Who is at risk and why?

Allergies can develop at any age, possibly even in the womb. They commonly occur in children but may give rise to symptoms for the first time in adulthood. Asthma may persist in adults while nasal allergies tend to decline in old age.

Why, you may ask, are some people “sensitive” to certain allergens while most are not? Why do allergic persons produce more IgE than those who are non-allergic? The major distinguishing factor appears to be heredity. For some time, it has been known that allergic conditions tend to cluster in families. Your own risk of developing allergies is related to your parents’ allergy history. If neither parent is allergic, the chance that you will have allergies is about 15%. If one parent is allergic, your risk increases to 30% and if both are allergic, your risk is greater than 60%.

Although you may inherit the tendency to develop allergies, you may never actually have symptoms. You also do not necessarily inherit the same allergies or the same diseases as your parents. It is unclear what determines which substances will trigger a reaction in an allergic person. Additionally, which diseases might develop or how severe the symptoms might be is unknown.

Another major piece of the allergy puzzle is the environment. It is clear that you must have a genetic tendency and be exposed to an allergen in order to develop an allergy. Additionally, the more intense and repetitive the exposure to an allergen and the earlier in life it occurs, the more likely it is that an allergy will develop.

There are other important influences that may conspire to cause allergic conditions. Some of these include smoking, pollution, infection, and hormones.

What are common allergic conditions and their symptoms and signs?

The parts of the body that are prone to react to allergies include the eyes, nose, lungs, skin, and stomach. Although the various allergic diseases may appear different, they all result from an exaggerated immune response to foreign substances in sensitive people. The following brief descriptions will serve as an overview of common allergic disorders.

Pandemic is a blink away

U of Maryland – A new study by University of Maryland researchers suggests that the potential for an avian influenza virus to cause a human flu pandemic is greater than previously thought. Results also illustrate how the current swine flu outbreak likely came about.
As of now, avian flu viruses can infect humans who have contact with birds, but these viruses tend not to transmit easily between
humans. However, in research recently published in the Proceedings of the National Academy of Sciences, Associate Professor Daniel Perez from the University of Maryland showed that after reassortment with a human influenza virus, a process that usually takes place in intermediary species like pigs, an avian flu virus requires relatively few mutations to spread rapidly between mammals by respiratory droplets.

“This is similar to the method by which the current swine influenza strain likely formed,” said Perez, program director of the University of Maryland-based Prevention and Control of Avian Influenza Coordinated Agricultural Project, AICAP. “The virus formed when avian, swine, and human-like viruses combined in a pig to make a new virus. After mutating to be able to spread by respiratory droplets and infect humans, it is now spreading between humans by sneezing and coughing.”
In his study, Perez used the avian H9N2 influenza virus, one that is on the list of candidates for human pandemic potential. Using reverse genetics, a technique whereby individual genes from viruses are separated, selected, and put back together, Perez and his team created a hybrid human-avian virus. Their research hybrid has internal human flu genes and surface avian flu genes from the H9N2 virus. Though it comes from a different strain of avian flu than the one that contributed to the
hybrid virus now causing the swine flu outbreak, Perez’s research virus is similar in origin to the swine flu virus, in that both involved a combination of avian and human influenza viruses.

Perez infected ferrets (considered a good model for human influenza transmission) with the virus he created, and allowed the virus to mutate in the species. Before long, healthy ferrets that shared air space but not physical space with the infected ferret had the virus, showing that the virus had mutated to spread by respiratory droplets.

When the genetic sequences of the mutant virus and original hybrid virus were compared, the only differences were five amino acid mutations, three on the surface, and two internally. Two of the surface mutations were determined to be solely responsible for supporting respiratory droplet transmission. Because so few mutations were necessary to make the hybrid H9N2 transmissible this way, they concluded that after an animal-human hybrid influenza virus forms in nature, a human pandemic of this virus is potentially just a few mutations away.

“We do not know if the mutations we saw in the lab are the same that have made the H1N1 swine flu transmissible by respiratory droplets,” Perez said. “We will be doing more research on the current swine flu strain to study its specific genetic mutations.”

Perez found that one of the two of the genetic mutations in his lab strain that enabled respiratory transmission between mammals was on the tip of the HA surface protein, one of the sites where human antibodies created in response to current vaccines would bind.

“Because the binding site of the mutant virus is different from the virus upon which the vaccine is modelled, it may mean that current vaccine stocks would not be as effective against the H9N2 mutant strain as previously anticipated,” said Perez. “We should keep this in mind when designing vaccines for an avian flu pandemic in humans.”

