Swine Influenza A H1/N1 (Swine Flu)

Swine Influenza A usually only infects pigs, however there have been isolated cases of infection in humans. From 1958 to 2005 approximately 37 cases of Swine Influenza were documented (1 case every 1-2 years on average) in humans and nearly half of them had exposure to pigs. Fortunately the infections did not readily spread from one human to another and the episodes ended without widespread illness in the community.

Interestingly from 2005 to 2009, there was an increase in reported Human Swine Influenza infections with 12 cases reported (2-3 cases per year on average) and 11 of 12 individuals had direct or indirect contact with pigs.

In the spring of 2009 (late March to early April) an outbreak of Human Swine Influenza A H1/N1 infection, with evidence of rapid human to human spread, was reported from North America.

The first documented cases were from Mexico and symptoms developed around 3/18, while documented cases in the United States developed symptoms on 3/28. The cases in Mexico were from 3 different districts: Federal District of Mexico City, Veracruz and Oaxaca, while those in the United States were from 2 counties in California that were 160 miles apart. The cases did not involve a history of exposure to pigs, nor was there a history of travel between Mexico and the United States. In addition, the viruses causing the illness in both countries have been found to be nearly genetically identical.

The fact that the viruses are nearly identical, that large clusters of people have developed illness in a very short period of time, in places that are far apart, with no direct exposure to pigs suggests a human to human spread.

Interestingly, investigators attention has focused on the village of La Gloria in Eastern Veracruz district, where nearly 600 people developed flu like upper respiratory symptoms from around March 3, 2009 to April 13, 2009. This village is in close proximity to a large pig farm and Swine Influenza A H1/N1 has been isolated from some of the people who were sick. However the pigs at the farm have not tested positive for the virus.

The illness is similar to the usual flu with symptoms of fever, chills, sore throat, runny nose, cough, body aches and muscle pains.

Where does it occur?

Swine Influenza A H1/N1 originated in North America (Mexico and United States) but has now spread to 30 countries as of May 11, 2009. They include: Argentina, Austria, Australia, Brazil, China, Columbia, Denmark, El Salvador, France, Germany, Guatemala, Ireland, Israel, Italy, Japan, Netherlands, New Zealand, Norway, Panama, Poland, Portugal, Republic of Korea, Spain, Sweden, Switzerland and the United Kingdom. For an updated list please go to http://www.who.int. Map

How is it transmitted?

Swine Influenza A H1/N1 is transmitted by exposure to respiratory secretions from infected individuals and probably through contact with surfaces contaminated with infected respiratory secretions and touching one’s nose and mouth.

Is it contagious from person to person?

It is contagious from person to person through respiratory secretions.

What is the risk for travelers?

At the present time the risk is moderate to high if traveling to countries where the outbreak is ongoing in large numbers.

How soon after exposure will one develop symptoms?

Symptoms start anywhere from 1– 7 days after exposure but usually within 4 days.

What are the signs and symptoms?

Symptoms are high fever, fever, chills, sore throat, runny nose, cough, body aches and muscle pains. This can progress to dry cough, shortness of breath, respiratory failure and death in a minority of cases. Nausea, vomiting and diarrhea are seen in some people.

Are there any lab tests to diagnose the illness?

There are tests that can be done on nasal and pharyngeal secretions; the single best test is RT- PCR, available through state health departments.

The virus can be cultured from respiratory secretions as well. Rapid influenza screening tests that test positive for Influenza A may be a quick screening test, but has problems with low sensitivity and does not distinguish from normal Human Influenza A H1/N1 cases.

Is there any treatment?

The anti-viral medications tamiflu (oseltamivir) and relenza (zanamavir) are the treatment of choice along with rest, fluids etc. Cases with respiratory compromise will need ICU support.

What preventive measures can be taken?

If there is an ongoing large scale outbreak of Swine Influenza A H1/N1, avoid travel to that area, check at outbreaks and http://www.cdc.gov/travel for any current outbreaks. If you are already in an area during an outbreak, avoid healthcare facilities and households with ill people. Wash hands frequently, since viruses can be present on surfaces. Avoid touching your nose and mouth with your hands for no good reason. Routine wearing of masks and gowns in public area is not recommended by the Centers for Disease Control (CDC).

What do we know about genetics of this new Swine Influenza A H1/N1 strain?

This novel strain seems to have the genetic elements of 4 different Influenza A H1/N1 viruses: Human Influenza A, North American Avian Influenza A, and 2 different Swine Influenza A strains.

The 2 different Swine Influenza A strains involved are: the North American Swine Influenza A strain that has been in the United States since 1998 and a Eurasian Swine Influenza strain that may be new to North America.

How do we think this virus formed?

The respiratory systems of pigs possess receptors that can attach Swine, Avian and Human Influenza viruses. This allows for mixing of the genetic sequences from all three viruses, a process called reassortment. Swine Influenza A H1/N1 virus was likely formed in this manner.

It also likely picked up the genetic material responsible for efficient human to human transmission from Human Influenza H1/N1 viruses that were co-habiting.

However it must be stated that this novel Influenza H1N1 strain has NOT been isolated from any pigs in any country so far!However pigs have not been routinely tested for Swine Influenza A virus.

Therefore at this time the origin of this virus remains a mystery.

So what is the best guess as to how this got started?

Since there was no routine testing of Swine Influenza A strains in infected pigs, it is possible that viral reassortment did in fact happen in pigs and a new strain that could readily spread from human to human formed. It initially infected those in close contact with pigs and subsequently was spread from them to the general population.

Since it now appears that this virus is not very virulent and only causes mild to moderately severe illness in the majority of people, it did not raise any alarm and symptomatic individuals were just felt to have the flu or another common viral respiratory illness and no further diagnostic testing was done.

However when flu like illnesses began to appear after the end of the normal flu season, it raised suspicions and further diagnostic testing was done and this novel strain was identified.

What is the big worry about this virus?

This is a virus that has not been seen in humans before and populations lack immunity, this can result in large numbers of people getting sick.

The virus is also readily transmissible from human to human; in fact nearly 25% of household contacts have contracted the illness. This can result in rapid spread of the illness in a community and world wide.

Since the virulence of the virus was unknown initially there was the fear that the illness could result in high mortality.

In short the worry is that nearly every one can get sick, can spread quickly and could be life threatening.

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