Family - Orthomyxoviridae

Abstract

This chapter focuses on Orthomyxoviridae family whose member genera include Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus, and Isavirus. The virions are spherical or pleomorphic, 80–120 nm in diameter and the virion envelope is derived from the cell membrane, incorporating virus glycoproteins (one to three in number) and nonglycosylated proteins (one or two in number). The virion surface glycoprotein projections are 10–14 nm in length and 4–6 nm in diameter and the virus genome is segmented, has helical symmetry, and consists of different size ribonucleoproteins (RNP), 50–150 nm in length. The structural proteins common to all genera include three polypeptides that form the viral RdRp (e.g., PA, PB1, PB2 in FLUAV); a nucleoprotein (NP), which is associated with each genome ssRNA segment to form the RNP; a hemagglutinin, which is an integral, type I membrane glycoprotein involved in virus attachment, envelope fusion and neutralization; and a nonglycosylated matrix protein (M1 or M). The lipids in the virion envelope constitute about 18–37% of the particle weight and they resemble lipids of the host cell plasma membrane. Carbohydrates in the form of glycoproteins and glycolipids constitute about 5% of the particle weight and they are present as N-glycosidic side chains of glycoproteins, as glycolipids, and as mucopolysaccharides. Certain influenzaviruses A naturally infect humans and cause respiratory disease whereas some particular influenzaviruses A infect other mammalian species and a variety of avian species. Influenza B virus strains appear to naturally infect mainly humans and cause epidemics every few years, whereas influenzaviruses C cause more limited outbreaks in humans and may also infect pigs.