However, scientists cannot predict what the actual mutations will look like if and when they occur in nature, or even which strain of avian influenza will mutate to infect mammals.

“This is just the tip of the iceberg,” said Perez. “Many more studies have to be done to see which combinations of mutations cause this type of transmission before we can design the appropriate vaccines.”

Perez will be talking this week with the NIH and the CDC to discuss his team’s role in researching the current swine flu virus strain. Perez will likely do studies related to vaccine development, virus transmission between humans and animals, and the pathogenesis of the virus.

A virus vaccine is derived from the virus itself. The vaccine consists of virus components or killed viruses that mimic the presence of the virus without causing disease. These prime the body’s immune system to recognize and fight against the virus. The immune system produces antibodies against the vaccine that remain in the system until they are needed. If that virus, or in some cases a closely similar one is later introduced into the system, those antibodies attach to viral particles and remove them before they have time to replicate, preventing or lessening symptoms of the virus.

The immune system also retains antibodies to a virus after being infected with it, so humans have general immunity to human strains of avian influenza strains. But humans do not generally have immunity to avian flu strains because they have not been infected by them before. The surface proteins are sufficiently different to escape the human immune response. Avian flu strains are therefore more dangerous for humans because the human immune system cannot recognize the virus or protect against it.

CDC on Swine Flu (H1N1) – What is the swine flu?

The swine influenza A (H1N1) virus that has infected humans in the U.S. and Mexico is a novel influenza A virus that has not previously been identified in North America. This virus is resistant to the antiviral medications amantadine (Symmetrel) and rimantadine (Flumadine), but is sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza). Investigations of these cases suggest that on-going human-to-human swine influenza A (H1N1) virus is occurring.

What is the prognosis (outlook) for patients who get swine flu (H1N1)?

The following is speculation on the prognosis for swine flu (H1N1) because this disease has only been recently diagnosed and the data is changing daily. This section is based on currently available information.

In general, the majority (about 90%-95%) of people who get the disease feel terrible (see symptoms) but recover with no problems, as seen in patients in both Mexico and the U.S. Caution must be taken as the swine flu (H1N1) is still spreading and has become a pandemic. So far, young adults have not done well, and in Mexico, this group currently has the highest mortality rate, but this data could quickly change.

People with depressed immune systems historically have worse outcomes than uncompromised individuals; investigators suspect that as swine flu (H1N1) spreads, the mortality rates may rise and be high in this population. Current data suggest that pregnant individuals, children under 2 years of age, young adults, and individuals with any immune compromise or debilitation are likely to have a worse prognosis. Unfortunately, the problem with the prognosis is still unclear. If the mortality is like the conventional flu that causes mortality rates of about 0.1%, the result would be about 36,000 deaths per year because of the huge number of people who get infected. If the Mexico swine flu (H1N1) ends up with a mortality rate of about 6% and infects the same number of millions of people as conventional flu viruses, the projected numbers could be as high as 2 million deaths in the U.S. alone. This is a bad prognosis for about 2 million people and their families; these potential deaths are major reasons that health officials are so concerned about the spread of this new virus. As of September 2009, the current estimates are that about 90,000 deaths will occur in the U.S. from novel H1N1 swine flu (estimated by the president’s advisory committee). As of October, these estimates have not been revised by the advisory committee or the CDC.

Another confounding problem with the prognosis of swine flu (H1N1) is that the disease is occurring and spreading in high numbers at the usual end of the flu season. Most flu outbreaks happen between November to the following April, with peak activity between late December to March. This outbreak is not following the usual flu pattern since novel H1N1 began its outbreak in April and had spread throughout the world by September. Some scientists think that swine flu (H1N1) will die down but return with many more cases in the fall, and still others speculate the current pandemic will eventually resemble the outcomes similar to the 1918 influenza pandemic. Some suggest it may resemble the SARS (severe acute respiratory syndrome caused by a coronavirus strain) outbreak in 2002-2003 in which the disease spread to about 10 countries with over 7,000 cases, over 700 deaths, and had a 10% mortality rate. Effective isolation of patients was done in this case, and many investigators think the outbreak was stopped due to this measure. Because swine flu (H1N1) is a new virus and does not seem to be following the usual flu disease pattern, any prognosis is speculative, although as of October 2009, the numbers of people with flu-like illness are higher than usual and the illness is affecting a much younger population than the conventional flu. As the pandemic progresses, this article will be updated. The best news about this novel H1N1 swine flu is that the majority of people, as of October 2009, who have caught the flu recover without medical treatment and have an excellent prognosis.

