Viral Taxonomy

Viruses are classified into Families, Subfamilies, Genera, and Species. The criteria used for classification are: 1) nucleic acid type and structure (linear single stranded (ss) DNA, circular double stranded (ds) DNA, linear ds DNA, positive strand RNA etc.), 2) morphology and size of the virion (naked vs enveloped icosahedral or helical nucleocapsids), and 3) viral replication strategy and gene expression.

FAMILIES OF ANIMAL VIRUSES

DNA VIRUSES

  1. Parvoviridae: 20 nm in diamenter, naked icosahedral nucleocapsids, ssDNA (5kb), narrow host range, replicate in rapidly dividing cells, can establish latent infections with viral DNA integrated within the host genome. Examples: Erythrovirus ( human parvovirus B19), erythemia infectiosum, hemolytic anemia.
    Site of replication: nucleus
    Site of budding: none

  2. Papovaviridae: 55 nm in diameter, naked icosahedral nucleocapsid, circular ds DNA (5-8kb), replicates in the nucleus and may transform cells, persists as an episome. Examples: Papillomavirus, cause warts and associated with cervical cancer.; Polyomavirus (BK and JC virus)
    Site of replication: nucleus
    Site of budding: none

  3. Adenoviridae: 70 nm in diameter, naked icosahedral nucleocapsids, ds linear DNA (37kb). Example: Adenovirus, associated with infections of the respiratory tract, eye, and intestines.
    Site of relication: nucleus
    Site of budding: none

  4. Herpesviridae: enveloped icosahedral nucleocapsids of 200 nm and naked icosahedral nucleocapsids of 100 nm, linear ds DNA (150kb), replicate in the nucleus, may establish latent infections. Examples: Herpes simplex, infection of skin, eye, and genitilia; Cytomegalovirus, congenital abnormalities; Epstein-Barr virus, infectious mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma.
    Site of replication: nucleus
    Site of budding: nucleus

  5. Poxviridae: largest vertebrate viruses, brick-shaped about 225 nm X 300 nm, linear ds DNA (130-250kb), replicates in the cytoplasm. Example: Variola virus, smallpox.
    Site of replication: cytoplasm
    Site of budding: golgi bodies

  6. Hepadnaviridae: 42 nm in diameter, enveloped icosahedral nucleocapsids, smallest of the viral gemomes (3kb), envelope ( Hbs Ag or surface antigen or Dane particles) can self associate forming particles of about 22 nm, this surface antigen can be group specific, replication cycle involves reverse transcription of viral RNA to DNA, infect liver cells. Example: Hepatitis B virus, hepatitis and liver cancer.
    Site of replication: cytoplasm
    Site of budding: endoplasmic reticulum

RNA VIRUSES

  1. Picornaviridae: 30 nm in diameter, naked icosahedral nucleocapsid, positive strand ss RNA (8kb), replicates in the cytoplasm. Examples: Enteroviruses (Poliovirus [poliomyelitis], Coxsackievirus B, Coxsackie virus A [striated mucles damage, meningitis, common cold, diarrhea in infants, hemorrhagic conjunctivitis], and Coxsackie virus B [primarily fatty tissue and CNS damage, upper respiratory infections]); Hepatovirus (hepatitis A virus), infectious hepatitis.
    Site of replication: cytoplasm
    Site of budding: none

  2. Caliciviridae: 40 nm in diameter, naked icosahedral nucleocapsids, positive ss RNA (8kb). Example: Norwalk agent, epidemic gastroenteritis; Hepatitis E virus, hepatitis.
    Site of replication: cytoplasm
    Site of budding: none

  3. Togaviridae and Flaviviridae (Arboviruses) : 40-50nm, enveloped icosahedral nucleocapsid, positive ss RNA (11kb-12kb), replicate in the cytoplasm. Examples: Alphaviruses and Flaviviruses, transmitted by mosquitoes and ticks, can cause encephalitis; also the Rubella virus (family Togaviridae), is not an arbovirus; can cause severe deformities of fetuses in the first trimester of pregnancy.
    Site of replication: cytoplasm
    Site of budding: plasma membrane

