🦠Virology Unit 7 – Animal Viruses – Overview and Key Families

What Are Animal Viruses?

• Obligate intracellular parasites that infect animal cells and tissues
• Consist of genetic material (DNA or RNA) encapsulated by a protein coat (capsid)
• Require host cell machinery for replication and propagation
• Cause a wide range of diseases in animals, including humans (influenza, rabies, HIV)
• Exhibit diverse morphologies, genome structures, and replication strategies
• Lack cellular structure and independent metabolism, relying entirely on host cells
• Evolve rapidly due to high mutation rates and short generation times
  • Allows for quick adaptation to new hosts and evasion of immune responses

Viral Structure and Classification

• Virion, the complete infectious particle, comprises genetic material, capsid, and sometimes an envelope
• Capsid, composed of proteins, protects the viral genome and facilitates cell entry
  • Icosahedral capsids have 20 triangular faces (adenoviruses, herpesviruses)
  • Helical capsids form a spiral structure around the nucleic acid (influenza virus, measles virus)
• Enveloped viruses have an additional lipid bilayer derived from the host cell membrane (HIV, influenza virus)
• Viral genomes can be DNA or RNA, single-stranded (ss) or double-stranded (ds), and linear or circular
• Baltimore classification system categorizes viruses based on genome type and replication strategy
  • Seven groups: dsDNA, ssDNA, dsRNA, (+)ssRNA, (-)ssRNA, ssRNA-RT, and dsDNA-RT viruses
• Viruses are also classified into families, genera, and species based on shared characteristics and evolutionary relationships

Lifecycle of Animal Viruses

• Attachment: Viral surface proteins (spikes, fibers) bind to specific receptors on the host cell surface
• Entry: Viruses penetrate the cell membrane through endocytosis or membrane fusion
• Uncoating: Viral capsid disassembles, releasing the genome into the cytoplasm or nucleus
• Replication: Viral genome is transcribed and replicated using host cell machinery and viral enzymes
  • DNA viruses typically replicate in the nucleus, while RNA viruses replicate in the cytoplasm
• Assembly: Newly synthesized viral components are packaged into progeny virions
  • Capsid proteins self-assemble around the viral genome
  • Enveloped viruses acquire their lipid bilayer by budding through host cell membranes
• Release: Mature virions are released from the host cell through lysis or budding, ready to infect new cells

Key Families of Animal Viruses

• Herpesviridae: Large, enveloped dsDNA viruses that establish latent infections (herpes simplex virus, varicella-zoster virus)
• Poxviridae: Large, complex dsDNA viruses that replicate entirely in the cytoplasm (smallpox virus, monkeypox virus)
• Adenoviridae: Non-enveloped dsDNA viruses with icosahedral capsids, causing respiratory and gastrointestinal infections
• Picornaviridae: Small, non-enveloped (+)ssRNA viruses (poliovirus, rhinovirus, foot-and-mouth disease virus)
• Orthomyxoviridae: Enveloped (-)ssRNA viruses with segmented genomes (influenza viruses)
• Paramyxoviridae: Enveloped (-)ssRNA viruses causing respiratory infections (measles virus, mumps virus)
• Retroviridae: Enveloped ssRNA-RT viruses that integrate into the host genome (HIV, feline leukemia virus)
  • Reverse transcriptase converts the viral RNA genome into DNA for integration and replication

Transmission and Host Range

• Animal viruses can be transmitted through various routes, depending on the virus and host
  • Respiratory droplets and aerosols (influenza virus, measles virus)
  • Fecal-oral route (rotavirus, norovirus)
  • Blood and body fluids (HIV, hepatitis B virus)
  • Vector-borne transmission by insects or ticks (dengue virus, Zika virus)
• Host range is determined by the presence of specific receptors and cellular factors required for viral replication
  • Some viruses have a narrow host range, infecting only one or a few closely related species (measles virus, canine parvovirus)
  • Others have a broad host range, capable of infecting multiple species (rabies virus, West Nile virus)
• Zoonotic viruses can cross species barriers and infect humans from animal reservoirs (SARS-CoV, Ebola virus)
  • Viral mutations and adaptations facilitate the emergence of new zoonotic threats

Pathogenesis and Disease

• Viral pathogenesis involves the complex interactions between the virus and the host, leading to disease
• Tissue tropism: Viruses preferentially infect and replicate in specific cell types or tissues
  • Respiratory viruses (influenza virus) target the epithelial cells of the respiratory tract
  • Neurotropic viruses (rabies virus) infect neurons and cause neurological disorders
• Cytopathic effects: Viruses can directly damage or kill infected cells, disrupting tissue function
  • Cell lysis, apoptosis, or fusion can result from viral replication and release
• Immune-mediated pathology: Host immune responses to viral infections can contribute to disease
  • Excessive inflammation and cytokine release (cytokine storm) can cause tissue damage
  • Autoimmune reactions triggered by viral antigens may lead to chronic conditions
• Viral persistence: Some viruses can establish long-term or lifelong infections in the host
  • Latent infections (herpesviruses) involve periods of dormancy with occasional reactivation
  • Chronic infections (HIV, hepatitis C virus) involve continuous viral replication and presence

Diagnosis and Detection Methods

• Clinical diagnosis based on symptoms, patient history, and physical examination
• Laboratory tests confirm the presence of the virus or the host's immune response
  • Viral culture: Isolation and growth of the virus in cell culture or embryonated eggs
  • Antigen detection: Identification of viral proteins using immunoassays (ELISA, immunofluorescence)
  • Nucleic acid amplification tests (NAAT): Detection of viral genetic material using PCR or other amplification methods
    • Highly sensitive and specific, allowing for rapid diagnosis and quantification of viral load
  • Serological tests: Measurement of virus-specific antibodies (IgM, IgG) in the host's serum
    • Indicates current (IgM) or past (IgG) exposure to the virus
• Imaging techniques (X-rays, CT scans) can reveal virus-induced pathology in affected tissues

Prevention and Control Strategies

• Vaccination: Administration of vaccines to stimulate protective immunity against viral infections
  • Live attenuated vaccines contain weakened viruses that replicate without causing disease (measles, mumps, rubella vaccine)
  • Inactivated vaccines use killed viruses or viral components to elicit an immune response (influenza vaccine, polio vaccine)
  • Subunit and recombinant vaccines contain specific viral antigens produced through genetic engineering (hepatitis B vaccine)
• Antiviral drugs: Medications that inhibit viral replication or block specific steps in the viral lifecycle
  • Nucleoside analogues (acyclovir) interfere with viral DNA synthesis
  • Protease inhibitors (lopinavir) prevent the cleavage of viral polyproteins
  • Entry inhibitors (enfuvirtide) block viral fusion and entry into host cells
• Infection control measures: Practices aimed at preventing the spread of viral infections
  • Hand hygiene, respiratory etiquette, and personal protective equipment (gloves, masks)
  • Isolation and quarantine of infected individuals to limit transmission
  • Disinfection and sterilization of contaminated surfaces and medical equipment
• Vector control: Strategies to reduce or eliminate the populations of insect vectors that transmit viruses
  • Insecticide spraying, bed nets, and environmental management to control mosquito populations (dengue, Zika)
• Surveillance and monitoring: Continuous tracking of viral infections and outbreaks to guide public health interventions
  • Reporting of cases, identification of risk factors, and early detection of emerging threats
  • Genomic sequencing to monitor viral evolution and the emergence of new variants