Is swine flu (H1N1) a cause of an epidemic or pandemic in 2009?

An epidemic is defined as an outbreak of a contagious disease that is rapid and widespread, affecting many individuals at the same time. The swine flu outbreak in Mexico fit this definition. A pandemic is an epidemic that becomes so widespread that it affects a region, continent, or the world. As of April 2009, the H1N1 swine flu outbreak did not meet this definition. However, as of June 11, 2009, WHO officials determined that H1N1 2009 influenza A swine flu reached WHO level 6 criteria (person-to-person transmission in two separate WHO-determined world regions) and declared the first flu pandemic in 41 years. To date, the flu has reached over 74 different countries on every continent except Antarctica in about three month’s time; fortunately, the severity of the disease has not increased.

Can H1N1 be prevented if the H1N1 flu vaccine is not readily available?

Although vaccination is the best way to “prevent” H1N1, currently (November 2009), there is not enough available for everyone who wants or needs H1N1 vaccination. Until H1N1 vaccine supplies meet demand, there are some things people can do to try and prevent infection. Without vaccination, the best strategy is to not allow H1N1 virus to contact a person’s mucus membranes because if the virus does not reach cells in which it can grow, it cannot cause infection. Quarantining H1N1-infected people is an extreme measure that may work in some instances (for example, China uses this method), but even with quarantining, the virus may still spread by people who have minimal or no symptoms.

The next step that is easier to be implemented by individuals is for people with the disease to self-quarantine until they become noninfectious (about seven to 10 days after flu symptoms abate). Infected people can wear surgical masks to reduce the amount of droplet spray from coughs and sneezes and throw away contaminated tissues. Unfortunately, these approaches depend on the compliance of many other people, and the likelihood that such methods will be highly successful in preventing H1N1 infections, at best, is only fair. Such methods have not stopped the current pandemic. Yet there are still some other methods available to individuals. Perhaps the best way for individuals to try to prevent H1N1 infection is a combination of methods that are aimed at fulfilling the very basic principle that if H1N1 doesn’t reach an individual’s mucus membrane cells, infection will be prevented. The methods are as follows:

1. Kill or inactivate the virus before it reaches a human cell by using soap and water to clean your hands; washing clothing and taking a shower will do the same for the rest of your body.

2. Use an alcohol-based hand sanitizer if soap and water are not readily available.

3. Use sanitizers on objects that many people may touch (for example, doorknobs, computer keyboards, handrails, phones).

4. Do not touch your mouth, eyes, nose, unless you first do items 1 or 2 above.

5. Avoid crowds, parties, and especially people who are coughing and sneezing (most virus-containing droplets do not travel more than 4 feet, so experts suggest 6 feet away is a good distance to stay). If you cannot avoid crowds (or parties), try to remain aware of people around you and use the 6-foot rule with anyone coughing or sneezing. Do not reach for or eat snacks out of canisters or other containers at parties.

6. Avoid touching anything within about 6 feet of an uncovered cough/sneeze, because the droplets that contain virus fall and land on anything usually within that range.

7. Studies show that individuals who wear surgical or N95 particle masks may prevent inhalation of some H1N1 virus, but the masks may prevent only about 50% of airborne exposures and offer no protection against surface droplets. However, masks on H1N1 infected people can markedly reduce the spread of infected droplets.

These seven steps can help prevent individuals from getting H1N1 infection, but for many people, adherence to them may be difficult at best. However, there are some additional strategies that may also help prevent H1N1 infections in unvaccinated people according to some investigators. Saline nasal washes and gargling with saline (or a commercial product) as a way to reduce or eliminate H1N1 virus from mucus membranes has been suggested. Proponents of these methods base their rationale on the fact that flu viruses usually take about two to three days to proliferate in nasal/throat cells. While nasal washes and gargling may be soothing to some people, there are no studies that indicate H1N1 is killed, inactivated, or completely removed by these methods; conversely, there are no data suggesting these methods cannot have any effect on H1N1. However, with long-term nasal washes using Neti pots, sinus infection with other pathogens may be encouraged.

Other investigators and physicians have offered additional methods that may help reduce exposure to H1N1 virus. For example, Dr. Gerberding, a former CDC director, had several suggestions about how to avoid H1N1 infection on an airplane. She suggested the following:

1. If a person is next to you or near (within 6 feet) and is coughing/sneezing, ask the flight attendant to offer the person a mask.

