1.2 Viral Pathogens: Respiratory, Herpesviruses, Hepatitis, and Emerging Viruses

Respiratory Viruses: Influenza, RSV, SARS-CoV-2, and Other Respiratory Pathogens

Respiratory viral pathogens represent the leading global cause of infectious acute respiratory illnesses, ranging from mild self-limiting upper respiratory infections to devastating acute respiratory distress syndrome (ARDS) and death.

Influenza A and B Viruses

Influenza viruses belong to the Orthomyxoviridae family and possess an enveloped, negative-sense, single-stranded RNA genome divided into discrete segments (8 distinct genomic segments in Influenza A and B). Two surface glycoproteins dominate the viral envelope:

• Hemagglutinin (HA): A trimeric glycoprotein that binds host cell surface sialic acid receptors (specifically alpha-2,6-linked sialic acid on human respiratory ciliated epithelial cells), mediating receptor-mediated endocytosis and viral-host membrane fusion upon endosomal acidification.
• Neuraminidase (NA): A tetrameric glycoprotein that enzymatically cleaves terminal sialic acid residues from host glycoconjugates and newly budded viral envelopes, preventing viral aggregation and permitting release and dissemination of progeny virions.

                    INFLUENZA VARIATION DYNAMICS

   ANTIGENIC DRIFT (Influenza A & B)          ANTIGENIC SHIFT (Influenza A Only)
──────────────────────────────────────     ──────────────────────────────────────
• Point mutations in HA / NA genes        • Reassortment of segmented RNA genomes
• Polymerase lacks proofreading            • Coinfection of single host (swine, avian)
• Causes gradual antigenic changes         • Yields completely novel HA ± NA subtypes
• Drives seasonal annual epidemics         • Population lacks preexisting immunity
• Requires annual vaccine reformulation    • Triggers worldwide catastrophic pandemics

Antiviral classes targeting influenza include:

• Neuraminidase Inhibitors (Oseltamivir, Zanamivir, Peramivir): Sialic acid transition-state analogs that bind the NA catalytic pocket, preventing progeny virion release. Active against both Influenza A and B. Resistance is driven by single amino acid substitutions, most notably the H275Y mutation (histidine-to-tyrosine substitution at codon 275) in the N1 neuraminidase subtype, which confers high-level resistance to oral oseltamivir and intravenous peramivir while leaving inhaled zanamivir fully active due to conformational differences.
• Cap-Dependent Endonuclease Inhibitors (Baloxavir marboxil): A small molecule that selectively inhibits the polymerase acidic (PA) protein subunit of the viral RNA polymerase complex, halting "cap-snatching" from host mRNA and aborting viral transcription. A single oral dose is effective, but mutational substitutions at codon I38 (I38T/M/F) can emerge during therapy and reduce susceptibility.
• Adamantanes (Amantadine, Rimantadine): M2 ion-channel blockers that inhibit endosomal viral uncoating. Universal resistance across circulating Influenza A strains and intrinsic inactivity against Influenza B render this class clinically obsolete.

Respiratory Syncytial Virus (RSV)

RSV is an enveloped, non-segmented, negative-sense single-stranded RNA virus belonging to the Pneumoviridae family. It is the leading cause of viral bronchiolitis and pneumonia in infants, and a major cause of cardiopulmonary morbidity in adults older than 60 years and immunocompromised hosts. Pathogenicity is driven by the surface Fusion (F) glycoprotein, which mediates viral penetration and forms multi-nucleated syncytia via cell-to-cell fusion. The F-protein transitions from a metastable, highly immunogenic prefusion (pre-F) conformation to an irreversible post-fusion (post-F) state. Modern stabilized pre-F vaccines (bivalent RSVpreF and adjuvanted pre-F) and long-acting recombinant monoclonal antibodies (nirsevimab, targeting antigenic site Ø on pre-F) have transformed infant and older-adult prophylaxis.

