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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 virus (referred to as Swine influenza viruses or SIV) refers to influenza cases that are caused by Orthomyxoviruses endemic to pig populations. SIV strains isolated to date have been classified either as Influenzavirus C or one of the various subtypes of the genus Influenzavirus A.

Swine flu infects people every year and is found typically in people who have been in contact with pigs, although there have been cases of person-to-person transmission. Symptoms include fever, disorientation, stiffness of the joints, vomiting, and loss of consciousness ending in death. 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. Most H3N2 virus isolates are triple reassortants, meaning that it contains genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages.

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. A combination of these two subtypes of the species known as the avian influenza virus in a country like China is a worst case scenario. In August 2004, researchers in China found H5N1 in pigs. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 in humans has increased to 91% in 2005.

In 2005 it was discovered that H5N1 could be infecting up to half of the pig population in some areas of Indonesia, but without causing symptoms. Chairul Nidom, a virologist at Airlangga University's tropical disease center in Surabaya, East Java, conducted an independent research; he tested the blood of 10 apparently healthy pigs housed near poultry farms in West Java where avian flu had broken out, Nature reported. 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.

History

1918 epidemic

In the spring of 1918, bird influenza mutated into a severe human form in just a few months. It was originally thought the strain evolved from a mix of swine flu which humans are more susceptible to and bird flu, with the two strains combining in a pig infected with both strains at the same time. But recovery of the actual 1918 strain from preserved tissue samples revealed it to be a mutated bird flu virus and not a swine flu virus combination.

In the U.S., approximately 675,000 people died. Worldwide, the number of casualties was between 50 and 100 million.

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. The vaccination program was plagued by delays and public relations problems, but about 24% of the population had been vaccinated by the time the program was canceled.

About 500 cases of Guillain-Barré syndrome, resulting in death from severe pulmonary complications for 25 people, were probably caused by an immunopathological reaction to the vaccine. Other influenza vaccines have not been linked to Guillain-Barré syndrome.

2007 Philippine outbreak

On August 20, 2007 Department of Agriculture officers investigated the outbreak of swine flu in Nueva Ecija and Central Luzon, Philippines. The mortality rate is less than 10% for swine flu, if there are no 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.

2009 H1N1 flu outbreak

In March and April 2009, more than 1,000 cases of swine flu in humans were detected in Mexico, and more than 80 deaths are suspected to have a connection with the virus. As of April 25, 2009 19:30 EDT there are 11 laboratory confirmed cases in the southwestern United States and in Kansas, and several suspected cases in the New York City metropolitan area. Following a series of reports of isolated cases of swine flu, the first announcement of the outbreak in Mexico was documented on April 23, 2009. Some of the cases have been confirmed by the World Health Organization to be due to a new genetic strain of H1N1. The new strain has been confirmed in 16 of the deaths and 44 others are being tested as of April 24, 2009. The Mexican fatalities are said to be mainly young adults, a hallmark of pandemic flu.

At 8 p.m. on Sunday, April 26, the New Zealand Minister of Health confirmed that 22 students returning from a school trip from Mexico had flu like symptoms (most likely swine flu). 13 of the students with flu like symptoms were tested and 10 tested positive for Influenza A, their cases strongly suspected to be the swine flu strain. However there is a possibility that the infected are not infected with the swine flu but other forms of the flu. The government has suggested that citizens of New Zealand with flu-like symptoms should see their G.P. immediately.

There have been five cases of possible swine flu in Canada, according to the Canadian Press. Two in British Columbia, and three in Nova Scotia. According to the Government of Nova Scotia, four students in Windsor, Nova Scotia have confirmed cases of swine flu.

The new strain appears to be a recombinant between two older strains. Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S. 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 U.S.

According to University of Virginia virologist Frederick Hayden, the most recent flu season was dominated by H1N1 viruses, and people who had received flu shots in the U.S. may have some protection against swine flu.

Veterinary swine flu vaccine

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.

The current vaccine against the seasonal influenza strain H1N1 is thought unlikely to provide protection. The director of CDC's National Center for Immunization and Respiratory Diseases said that the United States' 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 A virus subtype H1N1, and swine influenza virus typically found in Asia and Europe. On two cases, a complete genome sequence had been obtained. She said that the virus is resistant to amantadine and rimantadine, but susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza).

See also

Further reading

• "Swine Flu Cases Without Swine Exposure" Center for Biosecurity of UPMC
• The Swine Flu Affair: Decision-Making on a Slippery Disease Original 1978 U.S.A. Department of Health Education and Welfare review by Richard E. Neustadt and Harvey V Fineberg available from Louisiana State University Law Center Medical and Public Health Law Site.
• Surface sanitation and interruption of influenza using NAV-CO2
• The Swine Flu Episode and the Fog of Epidemics by Richard Krause in CDC's Emerging Infectious Diseases Journal Vol. 12, No. 1 January 2006 published December 20, 2005
• SWINE INFLUENZA by Carol G. Woodlief of College of Veterinary Medicine at North Carolina State University Overview, symptoms in pigs, treatment for pigs
• In California and Texas, 5 New Swine Flu Cases Washington Post, By Rob Stein (Staff Writer)
• Swine Flu In Mexico And U.S. May Lead To Pandemic, WHO Says AHN, Mayur Pahilajani (Staff Writer)
• News and information on the 2009 human swine flu outbreak

References

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6. PNAS Published online before print December 18, 2007, doi: 10.1073/pnas.0710286104 PNAS December 26, 2007 vol. 104 no. 52 20949-20954
7. Yu, H. (2008). "Genetic evolution of swine influenza A (H3N2) viruses in China from 1970 to 2006". Journal of Clinical Microbiology. 46 (3): 1067. doi:10.1128/JCM.01257-07. PMID 18199784. {{cite journal}}: Unknown parameter |month= ignored (help)
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10. "Economic impact of avian flu". The World Bank.
11. WHO (October 28, 2005). "H5N1 avian influenza: timeline" (PDF).
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13. . 31 March 2009 http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/mar3109swine-jw.html. {{cite news}}: Missing or empty |title= (help) report on Pigs as carriers
14. Pandemics and Pandemic Threats since 1900. U.S. Department of Health & Human Services
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17. "Influenza / Flu Vaccine". University of Illinois at Springfield. Retrieved 26 April 2009.
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34. "Update: Swine Influenza A (H1N1) Infections --- California and Texas, April 2009". MMWR. Centrers for Disease Control. 24 April 2009.
35. Steven Reinberg (24 April 2009). "Swine Flu Cases Now Total 7: CDC". ABC News.
36. Rob Stein (23 April 2009). "In California and Texas, 5 New Swine Flu Cases". Washington Post.
37. "CDC Briefing on Public Health Investigation of Human Cases of Swine Influenza". CDC online newsroom. 23 April 2009.

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