The 2009 swine flu outbreak is the spread of a new strain of H1N1 influenza virus that was first detected by public health agencies in March 2009. Local outbreaks of influenza-like illnessMexico, but the new strain was not clinically ascertained as such until a month later in cases in Texas and California, whereupon its presence was swiftly confirmed in various Mexican states and Mexico City; within days isolated cases elsewhere in Mexico, the U.S., and the Northern Hemisphere were also identified. By April 27, the new strain was confirmed in Canada, Spain, and the United Kingdom and suspected in many other nations, including New Zealand, with over 2,400 candidate cases, prompting the World Health Organization (WHO) to raise their pandemic alert level to 4. A level 4 warning officially means that the WHO considers that there is "sustained human to human transmission"; whereas levels 5 and 6 represent "widespread human infection". were first detected in three areas of
The new strain 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.
In April both the WHO and the United States Centers for Disease Control and Prevention (CDC) expressed serious concerns about the situation. It had the potential to become a flu pandemic On April 25, 2009, the WHO determined the situation to be a formal "public health emergency of international concern", with knowledge lacking in regard to "the clinical features, epidemiology, and virology of reported cases and the appropriate responses". Government health agencies around the world also expressed concerns over the outbreak and are monitoring the situation closely. because the strain was novel, transmitted from human to human against little immunity, and the Mexican mortality rate was unusually high.
As of April 26, 2009, Mexico City schools, universities, and all public events remained closed or suspended while other schools in the U.S. closed due to confirmed cases in students. On April 27, 2009, Mexican government officials announced a nationwide shut down of schools.
Prior influenza season
Prior to the outbreak, the Northern Hemisphere winter of 2008–2009 had been a comparatively mild season for flu infections, which typically cause 250,000–500,000 deaths worldwide yearly, mostly in the elderly, the very young, and persons with chronic illness. Up to April 8, 2009, the CDC had reported the deaths of 43 children from seasonal flu, compared to 68 in the previous flu season. The improvement was attributed in part to an improved Northern Hemisphere winter of 2008–2009 seasonal flu vaccine, for which a rare decision had been made to update all three strains (H1, H3, and B) simultaneously, which ultimately yielded a very good match to the strains of H1N1 and H3N2 which eventually circulated. (This followed the poor performance of the 2007–2008 vaccine, which offered only 2–20% protection rather than the 70–90% achieved in some years.) The improvement was also attributed to new recommendations including the vaccination of people 5–18 years of age, who potentially act as "super-spreaders" due to failure to take precautions such as hand-washing.
Furthermore, from December 2005 through February 2009, a total of twelve human infections with swine influenza were reported from ten states in the USA.
Initial outbreak
The earliest known case was at a Mexican pig farm nearby La Gloria whose nearby neighbors had been complaining about the smell and flies. 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. The first two cases identified (and confirmed) as swine flu were two children living in the United States, in San Diego County and Imperial County, California, who became ill on March 28 and 30. This new strain was promptly confirmed in Mexico, connecting the new strain to the ongoing outbreak of ILI. The first deadly case seems to go back to April 13, where it is believed that the first case was in a woman from Oaxaca. Some samples were sent to the U.S.-based CDC on April 18. News of the connection was broadcast live in Mexico on April 23, 2009.In March and April 2009, over 1000 cases of suspected swine flu in humans were detected in Mexico and the southwestern United States. The strain appears to be unusually lethal in Mexico, causing 152 deaths (20 confirmed) so far, mostly in Mexico City. There have also been cases reported in the states of San Luis Potosí, Hidalgo, Querétaro and Mexico State, all in central Mexico; Some cases in Mexico and the United States have been confirmed by the World Health Organization to be a never-before-seen strain of H1N1. The Mexican fatalities are mainly young adults of 25 to 45, a hallmark of pandemic flu. A new swine flu strain was confirmed in 16 of the deaths and at least 100 others were being tested as of April 24, 2009. 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." The origins of the new virus strain remain unknown. One theory is that Asian and European strains traveled to Mexico in migratory birds or in people, then combined with North American strains in Mexican pig factory farms before jumping over to farm workers. The Mexican health agency acknowledged that the original disease vector of the virus may have been flies multiplying in manure lagoons of pig farms near Perote, Veracruz, owned by Granjas Carroll, a subsidiary of Smithfield Foods. Smithfield Foods responded, saying that that it had found no clinical signs or symptoms of the presence of swine influenza in the company's swine herd, nor in its employees at its joint ventures in Mexico, and that it routinely administers influenza virus vaccination to their swine herds and conducts monthly testing for the presence of swine influenza.
Genetics and effects
The CDC has confirmed that American 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, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences." Pigs have been shown to act as a potential "mixing vessel" in which reassortment can occur between flu viruses of several species. This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, presumably due to superinfection in an individual human. Influenza viruses readily undergo reassortment because their genome is split between eight pieces of RNA (see Orthomyxoviridae).
The virus was resistant to amantadine and rimantadine, but susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza). Several complete genome sequences for U.S. flu cases were rapidly made available through the Global Initiative on Sharing Avian Influenza Data (GISAID).
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. The six genes from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses. While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S. The seasonal influenza strain H1N1 vaccine is thought to be unlikely to provide protection.