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)

SARS-CoV-2 (Coronaviridae) possesses a large (~30 kb), positive-sense single-stranded RNA genome enveloped in a lipid bilayer studded with the homotrimeric Spike (S) glycoprotein. The Spike S1 subunit houses the Receptor-Binding Domain (RBD), which binds human Angiotensin-Converting Enzyme 2 (ACE2), while the S2 subunit facilitates host membrane fusion upon cleavage by host proteases (TMPRSS2 or cathepsin L). Progressive mutational shifts in the RBD (e.g., Omicron sublineages BA.1, BA.5, XBB, JN.1, KP.2) enhance ACE2 affinity and mediate evasion from humoral polyclonal antibodies, rendering early anti-spike monoclonal antibodies ineffective. Antiviral therapeutics target two conserved viral enzymes:

1. Main Protease (3CLpro / Mpro): Cleaves the large polyprotein precursors (pp1a and pp1ab). Targeted by nirmatrelvir, which is co-formulated with low-dose ritonavir (a potent CYP3A4 inhibitor) to achieve therapeutic systemic concentrations.
2. RNA-Dependent RNA Polymerase (RdRp): Replicates viral RNA. Targeted by remdesivir (a 1'-cyano-substituted adenosine analog prodrug that causes delayed RNA chain termination) and molnupiravir (a mutagenic cytidine analog that induces viral lethal mutagenesis).

Adenoviruses and Parainfluenza Viruses

• Human Adenoviruses: Non-enveloped, double-stranded DNA viruses with icosahedral capsids equipped with projecting penton fibers. Extremely resilient to environmental drying and lipid disinfectants. Causes acute febrile respiratory pharyngitis, epidemic keratoconjunctivitis ("shipyard eye"), and severe necrotizing pneumonia or hemorrhagic cystitis in allogeneic hematopoietic cell transplant (HCT) recipients. Intravenous cidofovir (with oral probenecid) or brincidofovir is utilized for disseminated disease in transplant hosts.
• Human Parainfluenza Viruses (HPIV 1–4): Enveloped, single-stranded RNA viruses of the Paramyxoviridae family. HPIV-1 and HPIV-2 are the primary etiologic agents of pediatric croup (laryngotracheobronchitis), pathognomonic for inspiratory stridor, a barking "seal-like" cough, and subglottic tracheal narrowing on frontal neck radiographs ("steeple sign").

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The Herpesviridae Family: Structure, Latency, and Clinical Manifestations

The Herpesviridae family encompasses large, enveloped, double-stranded linear DNA viruses that share a common architecture: a core DNA genome encased in an icosahedral capsid, surrounded by an amorphous proteinaceous tegument, and enveloped in a lipid membrane containing viral glycoprotein spikes. All herpesviruses establish lifelong latent persistence in specific host cellular reservoirs with periodic episodic reactivation.

Alpha-Herpesviruses: HSV-1, HSV-2, and VZV

HSV-1 and HSV-2 infect mucosal epithelial cells, replicate actively (lytic cycle), and ascend retrograde along sensory axons to establish lifelong dormant latency in sensory ganglia. Upon reactivation, viral particles migrate anterograde back to mucosal sites, causing recurrent vesicular lesions or asymptomatic shedding.

• HSV Encephalitis: Caused predominantly by HSV-1. Characterized by acute onset of fever, altered mental status, seizures, and focal temporal/frontal lobe hemorrhagic necrosis. Cerebrospinal fluid analysis reveals a lymphocytic pleocytosis with elevated red blood cells and protein. Immediate initiation of high-dose intravenous acyclovir (10 mg/kg IV every 8 hours) is mandatory; delays in therapy markedly increase mortality and permanent neurocognitive deficits.
• Acyclovir Pharmacology and Resistance: Acyclovir is an acyclic guanosine analog prodrug that requires monophosphorylation by the viral thymidine kinase (TK), encoded by the UL23 gene. Host cellular kinases subsequently convert acyclovir monophosphate into acyclovir triphosphate, which selectively inhibits viral DNA polymerase (UL30) and causes irreversible DNA chain termination. More than 95% of clinical acyclovir resistance (predominantly in immunocompromised hosts) is mediated by point mutations or frame-shifts in the viral UL23 (TK) gene, producing a thymidine-kinase deficient or altered enzyme. TK-deficient mutants are cross-resistant to valacyclovir and famciclovir, but remain fully susceptible to foscarnet and cidofovir, which do not require phosphorylation by viral thymidine kinase.
• Varicella-Zoster Virus: Primary infection produces generalized varicella ("dewdrops on a rose petal" vesicles in varying stages of evolution). Following latency in cranial and spinal dorsal root ganglia, reactivation produces herpes zoster (shingles), a painful, unilateral vesicular eruption restricted to a specific dermatome. Complications include postherpetic neuralgia (debilitating pain persisting ≥ 90 days after rash healing), herpes zoster ophthalmicus (involvement of the V1 ophthalmic branch of the trigeminal nerve, heralded by vesicles on the tip of the nose [Hutchinson sign], risking corneal ulceration and blindness), and Ramsay Hunt syndrome (herpes zoster oticus, involving cranial nerves VII and VIII, causing facial paralysis and ear canal vesicles).

