Virology: Skin Lesions & Viral Properties

• Viruses are the smallest infectious agents and simplest life form, requiring living cells to replicate and containing either DNA or RNA, but not both. Infections may cause skin manifestations called Exanthems.

Forms of Skin Lesions

• Macule: Flat, reddened skin area with altered color, but not palpable.
• Papules: Raised, palpable, solid skin lesions smaller than 0.5 to 1.0 cm.
• Nodules: Raised, palpable lesions larger than papules, filled with pus.
• Vesicles: Raised, fluid-filled lesions smaller than 0.5 cm in diameter.
• Bullae: Blisters filled with clear fluid, larger than vesicles.
• Pustules: Vesicle-like lesions containing purulent material.

Properties of Viruses

• Viruses do not have cellular organization.
• They contain only one type of nucleic acid: DNA or RNA.
• Viruses are obligate intracellular parasites.
• Viruses lack necessary enzymes for protein and nucleic acid synthesis, relying on host cell machinery for replication.
• Viruses multiply via a complex process, not binary fission.
• Viruses are unaffected by antibacterial antibiotics.

Morphology of Virus

• Extracellular infectious particles are called VIRIONs.
• Viruses are much smaller than bacteria.
• Viruses are ultramicroscopic, thus can't be seen under a light microscope.
• Virus particles seen ultramicroscopically are called "elementary bodies".
• Virus size ranges from 20-300 nm.
• Parvovirus is the smallest at 20nm.
• Poxvirus is the largest at 300nm.

Viral Structure & Chemical Composition

• Virion is a complete infectious viral particle.
• Nucleic acid core is the genetic material/viral genome, consisting of single or double-stranded DNA or RNA.
• Capsid is a protein shell enclosing the nucleic acid genome, with either helical or icosahedral symmetry.
• Capsid functions:
  • Protects the viral genome from external destructive agents.
  • Transports the virus from host to another.
  • Provides a binding site enabling virus attachment to specific host cell sites.
  • Facilitates assembly and packaging of viral genetic information.
  • Act as antigens and specific for each virus.
  • Provides structural symmetry to the viral particle.

Viral Symmetry/Shape of Virus

• Viruses are in groups by the shape of the nucleocapsid symmetry

• It varies across different virus groups

• Icosahedral Symmetry (polyhedral):

  • Icosa is a Greek word meaning 20, which, for viruses means it is a polygon with 12 vertices or corners and 20 faces or sides
  • Each facet has the shape of an equilateral triangle
  • Pentagonal capsomers occur at the vertices (pentons), and hexagonal capsomers make up the facets (hexons), e.g., Adenovirus.

• Helical Symmetry:

  • Nucleic acid and capsomers winding into helix or spiral forms, e.g., Influenza, Parainfluenza, or Rabies viruses.

• Complex Symmetry:

  • Viruses lacking icosahedral or helical symmetry are because of their complex structure, e.g., Pox Virus.

• Capsomers: Complex morphologic subunits of the capsid consisting of identical or different protein molecules.

• Nucleocapsid: The capsid together with the enclosed nucleic acid.

• Peplos: Lipoprotein envelope covering the capsid.

• Enveloped:

  • Some viruses have envelopes from the host cell or nuclear membrane.
  • Outer covering contains lipids from the host cell's plasma membrane.
  • Glycoprotein in nature.
  • Susceptible to lipid solvents like ether, chloroform, and detergents.

• Non-enveloped:

  • Some viruses have no envelopes and are termed "naked."
  • More resistant to lipid solvents.

• Viral Origin of Non-enveloped viruses protect the viral genome

  • Less sensitive to extreme pH, and can enter via the GIT.
  • Less sensitive to heat, dryness, and simple disinfectants.
  • Easy target for the host's immune system.
  • Cell lysis is required and it's vulnerable to the effects of the host immune system

• Enveloped viruses originate from their host improving host's interaction

  • Very sensitive to extreme pH, usually, does not enter via the GIT.
  • Very sensitive to heat, dryness, and simple disinfectants and are vulnerable to hygienic measures.
  • Difficult target for the host's immune system.
  • No cell lysis is required

• A virion is only infectious if fully developed, so destruction of the envelope lowers its infectiousness.

