The 2009 swine flu outbreak is the epidemic spread of a new strain of influenza virus that was clinically identified in April 2009. The new virus strain is a type of influenza A (H1N1) virus, commonly called the swine flu.
By April 28, the new strain was confirmed to have spread to Spain, the United Kingdom, New Zealand, and Israel, and the virus was suspected in many other nations, with a total of over 4,400 candidate cases. As a result, WHO raised its alert level to“Phase 5” out of 6 possible, which it defines as a “signal that a pandemic is imminent. ”
In industrialized countries most of these annual deaths occur in people aged 65 or older.
Spread within Mexico
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. Some samples were sent to the U.S.-based CDC on April 18.
The high humidity of summer and the increase in exposure to ultraviolet light typically leads to the end of the flu season as the virus becomes less likely to spread.
Genetics and effects
|Genetic origins of the 2009 swine flu virus|
|HA||Hemagglutinin||swine (H1)||North America|
|PA||Polymerase Acid||avian||North America|
|PB1||Polymerase Basic Subunit 1, PB1-F2||human||1993 H3N2 strain|
|PB2||Polymerase Basic subunit 2||avian||North America|
|M||Matrix protein M1, M2||swine||Eurasia|
|NS||Non-structural proteins NS1, NEP||swine||North America|
In this video, Dr. Joe Bresee, with CDC’s Influenza Division, describes the symptoms of swine flu and warning signs to look for that indicate the need for urgent medical attention.
Pigs are susceptible to influenza viruses that can also infect both humans and birds, so they may act as a “mixing vessel” in which reassortment can occur between flu viruses of several species. Reassortment is a process that happens if two different types of influenza virus infect a single cell and it can produce a new strain of influenza.
This new strain appears to be a result of the reassortment of two swine influenza viruses, one from North America and one from Europe. But the North American pig strain was itself the product of previous reassortments, and has carried an avian PB2 gene for at least ten years and a human PB1 gene since 1993.
Symptoms and severity
The CDC does not fully understood why the U.S. cases’ symptoms were primarily mild while the Mexican cases had led to multiple deaths. However, research on previous pandemic strains has suggested that mortality can vary widely between different countries, with mortality being concentrated in the developing world.
International cases and responses
The new strain has spread widely beyond Mexico, with confirmed cases in eighteen countries and suspected cases in forty-two. Many countries have advised their inhabitants not to travel to infected areas. Countries including Australia, China, Iceland, India,Indonesia, Malaysia, Philippines, Singapore, South Korea and Thailand are monitoring visitors returning from flu-affected areas to identify people with fever and respiratory symptoms.
Government actions against pigs and pork
Although there is no evidence that the virus is transmitted by food, and influenza A viruses are generally killed by heating, some countries banned import and sale of pork products “as a precaution against swine flu”.
Several countries, including Serbia, China and Russia banned the import of pork products from North America in general as a response to the outbreak, despite assurances from the WHO that the disease is not spread through pork.
Possible transmission of flu virus from man to animal
In Alberta, Canada, provincial and federal officials announced on May 2 that a 2,200-head pig herd in central Alberta was under quarantine after preliminary findings indicated some of the animals were infected with swine flu.
WHO and CDC officials are concerned that this outbreak may become a pandemic, for the following reasons:
- New strain
The virus is a new strain of influenza, from which human populations have not been vaccinated or naturally immunized. In the United States, cases infected 25% of family members. Seasonal flu tends to sicken 5% to 20% of family members.
- Widespread human transmission
The virus infects by human-to-human transmission. Investigations of infected patients indicated no direct contact with swine, such as at a farm or agricultural fair.
All but one of the fatalities to date have been in Mexico. In Mexico, according to the New York Times, the deaths from the illness have primarily been young, healthy adults. Most other influenza strains produce the worst symptoms in young children, the elderly, and others with weaker immune systems.
- Lack of data
That other crucial factors are still largely unknown, such as transmission rates and patterns (epidemicity) and effectiveness of current influenza treatments, combined with the innate unpredictability of influenza strains, means that reliable forecasts cannot be made.
Since the outbreak began, there has been a week of near constant media attention.
Epidemiologists caution that the number of cases reported in the early days of an outbreak can be very inaccurate and deceptive. This can be due to several causes, including selection bias, media bias, and incorrect reporting by governments.
Furthermore, if national governments and local health care services for whatever reason do not accurately report their own data on suspected cases and deaths this will produce a garbage in garbage out bias in conclusions drawn from such data by any agency downstream, whether private or governmental, the media or official. For example, World Health Organization accused China of under-reporting cases of SARS during the 2002 SARS outbreak.
Gathering accurate data for the flu outbreak is further complicated by the possibility of further mutations of the virus, and because laboratory facilities to perform swift genetic tests on patient samples are not widely available.