6.5 Clinical Virology: Viral Structure, Specimen Handling, and Diagnostic Methods

Clinical Virology: Viral Structure, Specimen Handling, and Diagnostic Methods

Viruses make up about 10% of the microbiology portion of the board blueprint. Viruses are obligate intracellular parasites, so the laboratory cannot grow them on ordinary media. Diagnosis depends on correct collection and transport, detection of antigen or nucleic acid, demonstration of cytopathic effect or inclusions, and serology timed to the course of infection. This section covers viral architecture, specimen handling, diagnostic methods, the clinically important respiratory, herpes, blood-borne, enteric, and arthropod-borne viruses, and HIV, followed by a reference matrix and notes on Philippine reference laboratories.

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1. Viral Architecture, Specimen Handling, and Diagnostic Methods

Viruses are non-cellular obligate intracellular parasites comprising a nucleic acid genome (DNA or RNA) enclosed within a protein shell (capsid) composed of capsomers, arranged in helical or icosahedral (cubic) symmetry. Enveloped viruses acquire a host-derived lipid bilayer containing viral glycoprotein spikes, rendering them sensitive to heat, drying, detergents, and lipid solvents. Naked (non-enveloped) viruses possess protein capsids that resist drying, detergents, and gastric acidity (e.g., enteroviruses, rotavirus, norovirus, hepatitis A).

                        ┌────────────────────────────────────────────────────────┐
                        │   Clinical Virology Specimen Collection Protocols      │
                        └───────────────────────────┬────────────────────────────┘
                                                    │
         ┌──────────────────────────────────────────┴──────────────────────────────────────────┐
         ▼                                                                                     ▼
  ┌─────────────────────────────────────────┐                   ┌─────────────────────────────────────────┐
  │      Swab Selection & Transport         │                   │      Thermal Preservation Rules         │
  ├─────────────────────────────────────────┤                   ├─────────────────────────────────────────┤
  │ • ACCEPTABLE: Synthetic Dacron,         │                   │ • Immediate processing or store at 4°C  │
  │   polyester, or flocked nylon swabs on  │                   │   on wet ice for up to 48-72 hours      │
  │   plastic or flexible aluminum shafts   │                   │ • If delayed >72 hours:                 │
  │ • STRICTLY FORBIDDEN: Calcium alginate  │                   │   FREEZE AT -70°C (or liquid nitrogen)  │
  │   (toxic to viruses & cell culture) and │                   │ • CRITICAL EXAM TRAP: NEVER FREEZE AT   │
  │   wooden shafts (contain formaldehydes) │                   │   -20°C! Ice crystal formation causes   │
  │ • Media: Viral Transport Medium (VTM)   │                   │   complete loss of viral viability &    │
  │   with buffered salt, protein, and Abx  │                   │   membrane disruption                   │
  └─────────────────────────────────────────┘                   └─────────────────────────────────────────┘

Diagnostic Virology Methodologies

1. Cell Culture & Cytopathic Effect (CPE):
   • Primary Cell Cultures: Derived directly from animal tissue (e.g., Primary Rhesus Monkey Kidney, PRMK); low passage ($<2\text{ to }3$), ideal for myxoviruses and enteroviruses.
   • Low-Passage Diploid Cell Lines: Capable of 20 to 50 passages, maintaining normal diploid karyotypes (e.g., Human embryonic lung fibroblasts, MRC-5, WI-38); ideal for CMV and VZV.
   • Continuous Immortalized Cell Lines: Indefinite propagation (e.g., HEp-2, HeLa, Vero); ideal for RSV and Adenovirus.
   • Cytopathic Effect Patterns: Microscopic structural changes in monolayer cells: syncytia / multinucleated giant cells (RSV, Measles, HSV), cell rounding and grape-like clustering (Adenovirus), or cell shrinkage and lysis (Enteroviruses).
2. Shell Vial Centrifugation Culture: Specimen is inoculated onto a low-passage cell monolayer grown on a coverslip inside a small vial. The vial is centrifuged at $700\ \times\ g$ for 45 minutes (which mechanically forces viral attachment and penetration) and incubated for $24\text{ to }48\text{ hours}$. Viral antigens are detected via fluorescein-conjugated monoclonal antibodies long before visible CPE develops.
3. Molecular Real-Time RT-PCR: The contemporary gold standard for clinical virology, providing extraordinary analytical sensitivity and quantitative measurement of viral load (HIV, HCV, HBV, CMV, SARS-CoV-2).

