Virology: Virus Classification and Characteristics

• DNA viruses are classified based on criteria like host types, disease syndrome, viral structure & symmetry, nucleic acid types, mRNA production mechanism, and modern taxonomy (phylogenetic).
• The Baltimore classification of viruses is based on nucleic acid types and mRNA production mechanism.
• Viruses are classified into six groups/families based on the Baltimore classification.
• The six Baltimore groups of viruses:
  • ds-DNA viruses
  • ss-DNA viruses
  • ds-RNA viruses
  • ss-RNA viruses with positive strands (+ve polarity)
  • ss-RNA viruses with negative strands (-ve polarity)
  • ss-RNA viruses associated with the enzyme reverse transcriptase

Medical Important Double Stranded DNA Viruses

• Include Poxviridae, Herpesviridae, Hepadnaviridae, Adenoviridae, and Papovaviridae
• These viruses can be classified as enveloped or non-enveloped.

Poxviridae Overview

• This family contains many significant viruses, some of which cause human infection; others naturally infect animals.
• Incidental human infections, known as Zoonosis, are common with animal strains.
• Many Poxviridae members share antigenic determinants, allowing for cross-protection.
• Sometimes animal poxviruses are used in human vaccines.

Unique Properties of Poxviruses

• Poxviruses are the largest viruses, around 300nm in size.
• They are almost visible under a compound light microscope.
• They are enveloped but resistant to inactivation.
• Poxviruses have a dsDNA genome ranging from 130-375 kb.
• Characterized by bricked shape and a dumbbell core containing nucleic acid.
• They possess lateral bodies of unknown function.
• Produce more than 100 polypeptides and replicate in the cytoplasm.
• They are highly species-specific and are being explored for gene therapy.

Classification of Poxviridae Subfamilies

• Chordopoxvirinae are for vertebrates and include genera like Orthopoxvirus, Parapoxvirus, Avipoxvirus, Capripoxvirus, Leporipoxvirus, Suipoxvirus, Molluscipoxvirus, and Yatapoxvirus.
• Entomopoxvirinae are for insects and include Capripoxvirus A, B, and C.

Replication

• Divided into early (pre-replicative) and late (post-replicative) gene products.
• Replication begins with attachment and penetration, followed by viral and host cell membrane fusion.
• Naked DNA is not infectious.
• The viral core dissociates, releasing viral DNA and enzymes, including DNA-dependent RNA polymerase, which plays a key role.
• Virally-encoded RNA polymerase synthesizes early mRNA, which forms early proteins like DNA polymerase, thymidine kinase, and recombinase.
• This allows for uncoating, DNA replication, and synthesis of late mRNA.
• Initial uncoating is due to cellular enzymes. Full uncoating is by viral enzymes.
• Late mRNA leads to the synthesis of late proteins (structural proteins).
• Virus maturation follows assembly with synthesis and incorporation of viral membranes.
• Some mature viruses exit by budding (exocytosis), but most are released by cytolysis.
• 80-90% of the virus is released by cell rupture.

Transmission

• Peculiarities exist in the mode of transmission.
• Smallpox virus transmission is respiratory.
• Other poxviruses are largely transmitted by contact.
• Zoonotic transmission is common.
• Nosocomial transmission is possible.

Pathogenesis

• Poxvirus replication occurs locally in infected cells after acquisition.
• Dermal hyperplasia and leukocyte infiltration are observed..
• This is followed by primary viraemia and dissemination, especially to lymph nodes.
• Dissemination especially happens via lymphatics and cell (macrophages) mediated spread.
• The virus multiplies in seeded lymph nodes, leading to secondary viraemia, another dermal inoculation, and characteristic skin eruption (pocks).

Epidemiology

• Smallpox and molluscum contagiosum are strictly human viruses.
• The natural host for other poxviruses is vertebrates other than humans.
• Zoonosis occurs by accidental inoculation or occupational hazards.

Poxviruses Pathogenic for Humans

• Molluscum contagiosum primarily affects humans, causing multiple and long-lasting skin nodules.
• Milker's nodes virus affects cattle and creates skin lesions similar to cowpox.
• Monkeypox affects squirrels and resembles human smallpox.
• Smallpox primarily affects humans, leading to a generalized infection with a pustular rash.
• Vaccinia virus has no generalized host, leading to a rare complication of vaccination.

Clinical Manifestation

• Poxviruses produce characteristic skin rashes.
• Most poxviruses typically have a primary lesion.
• Generalized lesions develop in human monkeypox and molluscum.
• Cowpox and Parapox infections develop at the inoculation site (hands usually) and spread to other sites (face, genitals) by scratching.
• Human monkeypox resembles mild smallpox.
• Parapox and molluscum are painless and cause little disturbance.
• Human cowpox is very painful and causes fever and lymphadenopathy, often requiring hospitalization, especially in children.
• Encephalitis may complicate cowpox.
• Erythema multiforme is a complication of parapox.
• Severe infection with generalized illness can be precipitated by an immunocompromised state or eczema.

Diagnosis

• Clinical diagnosis involves watching for symptoms.
• Major Clinical indicators are medical history, contact, and occupational hazards.
• History of rash and its evolution must be evaluated.
• Definitive diagnosis involves:
  • Electron microscopy
  • ELISA for antigen and antibody capture
  • Viral isolation by culture
  • PCR

Treatment

• Treatment is largely supportive and directed at symptoms since the disease is self-limiting.
• Supportive care is used to prevent or manage severe and distressful disease complications.
• Management of comorbidity is also important.

Prevention

• Vaccination is the major prevention tool.
• Vaccinia vaccine was used to eradicate smallpox (Variola disease) and is being used in outbreaks for monkeypox.
• Nosocomial spread control is very important.

Explore virus classification based on evolutionary relationships and the Baltimore system. Learn about reverse transcription in RNA viruses and medically important DNA virus families like Poxviridae. Discover Poxvirus dissemination mechanisms.