Influenza (MS 654)☆

Influenza, commonly known as the flu, is an infectious disease of birds and mammals caused by an RNA virus, influenza A or B virus, of the family Orthomyxoviridae . In humans, common symptoms of influenza infection are fever, sore throat, muscle pains, severe headache, coughing, and weakness and fatigue. In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. Influenza A viruses are further classified into subtypes based on the antigenic properties of their surface spike glycoproteins, the hemagglutinin (HA) and neuraminidase (NA). There are 18 known HA subtypes (H1-H18) and 10 NA subtypes (N1-N10). HA H17, H18, and NA N10 are divergent species recently isolated from bats. Only H1, H2 and H3 are found in human influenza viruses but virus with H5, H7 and H10 HA have been isolated from humans but without human to human transmission of the viruses. Influenza virus is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds, particularly aquatic birds, through their droppings and contamination of lake water. Influenza spreads around the world in seasonal epidemics, killing millions of people in pandemic years and hundreds of thousands in non-pandemic years. Three influenza pandemics occurred in the 20th century (1918/9, 1957, and 1968) and to date one in the 21st century (2009). Each of these pandemics was caused by the appearance of a new strain of the virus in humans. These new strains resulted from the direct transfer of an avian virus to humans (1918) or the mixing of genes between human viruses and avian viruses. A highly pathogenic avian virus killed humans in Asia in 1997 and onwards. Up to May 2014 this deadly H5N1 strain has caused 650 cases with 386 fatalities. This virus has posed the greatest risk for a new influenza pandemic. However, this virus has not yet mutated to spread easily among humans. Also, in 2013/14 there have been sporadic isolates of H7N9 and H10N8 from humans in China. Vaccinations against influenza are given to humans. The most common human vaccine is inactivated virus. The vaccine is reformulated each year because influenza virus accumulates point mutations in its surface glycoproteins (antigenic drift) and different strains become dominant. Antiviral drugs can be used to treat influenza, with neuraminidase inhibitors (zanamivir and osteltamivir) being most effective.