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2. Clinically Important Viruses: Respiratory, Herpes, Blood-Borne, Enteric, Arboviruses, and HIV

Respiratory Viruses

• Influenza Viruses (Orthomyxoviridae): Enveloped, single-stranded negative-sense RNA genome composed of 8 distinct helical segments. Possesses two major surface glycoproteins: Hemagglutinin (HA) (mediates viral attachment to host sialic acid receptors) and Neuraminidase (NA) (cleaves sialic acid to release budding progeny virions; inhibited by oseltamivir and zanamivir).
  • Antigenic Drift: Minor point mutations in the genes encoding HA and NA occurring during viral replication, leading to gradual antigenic variations responsible for seasonal epidemics. Occurs in both Influenza A and Influenza B.
  • Antigenic Shift: Major, sudden, catastrophic genetic reassortment occurring when two distinct influenza strains (e.g., avian and human) co-infect a single animal host cell (e.g., swine), exchanging entire RNA segments. Yields novel HA or NA subtypes to which the global human population lacks immunity, resulting in worldwide pandemics. Occurs ONLY in Influenza A.
• Respiratory Syncytial Virus (RSV, Pneumoviridae): Enveloped, negative-sense ssRNA. The premier cause of severe fatal bronchiolitis and pneumonia in infants. F-protein mediates cell-to-cell fusion, forming multinucleated syncytia.

Herpesviridae: Diagnostic Inclusions and Serology

All herpesviruses feature icosahedral capsids enclosing linear double-stranded DNA and acquire an envelope from the host nuclear membrane, establishing lifelong latency.

Infectious Mononucleosis Diagnostic Serology

1. Heterophile Antibodies (Paul-Bunnell / Monospot Test):
   • IgM antibodies produced by non-specifically stimulated polyclonal B-cells that agglutinate sheep, horse, or bovine erythrocytes.
   • Davidsohn Differential Absorption Pattern:
     • Agglutinins of Infectious Mononucleosis are NOT absorbed by guinea pig kidney cells (antigen remains reactive; agglutination persists).
     • Agglutinins of Infectious Mononucleosis are COMPLETELY absorbed by beef (bovine) red blood cells (agglutination is extinguished).
     • (Forssman antibodies are absorbed by guinea pig kidney but not by beef RBCs; serum sickness antibodies are absorbed by both).
2. EBV-Specific Serology:
   • Anti-VCA IgM (Viral Capsid Antigen): Indicates acute primary infection (appears early, disappears in 2 to 3 months).
   • Anti-VCA IgG: Emerges during acute infection and persists for life.
   • Anti-EBNA (Epstein-Barr Nuclear Antigen): Appears late ($2\text{ to }4\text{ months}$ post-onset) and remains positive for life; indicates past/convalescent infection.

Blood-Borne, Enteric, and Arboviruses

• Rotavirus (Reoviridae): Naked, double-shelled icosahedral capsid enclosing 11 segments of double-stranded RNA. The leading cause of severe dehydrating diarrhea in infants and young children worldwide. Diagnosed by rapid stool enzyme immunoassay (EIA) or lateral flow immunochromatography.
• Dengue Virus (Flaviviridae): Single-stranded positive-sense enveloped RNA virus possessing 4 distinct antigenically related serotypes (DEN-1, DEN-2, DEN-3, DEN-4). Transmitted by the domestic day-biting mosquito Aedes aegypti (and Aedes albopictus).
  • Diagnostic Algorithm:
    • Days 1 to 5 of Fever (Acute Febrile Phase): High viremia. Diagnostic choice is Dengue Non-Structural Protein 1 (NS1) antigen detection in serum via rapid lateral flow or ELISA, alongside RT-PCR.
    • Day 5 Onwards (Convalescent/Critical Phase): Viremia declines as humoral immunity emerges. Diagnostic choice shifts to anti-dengue IgM and IgG capture ELISA. Primary infection demonstrates elevated IgM by day 5; secondary infection shows rapid, massive elevation of IgG with low or variable IgM.
  • Critical Clinical Warning (Antibody-Dependent Enhancement - ADE): Secondary infection with a heterologous dengue serotype triggers ADE. Pre-existing non-neutralizing IgG cross-reacts with the new serotype, enhancing viral uptake into Fc-receptor-bearing macrophages. This causes massive cytokine storms, severe vascular permeability, Dengue Hemorrhagic Fever (DHF), and Dengue Shock Syndrome (DSS) characterized by rapid platelet destruction ($<100,000/\mu\text{L}$) and hemoconcentration (hematocrit rise $\ge 20\%$).