Beta-Herpesviruses: CMV and HHV-6

Cytomegalovirus (CMV, HHV-5) is a major opportunistic pathogen in solid organ transplant (SOT) and allogeneic HCT recipients. Transmission occurs via bodily fluids, transplacentally, or via transplanted allografts.

• Transplant Risk Stratification: In solid organ transplantation, the donor-seropositive/recipient-seronegative (D+/R-) mismatch represents the highest risk category for primary CMV infection and severe end-organ disease, requiring universal prophylaxis (e.g., valganciclovir for 3 to 6 months post-transplant). Recipient-seropositive (R+) individuals carry intermediate risk due to reactivation. In allogeneic HCT, the D-/R+ match carries the greatest risk of lethal reactivation, typically managed via pre-emptive monitoring or letermovir prophylaxis.
• Clinical Syndromes: In SOT, CMV manifests as "CMV Syndrome" (fever, profound malaise, leukopenia, thrombocytopenia, and transaminitis) or tissue-invasive end-organ disease: CMV colitis (mucosal ulcerations, severe bleeding), hepatitis, or CMV pneumonitis (diffuse interstitial infiltrates carrying high mortality in HCT). In advanced HIV (CD4 <50 cells/$\mu$L), CMV presents characteristically as necrotizing CMV retinitis ("pizza-pie" hemorrhage and exudates on fundoscopy) risking retinal detachment and blindness.
• CMV Antiviral Targets and Resistance Mutations:
  • Ganciclovir / Valganciclovir: Phosphorylated initially by the CMV-encoded protein kinase homolog UL97. Ganciclovir triphosphate inhibits the viral DNA polymerase encoded by UL54. Ganciclovir resistance arises primarily via mutational substitutions in the UL97 kinase (common codons M460V/I, H520Q, C592G, L595S). Isolated UL97 mutations confer resistance to ganciclovir while retaining susceptibility to foscarnet and cidofovir.
  • UL54 DNA Polymerase Mutations: Mutations in the catalytic or exonuclease domains of UL54 arise after prolonged therapy, conferring high-level resistance to ganciclovir and cross-resistance to foscarnet, cidofovir, or both.
  • Letermovir: A first-in-class non-nucleoside inhibitor targeting the viral terminase complex (subunits pUL56, pUL51, and pUL89), preventing cleavage and packaging of viral unit-length DNA into procapsids. Because it acts downstream of DNA synthesis, it exhibits zero cross-resistance with ganciclovir, foscarnet, or cidofovir. Approved for prophylaxis in CMV-seropositive allogeneic HCT recipients and high-risk kidney transplant recipients.
  • Maribavir: An oral competitive inhibitor of the UL97 kinase ATP-binding site approved for post-transplant CMV infections refractory or resistant to ganciclovir, foscarnet, or cidofovir.
• Human Herpesvirus 6 (HHV-6B): Primary agent of exanthem subitum (roseola infantum) in toddlers (high spiking fever for 3–5 days that abruptly resolves, immediately followed by an asymptomatic blanching maculopapular rose-pink rash on the trunk). In allogeneic HCT recipients, HHV-6 reactivation causes post-transplant limbic encephalitis (acute anterograde amnesia, confusion, SIADH, and temporal lobe hyperintensity on MRI) and delayed marrow engraftment.

