Family Orthomyxoviridae

The family Orthomyxoviridae (ortho = normal or correct) contains the genera Influenzavirus A, which contains influenza virus A; Influenzavirus B, which contains influenza virus B; and Influenzavirus C, which contains influenza virus C (Table 4.7). Thogoto virus, a tick-borne virus of mammals, forms a fourth genus, Thogoto-like viruses. Influenza viruses A and B are closely related, but influenza A infects a wide spectrum of birds and mammals including humans, whereas influenza B infects only humans. Influenza C is more divergent. Eight segments of (-)RNA, totaling about 14 kb, comprise the genomes of influenza A and B viruses (Fig. 4.1) whereas influenza C has only 7 segments. Influenza viruses use sialic acid as a receptor, but the form used by influenza A and B viruses differs from that used by influenza C virus, and the enzymes encoded by the viruses to destroy receptors are correspondingly different. All three influenza viruses infect man and cause disease, but influenza A represents the most serious

TABLE 4.7 Orthomyxoviridae

Genus/members

Virus name abbreviation

Usual host(s)

Transmission

Disease

World distribution

Influenzavirus A Influenza A

Influenzavirus B Influenza B

Influenzavirus C

Influenza C Thogoto-like viruses Thogoto virus

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