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Chapter 31: Classification of Medically Important Viruses

PRINCIPLES OF CLASSIFICATION

The classification of viruses is based on chemical and morphologic criteria. The two major components of the virus used in classification are (1) the nucleic acid (its molecular weight and structure) and (2) the capsid (its size and symmetry and whether it is enveloped). A classification scheme based on these factors is presented in Tables 31–1 and 31–2 for DNA and RNA viruses, respectively. This scheme was simplified from the complete classification to emphasize organisms of medical importance. Only the virus families are listed; subfamilies are described in the chapter on the specific virus. Figures 31–1 and 31–2 show a classification outline for DNA viruses and RNA viruses, respectively, based on the type of genome, the nature of the nucleocapsid, and whether an envelope is present.

TABLE 31–1Classification of DNA Viruses
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TABLE 31–1 Classification of DNA Viruses
Virus Family Envelope Present Capsid Symmetry Virion Size (nm) DNA MW (×106) DNA Structure1 Medically Important Viruses
Parvovirus No Icosahedral 22 2 ss linear B19 virus
Polyomavirus No Icosahedral 45 3 ds circular, supercoiled JC virus, BK virus
Papillomavirus No Icosahedral 55 5 ds circular, supercoiled Human papillomavirus
Adenovirus No Icosahedral 75 23 ds linear Adenovirus
Hepadnavirus Yes Icosahedral 42 1.5 ds incomplete circular Hepatitis B virus
Herpesvirus Yes Icosahedral 1002 100–150 ds linear Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein–Barr virus
Poxvirus Yes Complex 250 × 400 125–185 ds linear Smallpox virus, molluscum contagiosum virus

1ss = single-stranded; ds = double-stranded.

2The herpesvirus nucleocapsid is 100 nm, but the envelope varies in size. The entire virus can be as large as 200 nm in diameter.

TABLE 31–2Classification of RNA Viruses
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TABLE 31–2 Classification of RNA Viruses
Virus Family Envelope Present Capsid Symmetry Particle Size (nm) RNA MW (×106) RNA Structure1 Medically Important Viruses
Picornavirus No Icosahedral 28 2.5 ss linear, nonsegmented, positive polarity Poliovirus, rhinovirus, hepatitis A virus
Hepevirus No Icosahedral 30 2.5 ss linear, nonsegmented, positive polarity Hepatitis E virus
Calicivirus No Icosahedral 38 2.7 ss linear, nonsegmented, positive polarity Norovirus
Reovirus No Icosahedral 75 15 ds linear, 10 or 11 segments Rotavirus
Flavivirus Yes Icosahedral 45 4 ss linear, nonsegmented, positive polarity Yellow fever virus, dengue virus, West Nile virus, hepatitis C virus, Zika virus
Togavirus Yes Icosahedral 60 4 ss linear, nonsegmented, positive polarity Rubella virus
Retrovirus Yes Icosahedral 100 72 ss linear, 2 identical strands (diploid), positive polarity HIV, human T-cell leukemia virus
Orthomyxovirus Yes Helical 80–120 4 ss linear, 8 segments, negative polarity Influenza virus
Paramyxovirus Yes Helical 150 6 ss linear, nonsegmented, negative polarity Measles virus, mumps virus, respiratory syncytial virus
Rhabdovirus Yes Helical 75 × 180 4 ss linear, nonsegmented, negative polarity Rabies virus
Filovirus Yes Helical 803 4 ss linear, nonsegmented, negative polarity Ebola virus, Marburg ...

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