• Peplomers are Lipoprotein subunits of the peplos.

• Spikes are Glycoprotein molecules linked in oligomer form and easily visible in the EM.

• Spikes function to:

  • Attach the virus to host cell receptors.
  • Attach to receptors on RBC causing agglutination.
  • Have enzymatic activity.
  • Have antigenic properties.

Classification of Viruses

• Based on Symptomatology:
  • Generalized Diseases: Spread throughout the body via the bloodstream with multiple-organ involvement, e.g., vaccinia or measles.
  • Diseases Primarily Affecting Specific Organs: Affects specific systems, with examples including:
    • Nervous System: causes Poliomyelitis, Rabies etc
    • Respiratory System: causes Influenza, Common Cold etc
    • Skin & Mucous Membranes: causes Herpes Simplex, Warts etc
    • Eyes: causes Adenovirus conjunctivitis, Herpes keratoconjunctivitis etc
    • Liver: causes Hepatitis A, B & C, yellow fever etc
    • Salivary Glands: causes Mumps or Cytomegalovirus (CMV)
    • GIT: causes Rotavirus, Norwalk virus, enteric adenovirus
    • Sexually Transmitted: causes Herpes simplex virus, Hepatitis B etc
• Based on Nucleic acid type:
  • Nucleic Acid composition can be DNA or RNA and single or double-stranded
  • DNA Viruses characteristics:
    • dsDNA-icosahedral, replicates in the nucleus unless:
      • ssDNA = Parvoviridae
      • Poxviridae - complex symmetry, replicates in cytoplasm
    • ENV = ether sensitive/ labile (Herpesviridae, Hepadnaviridae, Poxviridae)
    • Naked = ether resistant/ stable (Parvoviridae, Adenoviridae, Papoviridae)
    • Size: Largest DNA = Poxviridae, Smallest DNA = Parvoviridae
  • RNA Viruses characteristics:
    • ssRNA, helical, envelop, replicates in the cytoplasm except:
      • dsRNA Reoviridae
      • Icosahedral – Picornaviridae, Calciviridae, Reoviridae, Retroviridae
      • Naked - Picornaviridae, Calciviridae, Reoviridae
      • Replicate in nucleus – Retroviridae, Orthomyxoviridae
      • Size: Largest RNA – Paramyxoviridae, Smallest RNA – Enterovirus (Picornaviridae)
      • Arbovirus: arthropod-borne- Bunyaviridae, Flaviviridae, Togaviridae

