influenza C, this form of influenza does not cause pandemics in humans.[10]
Influenza A
Swine influenza is known to be caused by influenza A subtypes H1N1,[11] H1N2,[11] H3N1,[12] H3N2,[11] and H2N3.[13] In pigs, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are the most common strains worldwide.[14] 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.[15]
A/California/2009 (H1N1)
A/California/2009 (H1N1), 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 usually found separately, in humans, birds, and pigs. Preliminary data suggest 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 viruses present in European pigs. Viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, although there is no formal national surveillance system to determine what viruses are circulating in pigs in the United States.[16]
History
The H1N1 form of swine flu is one of the descendants of the Spanish flu that caused a pandemic in humans in 1918–1919.[18][19] 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.[18] However, direct transmission from pigs to humans is rare, with only 12 cases in the U.S. since 2005.[20]
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.[21] 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.[19]
For almost 60 years, from the first isolation in 1930 through 1998, SIV strains were almost exclusively H1N1. Then, between 1997 and 2002, novel viruses of three different subtypes and five different genotypes emerged as agents of influenza among pigs in North America. In 1997-1998, H3N2 strains emerged. These strains, which include genes derived by reassortment from human, swine and avian viruses, have become a major cause of swine influenza in North America. Reassortment between H1N1 and H3N2 produced H1N2. In 1999 in Canada, a strain of H4N6 crossed the species barrier from birds to pigs, but was contained on a single farm.[22]
Medical researchers worldwide, recognizing that swine influenza viruses might mutate into something as deadly as the Spanish flu, are carefully watching the 2009 H1N1 outbreak in humans 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.
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