Gamma-Herpesviruses: EBV and HHV-8

• Epstein-Barr Virus (EBV): Demonstrates strict tropism for B lymphocytes via binding of envelope glycoprotein gp350 to the host CD21 receptor. In immunocompetent adolescents, causes classic infectious mononucleosis (triad of fever, exudative pharyngitis, and posterior cervical lymphadenopathy, with splenomegaly and atypical Downey-cell lymphocytes on peripheral smear; heterophile antibody-positive via Monospot). In immunosuppressed SOT and HCT recipients, unchecked EBV-driven polyclonal and monoclonal B-cell proliferation leads to Post-Transplant Lymphoproliferative Disorder (PTLD), managed by tapering immunosuppression and administering rituximab (anti-CD20 monoclonal antibody).
• Human Herpesvirus 8 (HHV-8 / KSHV): Drives neoplastic vascular endothelial proliferation resulting in Kaposi Sarcoma (violaceous, non-blanching cutaneous plaques and visceral lesions in HIV/AIDS), Primary Effusion Lymphoma (PEL), and Multicentric Castleman Disease (driven by viral IL-6 homolog production).

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Viral Hepatitis: Hepatitis A, B, C, D, and E

Viral hepatitis comprises five distinct hepatotropic viruses that share a common clinical affinity for hepatocellular injury but exhibit divergent taxonomy, transmission dynamics, replication strategies, and chronic oncogenic potential.

                  HEPATITIS B VIRUS SEROLOGIC PATTERNS

Clinical Status            HBsAg   Anti-HBs  Anti-HBc Total  Anti-HBc IgM  HBeAg
─────────────────────────────────────────────────────────────────────────────────
Susceptible (Never exposed)  -        -            -              -          -
Vaccine-Induced Immunity     -        +            -              -          -
Natural Immunity (Resolved)  -        +            +              -          -
Acute Hepatitis B            +        -            +              +          +
Chronic HBV (High replica.)  +        -            +              -          +
Chronic HBV (Low replica.)   +        -            +              -          -
Isolated Core Antibody       -        -            +              -          -

Hepatitis B Virus (HBV)

HBV (Hepadnaviridae) is an enveloped, partially double-stranded relaxed circular DNA (rcDNA) virus. Upon entering hepatocytes via the sodium taurocholate cotransporting polypeptide (NTCP) receptor, the viral rcDNA is translocated to the nucleus and converted into covalently closed circular DNA (cccDNA). This cccDNA forms a highly stable, mini-chromosome episome that integrates with host histones, persisting indefinitely in the hepatocyte nucleus despite potent antiviral suppression. The cccDNA serves as the template for transcription of pregenomic RNA (pgRNA), which is subsequently reverse-transcribed by the viral polymerase into progeny rcDNA.

• Serologic Interpretation:
  • HBsAg (Surface Antigen): Hallmark of active infection (acute or chronic if present $>6$ months).
  • Anti-HBs (Surface Antibody): Marker of immunity and viral clearance. Present alone in vaccinated individuals; present alongside anti-HBc in resolved natural infection.
  • Anti-HBc Total (Core Antibody): Indicates past or present natural infection (never induced by vaccination). Persists for life.
  • Anti-HBc IgM: Indicates acute or recent infection (≤ 6 months) or severe acute flares of chronic HBV.
  • HBeAg (e-Antigen): Soluble secretory protein indicating active, high-level viral replication and high transmission risk.
  • Anti-HBe: Reflects seroconversion to lower replicative status, though precore/basal core promoter mutant strains can maintain high HBV DNA levels without HBeAg production.
• Immunosuppression and HBV Reactivation: Patients who are HBsAg-positive or anti-HBc-positive are at significant risk for fulminant viral reactivation, hepatic failure, and death when exposed to intense immunosuppression, particularly B-cell depleting agents (rituximab, obinutuzumab), chimeric antigen receptor T-cell (CAR-T) therapy, stem cell transplantation, or high-dose corticosteroids. Prophylactic antiviral therapy with nucleos(t)ide analogs possessing a high genetic barrier to resistance (entecavir, tenofovir disoproxil fumarate [TDF], or tenofovir alafenamide [TAF]) is mandatory before initiating high-risk therapy.