Human Immunodeficiency Virus (HIV-1 / HIV-2)

• Architecture: Enveloped retrovirus (Lentivirus) containing two identical copies of single-stranded positive-sense RNA, reverse transcriptase, integrase, and protease, enclosed in a cone-shaped capsid composed of p24 protein, surrounded by a lipid membrane with surface glycoprotein spikes (gp120 attachment protein and gp41 transmembrane protein).
• CDC / CLSI Diagnostic Algorithm:

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                         │     CDC/CLSI Standard HIV Laboratory Testing Algorithm │
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                         │ STEP 1: 4th-Generation HIV-1/2 Combo Immunoassay       │
                         │ (Detects HIV-1/2 antibodies AND free p24 antigen)      │
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                                ┌────────────────────┴────────────────────┐
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                         ┌──────────────┐                         ┌──────────────┐
                         │ NON-REACTIVE │                         │   REACTIVE   │
                         └──────┬───────┘                         └──────┬───────┘
                                │                                        │
                         Report Negative                         Proceed to STEP 2:
                         for HIV-1 & HIV-2                                │
                                                                          ▼
                                                ┌────────────────────────────────────────────────────────┐
                                                │ STEP 2: HIV-1 / HIV-2 Antibody Differentiation Assay   │
                                                └─────────────────────────┬──────────────────────────────┘
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                   ┌──────────────────────────────────────────────────────┼──────────────────────────────────────────────────────┐
                   ▼                                                      ▼                                                      ▼
        ┌─────────────────────────┐                            ┌─────────────────────────┐                            ┌─────────────────────────┐
        │ Reactive for HIV-1      │                            │ Reactive for HIV-2      │                            │ Non-reactive or         │
        │ antibodies              │                            │ antibodies              │                            │ Indeterminate           │
        └───────────┬─────────────┘                            └───────────┬─────────────┘                            └───────────┬─────────────┘
                    │                                                      │                                                      │
        Report: HIV-1 Positive                                 Report: HIV-2 Positive                                 Proceed to STEP 3:
                                                                                                                      Qualitative or Quantitative
                                                                                                                      HIV-1 RNA NAT (PCR)
                                                                                                                                  │
                                                                                 ┌────────────────────────────────────────────────┴────────────────────────────────────────────────┐
                                                                                 ▼                                                                                                 ▼
                                                                      ┌─────────────────────────┐                                                       ┌─────────────────────────┐
                                                                      │ HIV-1 RNA Detected      │                                                       │ HIV-1 RNA Not Detected  │
                                                                      └───────────┬─────────────┘                                                       └───────────┬─────────────┘
                                                                                  │                                                                                 │
                                                                      Report: Acute Early                                                               Report: False-positive
                                                                      HIV-1 Infection                                                                   initial screening test

• Immunological Monitoring: Flow cytometric enumeration of $CD4^+$ T-helper lymphocytes ($CD4:CD8$ ratio normally $>1.5\text{--}2.0$; inverted to $<0.5$ in untreated HIV). AIDS is diagnosed when the absolute $CD4^+$ count declines below $200\ \text{cells}/\mu\text{L}$ (or $CD4\% < 14\%$), placing the patient at catastrophic risk for opportunistic infections (Pneumocystis jirovecii, Cryptococcus neoformans, Toxoplasma gondii, Mycobacterium avium complex, and Cytomegalovirus).

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3. Clinical Virus Laboratory Reference Matrix

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4. Philippine Reference Laboratories and Timing Rules

• Research Institute for Tropical Medicine (RITM): The DOH national reference laboratory for many emerging and re-emerging infections, including dengue, influenza, measles and rubella, and rabies. Hospital laboratories send isolates and specimens to RITM for confirmation and surveillance.
• San Lazaro Hospital STD/AIDS Cooperative Central Laboratory (SACCL): The national reference laboratory for HIV, hepatitis, and syphilis testing. HIV confirmation in the Philippines follows the DOH rapid HIV diagnostic algorithm (rHIVda), covered in the infectious disease serology section.
• Dengue timing: NS1 antigen and RT-PCR detect dengue during the first 5 days of fever. Dengue IgM becomes reliably detectable from about day 5 onward, and a rising IgG (or a high IgG with a low IgM ratio) indicates secondary infection.
• Rabies: The direct fluorescent antibody test on brain tissue is the reference method. Negri bodies (eosinophilic cytoplasmic inclusions in neurons) are the classic histologic sign but are not seen in every case.
• Inclusion bodies at a glance: Intranuclear inclusions are typical of DNA viruses that replicate in the nucleus (Cowdry type A in herpes simplex and varicella-zoster, "owl's eye" in cytomegalovirus). Cytoplasmic inclusions are typical of most RNA viruses (Negri bodies in rabies) and of poxviruses (Guarnieri bodies).