Influenza also known as flu causes respiratory infection. It is more severe than those of the common cold. First influenza was reported in 1918 known as Spanish Influenza. It appeared during the First World War. Millions of young soldiers had got the infection and died. there were several factors enhance the distribution of the influenza at that time , such as: the quality of life was very poor, cold weather, the soldiers were under stress, fear, war gasses, they exposed to the elements, and contact with animals, both wild and domestic animals especially birds and pigs.( Erkoreka, 2009)
In 1933, influenza virus was isolated and called H1N1. It is influenza viruses type A which going to discuss about it more. Some patient with viral influenza develops life threatening which lead to death e.g. pregnant women, young children and immune compromised patient.
Influenza viruses are RNA viruses. It is caused by three species of orthomyxoviruses which grouped types A, B and C. These types can be distinguished by the antigenicity of their internal ribonucleprotein. Influenza viruses A cause epidemic and pandemic and its animal host are birds. Type B only cause epidemic influenza and does not require animal hosts. Finally influenza viruses C cause only minor respiratory illness (Mims & et al, 2004).
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The classification of influenza A viruses depend on the antigentic properties of their hemagglutinin (HA) and their neuraminidase (NA) surface glycoprotein. There are 16 HA (H1-H16) and 9 NA (N1-N9) subtypes have been identified. Most of them occurred in the birds and some in mammals. These subtypes of viruses have been isolated in more than 100 of wild bird species (Klenk, et al, 2008).
Influenza A and B viruses contains genome consisting of eight different single- stranded RNA molecules but influenza C virus contains only seven segments of ssRNA, with negative polarity. Each one of these segment encodes 1 or 2 proteins. These molecules surrounded by polymorphic lipid envelope which has hemagglutinin (HA) and neuraminidase (NA) spikes (Bauman, 2007). Hemagglutinin is a protein that play an important role in binding to the host cell ant enter the viral genome to the host cell, while neuraminidase release the virus from infected cell (Mims & et al, 2004).
M and NS genes of influenza A virus, NA, M, and NS genes of influenza B virus, and M and NS genes of influenza C virus encode two proteins ( Nakajima,1997).
Each type has same terminal nucleotide sequences of individual RNA segments. hemagglutinin(HA) and neuraminidase(NA), influenza A and B have same structure and character, however influenza C virus has hemagglutinin-esterase(HE), which is different(Nakajima, 1997).
Influenza A viruses undergoes two types of genetic changes. One is antigenic drift mutation that affecting H and N and causes gradual change of virus. These changes allow virus to multiply significantly in the individuals and produce new virus strain that may not be recognized by immune system of the individual. Because of that people can get infection more than one time. On the other hand, antigenic shift is sudden and major change that occurs in hemagglutinin and neuraminidase proteins. It is recombination between different virus strains when they infect the same target cell. It is less common than antigenic drift (Mims & et al, 2004).
In the spring of 2009, antigenic shift occurred when a new virus strain (H1N1) emerged to infect human. It tacked a short time to spread, causing a pandemic. Actually most people have low or no immunity against the new virus.
Influenza is more likely to occur in the coldest months of year. This is because, during winter, people stay long time inside their houses with limited air space; also at that time body resistance tend to decrease. Some people might have acute respiratory disease. Influenza has three mechanism of transmission: (1) transmission by direct contact with infected persons through coughing or sneezing directly in to the face another person ; (2) by using or dealing with contaminated objects (called fomites, such as toys, doorknobs); and (3) by inhalation droplets of virus-laden aerosols.
Virus enters into the upper or lower respiratory system through contaminated hand or nose, or by inhalation of small droplet. The virus attaches to epithelium cell through HA molecules which stick with cell-membrane receptors. Then virus start to multiply in the epithelium; it is then effuse into the secretions of respiratory. This process causes death of epithelial cell; cilia lose its function and reduced the formation of mucus. Moreover, other part of respiratory system from pharynx to alveoli may involve. Virus multiplies for 10 -14 days as primary infection. Due to viral infection of epithelium the patient can get pneumonia and develop life threading. Because of these changes patient can get secondary infection by bacteria (Katira, 2009).
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The incubation period of seasonal influenza is usually two days but can range from one to four days. Infected individual may develop sign and symptoms after get infection about 3- 4 days.
Patient with influenza will complain and suffer from Fever, headache, tiredness cough, sneeze, sore throat and body aches. Diarrhea and vomiting are more commonly seen with children. Actually having these symptoms does not mean that patient has got influenza because the symptoms are same as common cold and other illnesses.
Influenza viruses have ability change over time, RNA viruses try to tricks human immune system by mutation in RNA sequence. Due to this changing the viruses are able to invade the immune system of the host (humans, birds, and other animals). When the host gets the infection from influenza viruses, the immune system trays to defend the body by produces antibodies against that virus; but as the virus mutate the first antibody unable to recognize the new virus. As a result of this change the infection occurs because the host was unable to recognize the new virus strain. Actually in some cases the first antibody produced may provide partial immunity against new influenza virus. Unfortunately, most individuals don't have antibodies that will recognize influenza virus immediately.
Diagnosing the Flu
We cannot differentiate the flu from other infections depending on the symptoms alone. Laboratory examinations are needed to determine if the patient has flu within the first 3 days.There are some accurate and sensitive laboratory tests which used to detect influenza virus such as viral culture, polymerase chain reaction (PCR), rapid antigen testing, and immunofluorescence. PCR is the most accurate one but few laboratories may have this test. These tests spend days until the results get ready. The specimen should collects by the doctor from patient nose or back of his throat by using swab and blood sample does not require.
Furthermore, there is Rabid Influenza diagnostic test can detect influenza virus with half hour. Its principle is to detect influenza viral nucleoprotein antigen. But this test can give false negative and cannot distinguish between influenza A and B. However, sometimes and during outbreak, positive test is likely to indicate influenza.