Hepatitis D Virus (HDV)

HDV is a defective, circular single-stranded negative-sense RNA subviral satellite that requires the envelope proteins of Hepatitis B (HBsAg) for virion packaging, assembly, and transmission. It occurs either as a coinfection (simultaneous acquisition of HBV and HDV, causing acute hepatitis with higher rates of fulminant hepatic failure but low chronicity) or superinfection (acquisition of HDV in a chronic HBsAg carrier, leading to chronic hepatitis D in $>80\%$ of cases, rapid progression to cirrhosis, and marked risk of hepatocellular carcinoma). Bulevirtide is an entry inhibitor that blocks the NTCP receptor, preventing de novo hepatocyte infection.

Hepatitis C Virus (HCV)

HCV (Flaviviridae, genus Hepacivirus) is an enveloped, positive-sense single-stranded RNA virus (~9.6 kb) classified into 6 major genotypes (1 through 6). Because its life cycle occurs entirely within the cytoplasm without a DNA intermediate or nuclear persistence, true viral eradication and cure are achievable.

                    HCV REPLICATION & DAA TARGETS

   HCV Polyprotein:   [ Core | E1 | E2 | p7 | NS2 | NS3 | NS4A | NS4B | NS5A | NS5B ]
                                              ▲       ▲            ▲      ▲
                                              │       │            │      │
   NS3/4A Serine Protease ────────────────────┘       │            │      │
   • Cleaves polyprotein junctions                    │            │      │
   • Targeted by "-previr" agents (Glecaprevir, Voxilaprevir)      │      │
                                                                   │      │
   NS5A Replication Complex Phosphoprotein ────────────────────────┘      │
   • RNA replication, membrane web formation, virion assembly             │
   • Targeted by "-asvir" agents (Ledipasvir, Velpatasvir, Pibrentasvir)  │
                                                                          │
   NS5B RNA-Dependent RNA Polymerase ─────────────────────────────────────┘
   • Catalyzes viral RNA chain synthesis
   • Targeted by "-buvir" agents:
     - Nucleotide inhibitor: Sofosbuvir (High barrier to resistance, pangenotypic)
     - Non-nucleoside inhibitors: Dasabuvir

Hepatitis A and E Viruses

• Hepatitis A Virus (HAV): Non-enveloped, positive-sense single-stranded RNA picornavirus transmitted via the fecal-oral route through contaminated food/water or direct contact. Replicates in hepatocytes and is shed in high concentrations in feces prior to symptom onset. Causes acute self-limiting hepatitis, jaundice, and cholestasis; never causes chronic infection or a carrier state. Diagnosis relies on serum anti-HAV IgM.
• Hepatitis E Virus (HEV): Non-enveloped positive-sense RNA virus (Hepeviridae). Genotypes 1 and 2 cause large waterborne epidemics in developing regions; genotypes 3 and 4 cause zoonotic foodborne infections via undercooked pork or game meat in developed nations. While typically self-limited, HEV causes chronic hepatitis and rapid cirrhosis in solid organ transplant recipients, and carries an alarming 15–25% mortality rate in infected pregnant women during the third trimester secondary to fulminant hepatic necrosis.

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Neurotropic, Arboviral, and Emerging Pathogens

Neurotropic Enteroviruses

Non-polio enteroviruses (Coxsackieviruses A and B, Echoviruses, Enterovirus A71, and Enterovirus D68) are small, non-enveloped, positive-sense single-stranded RNA viruses of the Picornaviridae family. Transmitted via fecal-oral and respiratory secretions, they represent the predominant cause of aseptic viral meningitis in pediatric and adult populations (CSF demonstrates lymphocytic pleocytosis, normal glucose, and negative Gram stain). Enterovirus D68 (EV-D68) has emerged as a causative agent of Acute Flaccid Myelitis (AFM) in children, characterized by anterior horn motor neuron destruction producing asymmetric, flaccid limb weakness following a mild prodromal upper respiratory illness.