The CDC does not understand why the American cases were primarily mild disease while the Mexican cases had led to multiple deaths. However, research on previous pandemic strains have suggested that mortality can vary widely between different countries, with mortality being concentrated in the developing world. Differences in the viruses or co-infection are also being considered as possible causes. Only fourteen samples from Mexico had been tested by the CDC, with seven found to match the American strain. The virus likely passes through several cycles of infection with no known linkages between patients in Texas and California, and that containment of the virus is "not very likely". The U.S. embassy reported that a CDC investigative team arrived in Mexico City on April 25 to work with Mexican counterparts to study the virus. While the seasonal flu kills less than 1% of those infected, the Mexican fatality rate represents a 7% rate. This compares to the global rate of more than 2.5% for the Spanish flu pandemic of 1918.
Prior influenza season
Prior to the outbreak, the Northern Hemisphere winter of 2008–2009 had been a comparatively mild season for flu infections, which typically cause 250,000–500,000 deaths worldwide yearly, mostly in the elderly, the very young, and persons with chronic illness. Up to April 8, 2009, the CDC had reported the deaths of 43 children from seasonal flu, compared to 68 in the previous flu season. The improvement was attributed in part to an improved Northern Hemisphere winter of 2008–2009 seasonal flu vaccine, for which a rare decision had been made to update all three strains (H1, H3, and B) simultaneously, which ultimately yielded a very good match to the strains of H1N1 and H3N2 which eventually circulated. (This followed the poor performance of the 2007–2008 vaccine, which offered only 2–20% protection rather than the 70–90% achieved in some years.) The improvement was also attributed to new recommendations including the vaccination of people 5–18 years of age, who potentially act as "super-spreaders" due to failure to take precautions such as hand-washing.
Furthermore, from December 2005 through February 2009, a total of twelve human infections with swine influenza were reported from ten states in the USA.
Initial outbreak
The earliest known case was at a Mexican pig farm nearby La Gloria whose nearby neighbors had been complaining about the smell and flies. 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. The first two cases identified (and confirmed) as swine flu were two children living in the United States, in San Diego County and Imperial County, California, who became ill on March 28 and 30. This new strain was promptly confirmed in Mexico, connecting the new strain to the ongoing outbreak of ILI. The first deadly case seems to go back to April 13, where it is believed that the first case was in a woman from Oaxaca. Some samples were sent to the U.S.-based CDC on April 18. News of the connection was broadcast live in Mexico on April 23, 2009.In March and April 2009, over 1000 cases of suspected swine flu in humans were detected in Mexico and the southwestern United States. The strain appears to be unusually lethal in Mexico, causing 152 deaths (20 confirmed) so far, mostly in Mexico City. There have also been cases reported in the states of San Luis Potosí, Hidalgo, Querétaro and Mexico State, all in central Mexico; Some cases in Mexico and the United States have been confirmed by the World Health Organization to be a never-before-seen strain of H1N1. The Mexican fatalities are mainly young adults of 25 to 45, a hallmark of pandemic flu. A new swine flu strain was confirmed in 16 of the deaths and at least 100 others were being tested as of April 24, 2009. 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." The origins of the new virus strain remain unknown. One theory is that Asian and European strains traveled to Mexico in migratory birds or in people, then combined with North American strains in Mexican pig factory farms before jumping over to farm workers. The Mexican health agency acknowledged that the original disease vector of the virus may have been flies multiplying in manure lagoons of pig farms near Perote, Veracruz, owned by Granjas Carroll, a subsidiary of Smithfield Foods. Smithfield Foods responded, saying that that it had found no clinical signs or symptoms of the presence of swine influenza in the company's swine herd, nor in its employees at its joint ventures in Mexico, and that it routinely administers influenza virus vaccination to their swine herds and conducts monthly testing for the presence of swine influenza.
Genetics and effects
The CDC has confirmed that American 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, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences." Pigs have been shown to act as a potential "mixing vessel" in which reassortment can occur between flu viruses of several species. This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, presumably due to superinfection in an individual human. Influenza viruses readily undergo reassortment because their genome is split between eight pieces of RNA (see Orthomyxoviridae).
The virus was resistant to amantadine and rimantadine, but susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza). Several complete genome sequences for U.S. flu cases were rapidly made available through the Global Initiative on Sharing Avian Influenza Data (GISAID).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. The six genes from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses. While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S. The seasonal influenza strain H1N1 vaccine is thought to be unlikely to provide protection.
The CDC does not understand why the American cases were primarily mild disease while the Mexican cases had led to multiple deaths. However, research on previous pandemic strains have suggested that mortality can vary widely between different countries, with mortality being concentrated in the developing world. Differences in the viruses or co-infection are also being considered as possible causes. Only fourteen samples from Mexico had been tested by the CDC, with seven found to match the American strain. The virus likely passes through several cycles of infection with no known linkages between patients in Texas and California, and that containment of the virus is "not very likely". The U.S. embassy reported that a CDC investigative team arrived in Mexico City on April 25 to work with Mexican counterparts to study the virus. While the seasonal flu kills less than 1% of those infected, the Mexican fatality rate represents a 7% rate. This compares to the global rate of more than 2.5% for the Spanish flu pandemic of 1918.
At a press briefing on April 27, acting CDC director Dr. Richard Besser stated that out of 40 confirmed cases in the United States at that point, only one individual was hospitalized. He also revealed that the median age was 16 years "with a range in age of 7 to 54 years."
Unlike what usually happens in cases of influenza, which inflict a greater number of deaths between the elderly and the children, this strain resulted in deaths only in people between the ages of 25 and 50.
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