General Rules for Virus Classification

• DNA VIRAL GENOMES
  • All DNA viruses are dsDNA, except Parvoviridae.
  • All are linear except Papilloma, Polyoma, and Hepadnaviruses (circular).
• RNA VIRAL GENOMES
  • All RNA viruses are ssRNA except Reoviridae and Rotaviruses.
  • Positive-stranded RNA viruses: Retrovirus, Togavirus, Flavivirus, Coronavirus, Calicivirus, and Picornavirus.
• Virus Ploidy
  • All viruses are haploid except Retroviruses, which have 2 identical ssRNA molecules.
• Replication Location
  • All DNA viruses replicate in the nucleus except Poxvirus.
  • All RNA viruses replicate in the cytoplasm except Influenza and Retroviruses.
• Naked viruses: Calicivirus, Picornavirus, Reovirus, Parvovirus, Adenovirus, Papilloma, and Polyoma.
• Negative-stranded viruses transcribe a negative strand to positive and need their own RNA-dependent RNA polymerase such as arenavirus.
• Segmented viruses are all RNA viruses, including Bunyaviruses, Orthomyxoviruses, Arenaviruses, and Reoviruses (BOAR).
• Types of Viruses:
  • DNA ENVELOPED VIRUSES: Herpesviruses (HSV type 1 & 2, VZV, CMV, EBV), HBV, Smallpox virus = HHS.
  • DNA NUCLEOCAPSID VIRUSES: Papilloma virus, Adenovirus, Parvovirus = PAP.
  • RNA ENVELOPED VIRUSES: Influenza, Parainfluenza, RSV, Measles, Mumps, Rubella, Rabies, HTLV, and HIV.
  • RNA NUCLEOCAPSID VIRUSES: Enteroviruses (Poliovirus, Coxsackievirus, Echovirus, HAV), Rhinovirus, and Reovirus (rotavirus).
• Mnemonic:
  • ALL DNA VIRUSES: HHAPPPPy viruses (Hepadna, Herpes, Adeno, Pox, Parvo, Papilloma, and Polyoma).
  • Double-stranded (except Parvovirus).
  • Linear (except Papilloma & Polyoma and Hepadna = PPH).
  • Are icosahedral and replicate in the nucleus except Poxvirus.
• Viral Genomes:
  • DNA viruses have large genomes and are less host-dependent, with slow replication and DNA repair mechanisms and low mutability.
  • RNA viruses have small genomes and are strongly host-dependent, with rapid replication, no RNA repair mechanism, and high mutability.

Replication of Virus

• Viral replication involves the formation of biological viruses during the infection process in the target host cells., viruses need to first enter the cell for replication to occur
• DNA replication occurs in the nucleus and RNA replication occurs in the cytoplasm

Steps in Viral Replication Cycle

• Lytic Cycle:
  • Virus multiplies in the host cell, replicates, and is released.
  • Attachment: Viruses attach to the host cell membrane and inject DNA/RNA.
  • Penetration: Host cell membranes transpose virus particles.
  • Uncoating:
    • Early mRNA and protein synthesis.
    • Cell enzymes, especially lysozymes, breaks down the protein coat, releasing the genomic nucleic acid.
  • Viral genome replication
    • Processes called Transcription or mRNA Production.
    • Some RNA viruses create messengers from infected RNA and translate.
  • Assembly of virion: Newly synthesized nucleic acid, genomes, and proteins assemble to form new virus particles in cells cytoplasm.
  • Maturation: Virus particles start rupturing cells or gradually push out through cell membranes.
  • Release of Virion: Viruses are released from the host cell via budding, with residual viral proteins processed at the cell's surface.
• Lysogenic Cycle: Involves viral genome incorporation into the host cell genome.
• Cultivation of Virus: Can be grown in vivo (within a whole living organism, plant, or animal) or in vitro. This is used to:
  • Isolate and identify viruses in clinical samples.
  • Research viral structure, replication, genetics, and effects on the host cell.
  • Prepare viruses for vaccine production.

Specimen Collection, Transport and Storage

• Types include throat swabs, rectal swabs, skin scrapings, urine, body fluid or vesicle, and tissue biopsies.
• Collect early in the acute phase of infection (within 72 hours).
• Inoculate specimens into tissue culture within 2-4 hours for best virus isolation.
• Specimens should be kept at 4°C or on crushed iced until inoculated. Freeze specimens at -70°C if there is a delay of more than 4 days.

Methods of Viral Cultivation

• Animal inoculation:
  • Susceptible experimental animals are used, by injecting them. Key considerations:
    • Select healthy, communicable diseases free animals.
    • Route of inoculation also plays a role.
    • Suckling mice (less than 48 hours old) are commonly used.
    • After inoculation animals observed for symptoms of disease and death.
    • Virus is then isolated and purified from the tissue of these animals.
    • Live inoculation was first used on human volunteers for the study of yellow fever virus.
• Embryonated eggs or chick embryo method:
  • Viruses are inoculated into a chick embryo of 7-12 days old.
  • The shell surface must be disinfected with iodine and penetrated with a small sterile drill,. Subsequently, the opening needs to be sealed with gelatin or paraffin
  • Incubate at 36°C for 2-3 days.
  • After incubation virus is broken and can be isolated from the egg tissue.
  • Indicated by the death of the embryo, damage to the embryo cells, or the formation of typical pocks or lesions on the egg membranes.
  • Viruses can be cultivated in the chorioallantoic membrane, allantoic cavity, amniotic sac, and yolk sac.
• Tissue culture or cell culture:
  • Organ, Explants, and Cell cultures are the types of Tissue Culture
  • Cell culture is used to identify & grow viruses. The process occurs by which cells are grown under controlled conditions, Usually on glass or a treated plastic surface in a suitable growth medium

