VMC 211: General Veterinary Microbiology (1+1)

CLASSIFICATION

• Initially virus classifications were made based on the host normally infected and also based on trophism. Eg: neurotrophic virus ( having affinity for nerve tissue). Later when the methods for studying the physical, chemical and biological properties of viruses were developed more information on viruses became available to develop a classification system on the basis of these properties. The main properties used are nature of
• Nucleic acid (DNA or RNA, single or double stranded, single or segmented nucleic acid, positive or negative sense and molecular weight)
• Structure of virion ( helical, icosahedral or complex symmetry, enveloped or non enveloped, complexity; number of capsomeres for icsahedral, diameter of nucleocapsids for helical virions)
• Site of replication ( nucleus or cytoplasm)
• The other properties used are host range, trophism, mode of transmission, surface structures ( eg: antigenic properties).
• In 1971, David Baltimore proposed a virus classification based on how the viral genome is replicated and expressed. Viruses were grouped into six classes based on their method of transcription(m RNA synthesis). This grouping is based primarily on whether the genome is single or double stranded and whether the genome is converted to an intermediate form or not before the m RNA is produced.
• An International Committee on Nomenclature of Viruses (ICNV) was formed at the ninth International Congress for Microbiology in 1966. This committee is now called International Committee on Taxonomy of Viruses (ICTV).