Arboviruses and West Nile Virus

Arboviruses (arthropod-borne viruses) are transmitted to humans through the bites of hematophagous mosquitoes or ticks.

• West Nile Virus (WNV): An enveloped, positive-sense single-stranded RNA flavivirus maintained in an enzootic cycle between Culex mosquitoes and avian reservoir hosts (crows, jays, robins). Transmission also occurs via blood transfusions, solid organ transplantation, and breast milk. Approximately 80% of infections are asymptomatic, and 20% develop West Nile Fever (acute onset of fever, headache, retro-orbital pain, severe malaise, and a transient maculopapular rash). Less than 1% develop West Nile Neuroinvasive Disease (WNND), presenting as:
  1. West Nile Encephalitis: Marked lethargy, confusion, tremors, myoclonus, and parkinsonian extrapyramidal features.
  2. West Nile Meningitis: Classic meningismus, photophobia, fever.
  3. Acute Flaccid Paralysis: Sudden onset of asymmetric, polio-like motor paralysis secondary to selective destruction of anterior horn motor neurons in the spinal cord, progressing to respiratory failure while sensory perception remains preserved.

High-Consequence Emerging Pathogens

• Mpox (formerly Monkeypox): An enveloped, double-stranded DNA virus belonging to the genus Orthopoxvirus (Poxviridae). Clade I (Central Africa) exhibits higher fatality rates, whereas Clade II (specifically subclade IIb) was responsible for the 2022–2023 global outbreak, transmitted predominantly through close physical and sexual contact. Clinical manifestations begin with fever, headache, and prominent lymphadenopathy (cervical, axillary, inguinal; a critical hallmark distinguishing mpox from smallpox and chickenpox), followed by the emergence of deep-seated, well-circumscribed, umbilicated pustular lesions that progress synchronously from macules to papules, vesicles, pseudo-pustules, and crusts. Severe complications include debilitating proctitis, pharyngitis, and sight-threatening keratitis. The most studied antiviral is tecovirimat (TPOXX), which targets the viral envelope protein VP37, inhibiting the formation and egress of infectious extracellular virions. Two randomized placebo-controlled trials have since failed to show clinical benefit — PALM007 in clade I mpox and STOMP in clade II mpox (clinical resolution 83% vs 84% at day 29) — so tecovirimat is no longer supported for routine treatment and is reserved for severe disease or high-risk hosts, ideally within a trial or expanded-access protocol. Care is otherwise supportive, with brincidofovir and vaccinia immune globulin as additional options in severe cases.
• Filoviruses (Ebola and Marburg): Filamentous, enveloped, negative-sense single-stranded RNA viruses causing severe viral hemorrhagic fever. Transmitted through broken skin or mucous membranes in contact with infected blood, bodily fluids, or corpses. The virus infects dendritic cells and tissue macrophages, triggering systemic endothelial disruption, massive vascular leakage, impaired synthesis of coagulation factors, and catastrophic disseminated intravascular coagulation (DIC). Advanced therapies include monoclonal antibodies targeting the surface glycoprotein, such as ansuvimab (mAb114) and Inmazeb (a cocktail of atoltivimab, maftivimab, and odesivimab).
• Highly Pathogenic Avian Influenza (HPAI H5N1): A global clade (2.3.4.4b) affecting wild birds and domestic poultry with spillover into multiple mammalian species (including dairy cattle via unpasteurized milk). Human infections range from conjunctivitis to lethal primary necrotizing viral pneumonia and ARDS. H5N1 hemagglutinin preferentially binds alpha-2,3-linked sialic acid receptors deep in the lower human respiratory tract, raising persistent pandemic surveillance concerns regarding adaptive mutations (such as PB2 polymerase gene mutations E627K) that could facilitate efficient human-to-human airborne transmission.