Classification of medically important viruses II: RNA viruses

Abstract

The early attempts to classify viruses were based on diseases they caused or on the site of their recovery. They were grouped according to the assumed trophism or affinity for different systems or organs of the body. Thus, human viruses were classified as neurotropic--i.e., those affecting the nervous system (poliomyelitis, rabies), pneumotropicm i.e., those affecting the respiratory system (influenza, common cold), dermotropic i.e., those producing the skin lesions (smallpox, chickenpox, measles), and viscerotropic---i.e., those affecting the visceral organs (yellow fever, hepatitis). This system did not gain acceptance because different viruses could cause similar symptoms. In 1941, Bawden suggested that the viral nomenclature and classification should be based on the properties of the viruses and not upon the host responses. From early 1950s, viruses began to be classified into groups on the basis of their physicochemical and structural features. Currently, viruses are classified into two major groups on the basis of the type of nucleic acid they possess: the riboviruses (RNA-containing) and deoxyriboviruses (DNA-containing). Further classification is based on the other properties of the viruses such as the strandedness of nucleic acid (single or double), polarity of nucleic acid (positive or negative), capsid symmetry (icosahedral, helical, or complex), site of capsid assembly (nucleus or cytoplasm), envelope (presence or absence), site of envelopment (surface membrane, nuclear membrane, or intracytoplasmic membrane), virion and capsid diameter, number of capsomeres, and molecular weight of nucleic acid in the virion. Another striking feature of the present classificatiou is that the virus family names end in ...viridae, subfamily names end in ...virinae, and genus names end in ...virus. So far, twenty-one families of animal viruses have been described. As the knowledge increases, periodic revisions among different groups are made by an international nomenclature committee. The committee, originally established in 1966 as the International Committee on Nomenclature of Viruses (ICNV), was changed in 1974 to International Committee on Taxonomy of Viruses 0CTV). Information on classification of animal viruses will appear in two successive articles in this newsletter, starting in this issue with Part I, the DNA viruses. The chemical and mo~phologic properties of DNA-containing animal virus families are shown in Table 1.