2. If there are available seats 6 feet or more away from the coughing/sneezing person, ask to change your seat (planes are good means of travel because the air is recirculated through HEPA filters that can capture viruses, but even the filters will not help if people touch areas where droplets have landed; HEPA filters are usually not available in buses, cars, ships, or trains).

3. Turn away from the coughing/sneezing person and turn the air vent toward the person to blow the droplets away from yourself.

Variations of her suggestions may be applicable in many different social, work, or travel situations, but there are no data to prove these methods are effective. In addition, common-sense precautions such as not drinking or eating things touched by others, avoiding casual physical contacts (for example, handshakes, social hugs or kisses, public water fountains [these are OK if you touch nothing and lips only touch flowing water], banisters on stairways, and restroom door handles) will limit exposure to H1N1. Again, these common sense suggestions lack data substantiation.

Many investigators suggest that people stay well hydrated, take vitamins, and get plenty of rest, but these precautions will not prevent H1N1 infections although they may help reduce the effects of infection by strengthening the person’s immune system to fight infection. Similarly, current antiviral medications (described in the preceding section) act on H1N1 viruses that have already infected cells; they work by preventing or reducing viral particles from aggregating and being released from infected cells. Timing is important; if only a few cells are infected and the antiviral medications are administered quickly (usually before flu symptoms develop or within 48 hours), the viruses are reduced in number (they cannot easily bud out from the cell surface), so few, if any, other respiratory or mucus membrane cells become infected. This can result in either no flu symptoms or, if a larger number of cells were initially infected, less severe symptoms. The overall effect for the person is that the H1N1 infection was prevented (it was not; the symptoms were prevented from developing) or that symptoms were reduced.

In the strictest sense of the word prevention, even effective vaccines do not “prevent” infections. What they do accomplish is to alert the immune system to be on guard for certain antigens that are associated with a pathogen (for example, H1N1 virus, pneumococcal bacteria). When the pathogen first infects the host, its antigens are recognized, and these cause a rapid immunoprotective response to occur that prevents the pathogen from proliferating and developing symptoms in the host. People, including physicians and researchers, often term this complex response to vaccination as “prevention of infection” but what actually occurs is the prevention of further infection so well that symptoms do not develop or are minimal in the host.

In summary, if H1N1 viruses fail to contact cells they can infect, the disease will be prevented. As stated above, this is difficult, but not impossible, to do in almost all societies. Prevention of H1N1 symptoms of infection is possible with antiviral medications if these are given very early in the infection. There are many other methods that may reduce the chance of getting the virus on a person’s mucosal surface, but most methods have not been backed up with objective data. Most doctors and investigators suggest that items that help boost or allow the immune response to function well will help people resist H1N1 infections and reduce symptoms, but these also do not prevent infections. Consequently, while waiting for H1N1 vaccine, these are some ways individuals can improve their chances of preventing or reducing the symptoms of H1N1 infections.

What is the history of swine flu (H1N1) in humans?

In 1976, there was an outbreak of swine flu at Fort Dix. This virus is not the same as the 2009 outbreak, but it was similar insofar as it was an influenza A virus that had similarities to the swine flu virus. There was one death at Fort Dix. The government decided to produce a vaccine against this virus, but the vaccine was associated with neurological complications (Guillain-Barré syndrome) and was discontinued. Some individuals speculate that formalin, used to inactivate the virus, may have played a role in the development of this complication in 1976. There is no evidence that anyone who obtained this vaccine would be protected against the 2009 swine flu. One of the reasons it takes a few months to develop a new vaccine is to test the vaccine for safety to avoid the complications seen in the 1976 vaccine. New vaccines against any flu virus type are usually made by growing virus particles in eggs. A serious side effect (allergic reaction such as swelling of the airway) to vaccines can occur in people who are allergic to eggs; these people should not get flu vaccines. Individuals with active infections or diseases of the nervous system are also not recommended to get flu vaccines.

Can novel H1N1 swine flu be prevented with a vaccine?

The best way to prevent novel H1N1 swine flu would be the same best way to prevent other influenza infections, and that is vaccination. The CDC has multiple recommendations for vaccination based on who should obtain the first doses when the vaccine becomes available (to protect the most susceptible populations) and according to age groups. The CDC based the recommendations on data obtained from vaccine trials and infection reports gathered over the last few months. The current (October 2009) vaccine recommendations from the CDC say the following groups should get the vaccine as soon as it is available:

* pregnant women,

* people who live with or provide care for children younger than 6 months of age,

* health-care and emergency medical services personnel,

* people between 6 months and 24 years of age, and

* people from the ages of 25 through 64 who are at higher risk because of chronic health disorders such as asthma, diabetes, or a weakened immune system.

