Swine influenza

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Typical victim of Pig Flu.

Swine influenza virus (referred to as Swine influenza viruses or SIV) refers to influenza cases that are caused by Orthomyxoviruses endemic to populations of pigs. 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 conciousness ending in death. Influenza A virus subtypes H1N1, H1N2, H3N1 and H3N2 are all known to cause SIV infections. 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

Main article: Spanish Flu

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.

Some of the victims of the 1918 flu became severely ill and died, while the rest suffered from non-fatal symptoms. In the U.S., the first deaths were recorded among sailors in Boston in August 1918, and the epidemic quickly spread to all parts of the country. Between the autumn of 1918 and the spring of 1919, 548,452 people died of this flu in the U.S. In the U.K., France and Germany, around 600,000 people died. Worldwide, the number of casualties was estimated to be between 20 and 100 million . The puzzling fact is that the epidemic erupted almost simultaneously at distant locations. Most of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients. The pandemic lasted from March 1918 to June 1920, At the military prison at Deer Island (Massachusetts) in Boston Harbor there was an attempt to develop a vaccine during the 1918 outbreak.

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

Main article: 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.

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.