Saturday, February 28, 2015

Swine Flu

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What is the swine flu?

Swine flu (swine influenza) is a respiratory disease caused by viruses (influenza viruses) that infect the respiratory tract of pigs, resulting in nasal secretions, a barking cough, decreased appetite, and listless behavior. Swine flu produces most of the same symptoms in pigs as human flu produces in people. Swine flu can last about one to two weeks in pigs that survive. Swine influenza virus was first isolated from pigs in 1930 in the U.S. and has been recognized by pork producers and veterinarians to cause infections in pigs worldwide. In a number of instances, people have developed the swine flu infection when they are closely associated with pigs (for example, farmers, pork processors), and likewise, pig populations have occasionally been infected with the human flu infection. In most instances, the cross-species infections (swine virus to man; human flu virus to pigs) have remained in local areas and have not caused national or worldwide infections in either pigs or humans. Unfortunately, this cross-species situation with influenza viruses has had the potential to change. Investigators decided the 2009 so-called "swine flu" strain, first seen in Mexico, should be termed novel H1N1 flu since it was mainly found infecting people and exhibits two main surface antigens, H1 (hemagglutinin type 1) and N1 (neuraminidase type1). The eight RNA strands from novel H1N1 flu have one strand derived from human flu strains, two from avian (bird) strains, and five from swine strains.

Swine flu is transmitted from person to person by inhalation or ingestion of droplets containing virus from people sneezing or coughing; it is not transmitted by eating cooked pork products. The newest swine flu virus that has caused swine flu is influenza A H3N2v (commonly termed H3N2v) that began as an outbreak in 2011. The "v" in the name means the virus is a variant that normally infects only pigs but has begun to infect humans. There have been small outbreaks of H1N1 since the pandemic; a recent one is in India where at least three people have died.
What causes swine flu?

The cause of the 2009 swine flu was an influenza A virus type designated as H1N1. In 2011, a new swine flu virus was detected. The new strain was named influenza A (H3N2)v. Only a few people (mainly children) were first infected, but officials from the U.S. Centers for Disease Control and Prevention (CDC) reported increased numbers of people infected in the 2012-2013 flu season. Currently, there are not large numbers of people infected with H3N2v. Unfortunately, another virus termed H3N2 (note no "v" in its name) has been detected and caused flu, but this strain is different from H3N2v. In general, all of the influenza A viruses have a structure similar to the H1N1 virus; each type has a somewhat different H and/or N structure.

Why is swine flu now infecting humans?

Many researchers now consider that two main series of events can lead to swine flu (and also avian or bird flu) becoming a major cause for influenza illness in humans.

First, the influenza viruses (types A, B, C) are enveloped RNA viruses with a segmented genome; this means the viral RNA genetic code is not a single strand of RNA but exists as eight different RNA segments in the influenza viruses. A human (or bird) influenza virus can infect a pig respiratory cell at the same time as a swine influenza virus; some of the replicating RNA strands from the human virus can get mistakenly enclosed inside the enveloped swine influenza virus. For example, one cell could contain eight swine flu and eight human flu RNA segments. The total number of RNA types in one cell would be 16; four swine and four human flu RNA segments could be incorporated into one particle, making a viable eight RNA-segmented flu virus from the 16 available segment types. Various combinations of RNA segments can result in a new subtype of virus (this process is known as antigenic shift) that may have the ability to preferentially infect humans but still show characteristics unique to the swine influenza virus (see Figure 1). It is even possible to include RNA strands from birds, swine, and human influenza viruses into one virus if a single cell becomes infected with all three types of influenza (for example, two bird flu, three swine flu, and three human flu RNA segments to produce a viable eight-segment new type of flu viral genome). Formation of a new viral type is considered to be antigenic shift; small changes within an individual RNA segment in flu viruses are termed antigenic drift (see figure 1) and result in minor changes in the virus. However, these small genetic changes can accumulate over time to produce enough minor changes that cumulatively alter the virus' makeup over time (usually years).

Second, pigs can play a unique role as an intermediary host to new flu types because pig respiratory cells can be infected directly with bird, human, and other mammalian flu viruses. Consequently, pig respiratory cells are able to be infected with many types of flu and can function as a "mixing pot" for flu RNA segments (see figure 1). Bird flu viruses, which usually infect the gastrointestinal cells of many bird species, are shed in bird feces. Pigs can pick these viruses up from the environment, and this seems to be the major way that bird flu virus RNA segments enter the mammalian flu virus population. Figure 1 shows this process in H1N1, but the figure represents the genetic process for all flu viruses, including human, swine, and avian strains.

What are the symptoms of swine flu?

Symptoms of swine flu are similar to most influenza infections: fever (100 F or greater), cough, nasal secretions, fatigue, and headache, with fatigue being reported in most infected individuals. Some patients may also get a sore throat, rash, body aches, headaches, chills, nausea, vomiting, and diarrhea. In Mexico, many of the initial patients infected with H1N1 influenza were young adults, which made some investigators speculate that a strong immune response, as seen in young people, may cause some collateral tissue damage.The incubation period from exposure to first symptoms is about one to four days, with an average of two days. The symptoms last about one to two weeks and can last longer if the person has a severe infection.

Some patients develop severe respiratory symptoms and need respiratory support (such as a ventilator to breathe for the patient). Patients can get pneumonia (bacterial secondary infection) if the viral infection persists, and some can develop seizures. Death often occurs from secondary bacterial infection of the lungs; appropriate antibiotics need to be used in these patients. The usual mortality (death) rate for typical influenza A is about 0.1%, while the 1918 "Spanish flu" epidemic had an estimated mortality rate ranging from 2%-20%. Swine (H1N1) flu in Mexico had about 160 deaths and about 2,500 confirmed cases, which would correspond to a mortality rate of about 6%, but these initial data were revised and the mortality rate worldwide was estimated to be much lower. Fortunately, the mortality rate of H1N1 remained low and similar to that of the conventional flu (average conventional flu mortality rate is about 36,000 per year; projected H1N1 flu mortality rate was 90,000 per year in the U.S. as determined by the president's advisory committee, but it never approached that high number).

Fortunately, although H1N1 developed into a pandemic (worldwide) flu strain, the mortality rate in the U.S. and many other countries only approximated the usual numbers of flu deaths worldwide. Speculation about why the mortality rate remained much lower than predicted includes increased public awareness and action that produced an increase in hygiene (especially hand washing), a fairly rapid development of a new vaccine, and patient self-isolation if symptoms developed.

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