Currently, the CDC is stating that people ages 10 and above are likely to need only one vaccine shot to provide protection against novel H1N1 swine flu and further suggest that these shots will be effective in about 76% of people who obtain the vaccine. New vaccine trial data showed that healthy adults produce protective antibodies in about 98% of people in 21 days. Unfortunately, the vaccine shot in children ages 6 months to 9 years of age is not as effective as it is in older children and adults. Consequently, the CDC currently recommends that for ages 6 months up to and including 9 years of age, the children obtain two shots of the novel H1N1 vaccine, the second shot 21 days after the first shot.

Pregnant women are strongly suggested to get vaccinated as stated above. Although some vaccine preparations (multidose vials) contain low levels of thimerosal preservative (a mercury-containing preservative), the CDC still considers the vaccine safe for the fetus and mother. However, some vaccine preparations that are in single-dose vials will not have thimerosal preservative, so those pregnant individuals who are concerned about thimerosal can get this vaccine preparation when it is available.

Another type of vaccine (currently named Influenza A [H1N1] 2009 Monovalent Vaccine Live, Intranasal) has been made available during the first week in October 2009. It is a live attenuated novel H1N1 flu vaccine that contains no thimerosal, is produced by MedImmune, LLC, and is sprayed into the nostrils. This vaccine is only for healthy people 2-49 years of age, and some data suggest that it is less effective in generating an immune response in adults than the vaccine injection. The dosing schedule is as follows:

* Children 2-9 years of age should receive two doses (0.1 ml in each nostril; total equals 0.2 ml per dose) — the second dose should be given the same way about one month after the first dose

* Children, adolescents and adults, 10-49 years of age should receive one dose — (0.1 ml in each nostril; total equals 0.2 ml per dose)

The CDC occasionally makes changes and updates its information on vaccines and other recommendations about the current flu pandemic. The CDC states, “for the most accurate health information, visit http://www.cdc.gov or call 1-800-CDC-INFO, 24/7.” Caregivers should check the vaccine package inserts for more detailed information on the vaccines when they become available. This article has an updated timeline for novel H1N1 swine flu attached (see below) and provides the reader with current details about the pandemic. The following is a list of the CDC-approved H1N1 vaccines and the companies that name and manufacture them as of 10/29/09:

* Influenza A (H1N1) 2009 Monovalent Vaccine by Sanofi Pasteur

* Influenza A (H1N1) 2009 Monovalent Vaccine by Novartis

* Influenza A (H1N1) 2009 Monovalent Vaccine Live, Intranasal by MedImmune, LLC

* Influenza A (H1N1) 2009 Monovalent Vaccine by CSL Limited

The CDC says that a good way to prevent any flu disease is to avoid exposure to the virus; this is done by frequent hand washing, not touching your hands to your face (especially the nose and mouth), and avoiding any close proximity to or touching any person who may have flu symptoms. Since the virus can remain viable and infectious for about 48 hours on many surfaces, good hygiene and cleaning with soap and water or alcohol-based hand disinfectants are also recommended. Some physicians say face masks may help prevent getting airborne flu viruses (for example, from a cough or sneeze), but others think the better use for masks would be on those people who have symptoms and sneeze or cough. The use of Tamiflu or Relenza may help prevent the flu if taken before symptoms develop or reduce symptoms if taken within about 48 hours after symptoms develop. Some investigators say that administration of these drugs is still useful after 48 hours, especially in high-risk patient populations .However, taking these drugs is not routinely recommended for prevention for the healthy population because investigators suggest that as occurs with most drugs, flu strains will develop resistance to these medications. Recently, the CDC made further suggestions about the use of these antiviral medications. Dr. Schuchat, a CDC official, indicated that three modifications were being suggested (Sept. 8, 2009) to the interim guidelines for use of Tamiflu and Relenza:

1. Patients with high-risk factors should discuss flu symptoms and when to use antiviral medications; doctors should provide a prescription for the antiviral drug for the patient to use if the patient is exposed to flu or develops flu-like symptoms without having to go in to see the doctor.

