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Influenza viruses are a group (genus) of germs that infect a wide variety of birds and mammals. The viruses are in 3 groups that are termed A, B and C. Although very small, influenza viruses have a number of components. One of these components is called H (from the word Hemaglutinin) and another is called N (from the word Neuramidase). I mention these two because they are used to give names to particular viruses. These are present at the same time just like we can dress in different pieces of clothing for example a coat and a trouser. Each of these components, i.e. H and N, can take on various forms (like coats can be of different colors and designs), which are identified using numbers e.g. H1, H2, N1, N2, etc.
Influenza viruses have different combinations of these different “coats” and “trousers”, and depending on what they have, scientists design the name for that particular influenza virus. For example H5N1 the Avian influenza virus that affects people and H1N1 the virus that has caused the flu outbreak in humans in Mexico and USA in 2009. These different influenza viruses are called strains. Different influenza strains normally prefer to inhabit a given bird or mammalian species – and the name of that species is usually added to the name of the virus for example we say Avian Influenza H5N1, Swine Influenza H1N1, etc. The table below shows the normal host for some common influenza viruses.
Swine: H1N1, H1N2, H3N1 and H3N2
Horse: H7N7, H3N8
Human: H1N1, H1N2, and H3N2
Duck: H1N1, H1N8, H2N2, H2N9, H3N8, H4N6 H4N3, H6N1, H9N6, H11N6, H11N9, H12N5, H14N4, H15N4
Turkey: H3N2, H5N9, H6N2 H6N8 H8N4H9N2
Chicken: H5N1, H7N7, H7N1, H7N3 H10N7
Influenza viruses are always around us. Although spreading very first through a population most often they cause only mild or no illness in affected humans and animals. These are generally called Low Pathogenic Influenza. It is possible for a number of different types of virus to infect one animal at ago. When this happens, the viruses can borrow pieces of coats and trousers from each other, coming up with new combinations. Sometimes these new combinations result in new viruses that either have increased ability to cause serious illness in the same host or are able to cross and infect a new species. These are called Highly Pathogenic Influenza. Even when the virus has crossed to a new host, we still maintain its name, thus even when isolated from human beings, H5N1 is still called avian influenza. However there are times when the same type is found in two or more species, for example H1N1 circulates in both humans and pigs, and then we refer to the virus according to where it is isolated.
Why the concerns about Influenza (flu)
Flu is a common respiratory illness caused by influenza viruses that usually spreads through certain populations at certain times of the year. It can cause mild to severe illness, and at times can lead to death. Some people especially very young children, older age people, and people with certain health conditions (such as asthma, diabetes, heart disease or AIDS), are at high risk for serious flu complications. Because it tends to occur in certain seasons, this is referred to as Seasonal (or common) flu. In the past it was believed that seasonal flu only occur in temperate zones (where they have winter) but there is evidence now that there are actually seasons of increased flu incidence in many countries in Africa. Vaccines against flu do exist but are rarely available in developing countries.
Pandemic flu refers to a worldwide outbreak of a severe form of flu that results in serious illnesses and sometimes many deaths. This usually occurs when a new type of Highly Pathogenic Influenza virus is introduced into the human population. For a pandemic to occur, this virus must be able to spread easily from person to person and it should be a type that the population has not been exposed to before. At the moment there is no flu pandemic. However past pandemics have caused such serious problems that the World Health Organization (WHO) and all those involved in health such as the American Centers for Disease Control and Response (CDC) continuously watch the types of viruses to see if any is threatening to cause a pandemic. Therefore most of the recent efforts have been directed at types of flu viruses that were detected and showed signs that they might become a world wide problem. Currently there are two types that are being watched; the avian influenza type H5N1 and the H1N1 Influenza.
Avian flu (AI) or Bird Flu is caused by influenza viruses that occur naturally among domestic and wild birds. Low pathogenic AI is common in birds and causes few problems. However the strain H5N1, which does not cause disease in ducks, but is highly pathogenic in chicken, can be transmitted from birds to humans causing a deadly disease. It is feared that this virus has potential to cause a pandemic. However it is not yet able to be transmitted from one human being to another. There have been extensive efforts worldwide towards prevention and preparedness against this type of flu.
H1N1 Influenza (H1N1flu) occurs naturally in both humans and pigs. In pigs it regularly causes outbreaks of respiratory disease especially in temperate countries. Apart from H1N1 there are other types of swine flu, including H3N2, H1N2, and H3N1. People do not normally get swine flu, but human infections can and do happen. In April 2009, an outbreak of severe human flu started in Mexico and USA. The virus causing the outbreak was identified as H1N1. Because of its close resemblance to the virus that occurs in pigs, it was at first named Swine Influenza. It was later found that this particular version of virus had not been isolated in pigs, and so the name was changed to Human Influenza (A) H1N1. There is no evidence that the virus is transmitted from pigs to humans but some reports show that actually human beings can infect pigs.