Pathogenesis of Virus

• Defined as the mechanism by which viruses cause a disease.
• Pathogenic mechanisms include:
  • Implantation of the virus at the portal of entry via respiratory, gastrointestinal, skin-penetration, and genital routes. The final outcome depends on the dose and location of the virus.
  • Local replication: Most viruses spread among cells extracellularly/ intracellularly. Local infection establishment may lead to localized disease and localized shedding of the virus.
  • Spread to target organs: Depending on the balance of virus and host defenses, virus multiplication in the target organ may cause disease and death.
  • Spread to sites of shedding of virus into the environment: The respiratory tract, alimentary tract, urogenital tract, and blood are the most frequent sites of shedding, but diverse viruses may be shed virtually everywhere.
• Factors that Affect Pathogenic mechanisms:
  • Accessibility of virus to tissue
  • Cell susceptibility to virus multiplication
  • Natural selection favors the dominance of low-virulence virus strains
• Modes of Transmission:
  • Transmission can happen through are respiratory (Aerosol), Fecal-Oral, and Sexual routes. Other:
    • Vertical (parent to offspring), Horizontal (one individual to another), Via Vectors, Reservoirs, Zoonoses, and Arboviruses.

Common Viral Infections

• Infections associated with Maculopapular Exanthem
  • MEASLES (Rubeola): A highly contagious childhood infection associated with serious complications.
    • Etiologic Agent: Rubeola Virus – single-stranded RNA virus with an envelope.
    • MOT: Inhalation of respiratory droplets.
    • Clinical Findings:
      • High grade fever with cough, coryza and conjunctivitis with Photophobia
      • Highly infectious stage
      • Koplik's spots develop (after 2 days of prodrome) lasts for only 24 to 48 hours found in conjunctivae and vagina
      • Persistent fever and Appearance of maculopapular rashes
    • Complications: Pneumonia, Otitis media, Subacute Sclerosing Panencephalitis (manifested by behavior and personality changes, spasticity, myoclonic jerks, and blindness)
    • Treatment & Prevention: Prevention is done through administration of a live attenuated vaccine alongside Mumps and Rubeola (MMR) at 2 years of age.
  • GERMAN MEASLES/ 3-Day Measles (Rubella)
    • Most common infections in childhood together with measles, chicken pox, fifthe disease and roseola. More severe in adults (acquired during pregnancy of first 2 weeks AOG).
    • Etiologic Agent: Rubella Virus (single RNA virus under the genus Rubivirus in the Togavirus Family)
    • Transplacental transmission or Inhalation of respiratory droplets are ways to contract its transmission.
    • Clinical Findings:
      • Maculopapular rashes with Fever, which, last for 3-days and are and pruritic and conjunctivitis
      • Is most serious when has Congenital Rubella. Manifestations includes microcephaly, mental retardation, intrauterine growth retardation, cataracts, deafness, failure to thrive and congenital heart disease.
    • Diagnosis: Anti-Rubella-specific IgM is used for Laboratory Diagnosis and clinical manifestations
    • Treatment & Prevention: Administration of vaccine at 2 years of age. and is Contraindicated with pregnant women.
  • ROSEOLA INFANTUM is caused by Human Herpes Virus 6 belonging to family Herpesviridae that primarily Lymphocytes particularly CD4+T cells.:
    • The agent is unknown, but virus replicates in the salivary glands (through respiratory transmission and oral secretions)
    • Clinical Findings: Fever and rash that can last for 2 days. Can also cause Encephalitis and hepatitis.
    • treatment is Symptomatic
  • ERYTHEMA INFECTIOSUM (Fifth Disease):
    • The etiologic Agent is Parvovirus B-19. Transmitted the most during the lytic stage, but can also be transmitted through Blood transfusions. A definitive diagnosis can be accomplished through ELISA and PCR.
• Infections associated with Vesicular Exanthem
  • VARICELLA (Chickenpox): Highly communicable infection, double-stranded, enveloped DNA virus of the mucoepithelial cells and establishes latency in nerve ganglia.. Inhalation of respiratory droplets, direct contact with the lesions, the skin, and Mucous membranes can contract Chicken pox. Can be demonstrated through Tzanck smear of skin scrapings or swab from the vesicle to demonstrate the Cowdry type A inclusions and multinucleated giant cells for Labratory Diagnoses. Treatment & Prevention: Acyclovir reduces the course of the disease only., use the live attenuated VZV vaccine
  • HERPES ZOSTER (Shingles):
    • caused by the same Virus for chickenpox but occurs in adults or immunocompromised individuals. Causes Severe radicular pain over the skin. The main type of complication that many have is post-herpetic neuralgia.
  • VARIOLA (Smallpox):
    • A contagious infection with high fatality rate worldwide but controlled through inoculation with high risk individuals with live virulent virus.
    • Variola virus member of human Poxviruses is the Etiologic agent. Transmitted Through dry materials like clothing.

