Animal viruses come in many shapes and sizes, but they all share one goal: to infect and replicate inside host cells. From tiny parvoviruses to massive poxviruses, these microscopic invaders use clever tricks to sneak into our bodies and hijack our cellular machinery.

Understanding how viruses are classified helps us make sense of their diverse strategies. Whether they use DNA or RNA, envelopes or naked capsids, each viral family has evolved unique ways to spread and cause disease. Let's break down the key players and their sneaky tactics.

Animal Virus Classification

Nucleic Acid-Based Classification

• Animal viruses classified primarily by nucleic acid type (DNA or RNA) and strand configuration (single-stranded or double-stranded)
• Baltimore classification system categorizes viruses into seven groups based on genome and replication strategy
  • Group I: dsDNA viruses (Herpesviridae)
  • Group II: ssDNA viruses (Parvoviridae)
  • Group III: dsRNA viruses (Reoviridae)
  • Group IV: (+)ssRNA viruses (Coronaviridae)
  • Group V: (-)ssRNA viruses (Rhabdoviridae)
  • Group VI: ssRNA-RT viruses (Retroviridae)
  • Group VII: dsDNA-RT viruses (Hepadnaviridae)
• Viral replication strategies determined by genome nature
  • Direct translation for (+)ssRNA viruses
  • Complementary mRNA production for (-)ssRNA and dsRNA viruses
  • Reverse transcription for retroviruses and hepadnaviruses

Taxonomic Classification System

• International Committee on Taxonomy of Viruses (ICTV) provides standardized classification system
• ICTV incorporates genetic, structural, and biological properties for comprehensive classification
• Hierarchical classification system organizes viruses into families, subfamilies, genera, and species
• Virus families subdivided based on shared characteristics and evolutionary relationships
• Examples of virus families: Orthomyxoviridae (influenza viruses), Flaviviridae (dengue virus, Zika virus)

Animal Virus Structure

Basic Structural Components

• Viral structures consist of nucleic acid genome, protein capsid, and sometimes lipid envelope
• Capsid symmetry serves as key morphological feature
  • Icosahedral symmetry (adenoviruses, polioviruses)
  • Helical symmetry (influenza viruses, measles virus)
• Enveloped viruses acquire lipid bilayer from host cell membranes
  • Viral glycoproteins embedded in envelope crucial for cell entry (hemagglutinin in influenza viruses)
• Specific structural proteins play important roles
  • Matrix proteins in some enveloped viruses aid in virion assembly and stability (M1 protein in influenza viruses)

Complex Virus Structures

• Complex viruses possess additional structural elements
  • Poxviruses have lateral bodies and surface tubules
  • Herpesviruses contain tegument layer between capsid and envelope
• Virus size varies greatly among families
  • Small non-enveloped viruses: 20-30 nm (parvoviruses)
  • Large, complex viruses: 200-400 nm (poxviruses, mimiviruses)
• Unique structural features in certain virus families
  • Filoviruses (Ebola virus) have filamentous morphology
  • Rhabdoviruses (rabies virus) exhibit bullet-shaped structure

Animal Virus Replication

General Replication Stages

• Attachment: virus binds to specific receptors on host cell surface
• Entry: viruses enter cells through various mechanisms (endocytosis, membrane fusion)
• Uncoating: viral genome released into host cell
• Gene expression: viral genes transcribed and translated using host or viral machinery
• Genome replication: viral genomes copied using host or viral enzymes
• Assembly: viral components come together to form new virions
• Release: mature virions exit the cell through budding or cell lysis

Replication Strategies of Different Virus Families

• DNA virus families typically replicate in nucleus
  • Herpesviridae use host RNA polymerase II for transcription
  • Adenoviridae encode their own DNA-dependent DNA polymerase
• RNA virus families often replicate in cytoplasm
  • Flaviviridae use virus-encoded RNA-dependent RNA polymerase
  • Orthomyxoviridae replicate in nucleus, unique among RNA viruses
• Retroviruses (Retroviridae) employ unique replication strategy
  • Reverse transcription of RNA genome into DNA
  • Integration of viral DNA into host genome as provirus
• Some virus families use complex replication strategies
  • Hepadnaviridae involve both DNA and RNA intermediates
  • Reverse transcription of pregenomic RNA to produce DNA genome

Temporal Regulation of Viral Gene Expression

• Viral gene expression often temporally regulated
  • Immediate-early genes: expressed first, often regulatory proteins
  • Early genes: involved in genome replication and protein production
  • Late genes: typically structural proteins for virion assembly
• Examples of temporal regulation:
  • Herpesviruses have distinct immediate-early, early, and late gene phases
  • Adenoviruses express early genes before DNA replication, late genes after

Animal Virus Pathogenesis

Virus-Host Interactions

• Virus tropism determines ability to infect specific cell types or tissues
  • Influenced by receptor specificity (CD4 receptor for HIV)
  • Cellular factors affect replication (transcription factors, enzymes)
• Transmission modes vary among virus families
  • Respiratory droplets (influenza viruses)
  • Bodily fluids (HIV, hepatitis B virus)
  • Vector-borne transmission (dengue virus, Zika virus)
• Viral pathogenesis mechanisms include:
  • Direct cell lysis (poliovirus in motor neurons)
  • Immune-mediated damage (liver damage in hepatitis B infection)
  • Alteration of host cell function (oncogenic viruses)

Infection Patterns and Disease Outcomes

• Some virus families establish latent infections
  • Herpesviridae (herpes simplex virus, varicella-zoster virus)
  • Periodic reactivation leads to recurrent symptoms
• Other families cause acute infections with rapid clearance
  • Picornaviridae (common cold viruses)
  • Short-lived symptoms followed by recovery
• Certain virus families associated with oncogenesis
  • Papillomaviridae (human papillomavirus causing cervical cancer)
  • Retroviridae (human T-cell lymphotropic virus causing leukemia)
• Mechanisms of viral oncogenesis:
  • Integration into host genome disrupting tumor suppressor genes
  • Expression of viral oncoproteins (E6 and E7 in HPV)

Immune Response and Viral Evasion

• Immune response to viral infections varies among families
  • Innate immune responses (type I interferons, natural killer cells)
  • Adaptive immune responses (antibodies, cytotoxic T cells)
• Viruses evolve mechanisms to evade or suppress host immunity
  • Antigenic drift in influenza viruses
  • Immune exhaustion in chronic hepatitis B virus infection
• Examples of immune evasion strategies:
  • Herpesviruses encode proteins that interfere with antigen presentation
  • HIV targets and depletes CD4+ T cells, compromising immune function