  4. Flaviviridae:: 55 nm in diameter, enveloped icosahedral nucleocapsid, 10 kb gnome. Examples: Yellow fever virus, transmitted by mosquitoes; Hepatitis C virus, transmitted sexually and by human blood/products.
    Site of replication: cytoplasm
    Site of budding: endoplasmic reticulum

  5. Coronaviridae: 120 nm in diameter, enveloped helical nucleocapsid, positive strand ss RNA (30kb). Examples: Human coronavirus, upper respiratory infections.
    Site of replication: cytoplasm
    Site of budding: golgi bodies

  6. Paramyxoviridae: 150 nm in diameter, enveloped helical nucleocapsid, negative strand ss RNA (15kb), the envelop contains a glycoprotein with hemagglutinin/neuraminidase activity and a glycoprotein with membrane fusing activity. Examples: Parainfluenza virus, respiratory infections; Mumps virus; Morbillivirus, measles; Respiratory syncytial virus, pneumonia/bronchiolitis in infants and children, common cold.
    Site of replication: cytoplasm
    Site of budding: plasma membrane

  7. Rhabdoviridae: bullet-shapped enveloped helical nucleocapsis, 180 nm X 75 nm, negative strand ss RNA (15kb), natural host not humans but disease producing when a human is infected. Examples: Vesicular stomatitis virus, lesions of the mouth; Rabies virus, fatal encephalitis.
    Site of replication: cytoplasm
    Site of buding: plasma membrane

  8. Filoviridae: 80 nm in diameter as a filament of 800 nm in length, shape is very polymorphic, negative strand ss RNA (13kb). Examples: Marburg virus, hemorrhagic fever that is usually fatal; Ebola virus, hemorrhagic fever that is usually fatal.

  9. Orthomyxoviridae: 120 nm diameter, enveloped helical nucleocapsid, segmented genome of spherical negative strand RNA ( seven to eight segmnets; 13kb),. envelop with hemagglutinin/neuraminidase activity. Examples: Influenza A , B, and C viruses, respiratory infections.
    Site of replication: nucleus
    Site of budding: plasma membrane

  10. Arenaviridae: 130 nm in diamenter, enveloped helical nucleocapsid, genome of two segments of circular negative ss RNA (13kb). Examples: Lymphocytic choriomemingitis virus, sometimes fatal meningitis; Lassa virus (natural host rodents), frequently fatal hemorrhagic fever.
    Site of replication: cytoplasm
    Site of budding: plasma membrane

  11. Bunyaviridae: 100 nm in diameter, enveloped helical nucleocapsid, genome of three negative strands of ss RNA (14-20kb) in a circular configuration. Examples: Rift Valley fever virus, fever sometimes fatal; Hantaan virus, hemorrhagic fever sometimes fatal.
    Site of replication: cytoplasm
    Site of budding: golgi bodies

  12. Reoviridae: 75 nm in diameter, naked icosahedral symmetry with two capsid shells, genome of 10-12 segments of ds RNA (18-27kb). Examples: Mammalian reovirus, pathogenicity not determined in humans; Colorado tick fever virus, encephalitis; Rotavirus, gastroenteritis in infants.
    Site of replication: cytoplasm
    Site of budding: none

  13. Retroviridae: 100 nm in diameter, enveloped helical nucleocapsid within a perhaps icosahedral core shell, genome is diploid consisting of two identical positive strands of RNA (7-10kb), each virion contains the enzymes reverse transcriptase, intergrase, and protease, the life cycle consists of a stage at which viral RNA is transcribed to viral DNA which can intergrate within the host genome, can cause latent infections. Examples: HIV, causes AIDS; Human T-cell leukemia (lymphotropic) virus 1 (HTLV-1), Adult T-cell leukemia; HTLV-2, hairy cell leukemia.
    Site of replication: nucleus
    Site of budding: plasma membrane

OTHER VIRUSES AND INFECTIOUS AGENTS

  1. Hepatitis D virus (Deltavirus): replication dependent on co-infection with Hepatitis B virus, 32 nm in diameter, genome consists of circular negative ss RNA (1.7kb) with ribozyme (ribonuclease activity).
  2. Proteinaceous Infectious Particles (PRIONS)