2. “Watchful waiting” was added as a response to taking antiviral medications, with the emphasis on the fact that those people who develop fever and have a preexisting health condition should then begin the antiviral medication.

3. The antiviral medications are the first-line medicines for treatment of novel H1N1 swine flu, and most current cases of flu are novel H1N1 and are, to date, susceptible to Tamiflu and Relenza.

Your doctor should be consulted before these drugs are prescribed.

In general, preventive measures to prevent the spread of flu are often undertaken by those people who have symptoms. Symptomatic people should stay at home, avoid crowds, and take off from work or school until the disease is no longer transmittable (about two to three weeks) or until medical help and advice is sought. Sneezing, coughing, and nasal secretions need to be kept away from other people; simply using tissues and disposing of them will help others. Quarantining patients is usually not warranted, but such measures depend on the severity of the disease. The CDC recommends that people who appear to have an influenza-like illness upon arrival at work or school or become ill during the day be promptly separated from other people and be advised to go home until at least 24 hours after they are free of fever (100 F [37.8 C] or greater), or signs of a fever, without the use of fever-reducing medications. The novel H1N1 swine flu disease takes about seven to 10 days before fevers stop, but new research data (Sept. 14, 2009) suggests waiting until the cough is gone since many people are still infectious about one week after fever is gone. The CDC has not yet extended their recommendations to stay home for that extra week.

What treatment is available for swine flu (H1N1)?

The best treatment for influenza infections in humans is prevention by vaccination. Work by several laboratories has recently produced vaccines. The first vaccine released in early October 2009 was a nasal spray vaccine. It is approved for use in healthy individuals ages 2 through 49. This vaccine consists of a live attenuated H1N1 virus and should not be used in anyone who is pregnant or immunocompromised. The injectable vaccine, made from killed H1N1, became available in the second week of October. This vaccine is approved for use in ages 6 months to the elderly, including pregnant females. Both of these vaccines have been approved by the CDC only after they had conducted clinical trials to prove that the vaccines were safe and effective. However, caregivers should be aware of the vaccine guidelines that come with the vaccines, as occasionally, the guidelines change. Please see the sections below titled “Can novel H1N1 swine flu be prevented with a vaccine?” and the timeline update for the current information on the vaccines.

Two antiviral agents have been reported to help prevent or reduce the effects of swine flu. They are zanamivir (Relenza) and oseltamivir (Tamiflu), both of which are also used to prevent or reduce influenza A and B symptoms. These drugs should not be used indiscriminately, because viral resistance to them can and has occurred. Also, they are not recommended if the flu symptoms already have been present for 48 hours or more, although hospitalized patients may still be treated past the 48-hour guideline. Severe infections in some patients may require additional supportive measures such as ventilation support and treatment of other infections like pneumonia that can occur in patients with a severe flu infection. The CDC has suggested in their interim guidelines that pregnant females can be treated with the two antiviral agents.

How is swine flu (H1N1) diagnosed?

Swine flu is presumptively diagnosed clinically by the patient’s history of association with people known to have the disease and their symptoms listed above. Usually, a quick test (for example, nasopharyngeal swab sample) is done to see if the patient is infected with influenza A or B virus. Most of the tests can distinguish between A and B types. The test can be negative (no flu infection) or positive for type A and B. If the test is positive for type B, the flu is not likely to be swine flu (H1N1). If it is positive for type A, the person could have a conventional flu strain or swine flu (H1N1). However, the accuracy of these tests has been challenged, and the U.S. Centers for Disease Control and Prevention (CDC) has not completed their comparative studies of these tests. However, a new test developed by the CDC and a commercial company reportedly can detect H1N1 reliably in about one hour; as of October 2009, the test is only available to the military.

Swine flu (H1N1) is definitively diagnosed by identifying the particular antigens associated with the virus type. In general, this test is done in a specialized laboratory and is not done by many doctors’ offices or hospital laboratories. However, doctors’ offices are able to send specimens to specialized laboratories if necessary. Because of the large number of novel H1N1 swine flu cases (as of October 2009, the vast majority of flu cases [about 99%] are due to novel H1N1 flu viruses), the CDC recommends only hospitalized patients’ flu virus strains be sent to reference labs to be identified.

Why is swine flu (H1N1) 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 (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 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 in an individual RNA segment in flu viruses are termed antigenic drift and result in minor changes in the virus. However, these can accumulate over time to produce enough minor changes that cumulatively change the virus’ antigenic 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 seem to be the major way that bird flu virus RNA segments enter the mammalian flu virus population.

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