HUMAN IMMUNODEFICIENCY VIRUS (HIV)

• HIV is a retrovirus of the lentivirus group, also called (AIDS). It is transmitted via: Transfusion, Sexual contact, and Blood contact. HIV has been isolated from mother's milk (however to date, transmission by fluids other than the first 3 has not been reported).
• Phases include:
  • Clinical Manifestations in AIDS: The primary site of destruction is the host immune system, this makes patients suffer from numerous opportunistic infections
  • Opportunistic infections; the common are:
    • Esophageal & disseminated candidiasis
    • Lyme
    • Disseminated aspegillosis
    • Chronic cryptosporidiosis (enteritis)
    • Pneumocystis carinii pneumonia
  • Treatment includes:
    • Culture, Screening Test (Western Blot) and Polymerase Chain Reaction (PCR) methods
    • Treatment is only 3'-azidothymidine (AZT) or zidovudine, because, currently there is no antivirus agent that will eradicate or prevent HIV-1 infection.

Hepatitis Viruses

• Hepatitis refers to an inflammation of the liver and commonly the result of a viral infection. The five main viral classifications of hepatitis are hepatitis A, B, C, D, and E
• HEPATITIS A VIRUS (HAV): Transmitted via fecal-oral route or Hepatitis A or Infectious Hepatitis. Lab diagnosis can include: anti-HAV Detection, Demonstration of virus.
• HEPATITIS B VIRUS (HBV): a hepadnavirus containing the infectious Dane particle. Diseases produced include Hepatitis B or Serum Hepatitis. Transmitted Predomiantly parental and can also be a serological exposure.
• HEPATITIS C VIRUS (HCV): Related to the togavirus and, flavivirus family. Lab test carried by the Anti-HCV test.
• HEPATITIS D VIRUS (DELTA HEPATITIS VIRUS): Possesses an envelope composed of HBsAg and a very small viroid-like RNA molecule. The disease replicated only in HBV infected cells; requires coinfection with hepatitis B for damage to occur.
• HEPATITIS E VIRUS (HEV): Hepatitis E is a waterborne Hepatitis E. A Lab test has diagnosable exclusion.

COVID-19 (novel COronaVirus Disease-2019)

• Etiologic Agent: SARS-CoV-2 and has many signs and symptoms.
• Day 4: the patient faces severe Dizziness, throat pain, shortness
• Day 7: has been noticed to cause Fever.