A/VERACRUZ/2009 (H1N1) - Swine influenza

Swine influenza (also called swine flu, pigfluenza, 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). 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.

Transmission of swine influenza virus from pigs to humans is not common. 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 first identification of an influenza virus as a cause of disease in pigs occurred in 1930.[20] For the following 60 years, swine influenza 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.

The 2009 flu outbreak in humans that is widely known as "swine flu" is due to an apparently virulent new strain of influenza A virus subtype H1N1 that was produced by resentment from one strain of human influenza virus, one strain of avian influenza virus, and two separate strains of swine influenza. 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. It passes with apparent ease from human to human, an ability attributed to an as-yet unidentified mutation. This 2009 H1N1 strain causes the normal symptoms of influenza, such as fever, coughing and headache.

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. 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. Transmission between pigs and humans have occurred in the past. For example, influenza C caused a small outbreaks of a mild form of influenza amongst children in Japan, and California. 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.

Influenza A
Swine influenza is known to be caused by influenza A subtypes H1N1, H1N2, H3N1, H3N2, and H2N3. In pigs, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are the most common strains worldwide. 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.

A/VERACRUZ/2009 (H1N1)

A/Veracruz/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.

Main symptoms of swine flu in humans

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. Of these six people, one was pregnant, one had leukemia, one had Hodgkin disease and two were known to be previously healthy. 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.

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. The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from pigs to humans, but from person to person.

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.

Prevention in humans
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.

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.

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.

TREATMENT

In swine
As swine influenza is not usually fatal to pigs, little treatment is required; instead veterinary efforts are focused on preventing the spread of the virus throughout the farm, or to other farms. Vaccination and animal management techniques are most important in these efforts. The modern pork industry also uses antibiotic, which although they have no effect against the influenza virus, do help prevent bacterial pneumonia and other secondary infections in influenza-weakened herds.

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.

2009 outbreak in humans

The 2009 flu outbreak is due to a new strain of influenza, an apparent reassortment of at least four strains of influenza A virus subtype H1N1, including one strain endemic in humans, one endemic in birds, and two endemic in swine. Although initial reports identified the new strain as swine influenza (ie, a zoonosis), the new strain has not been reported in swine