Herpes simplex virus 1: Questions

A dendritic corneal ulcer is the classic sign of HSV keratitis, and recurrent herpetic keratitis is, after trauma, the leading infectious cause of corneal blindness.

Adenovirus causes a follicular keratoconjunctivitis without a true dendrite, bacterial keratitis a suppurative ulcer, and cytomegalovirus retinitis and VZV acute retinal necrosis affect the retina rather than the corneal surface.

Disseminated HSV infection of eczematous skin, called eczema herpeticum (Kaposi varicelliform eruption), produces clusters of monomorphic punched-out vesicles and can be severe.

Impetigo gives honey-crusted lesions, chickenpox a centripetal rash with lesions at different stages, hand, foot and mouth disease acral and oral vesicles, and molluscum contagiosum painless umbilicated papules without fever.

Glycoprotein D binds nectin-1, an adhesion molecule on epithelial cells and neurons, and also the herpesvirus entry mediator and 3-O-sulfated heparan sulfate. This receptor engagement triggers the fusion machinery; ordinary heparan sulfate is only the initial attachment site, via glycoproteins B and C.

The distractors are the receptors of other viruses: CD4 for HIV, sialic acid for influenza, the mannose-6-phosphate receptor for varicella-zoster virus, and ACE2 for SARS-CoV-2.

As childhood oral acquisition of HSV-1 declines, more young adults reach sexual debut without HSV-1 antibody and acquire it genitally, so HSV-1 now causes more than half of new genital herpes in many such settings.

HSV-2 remains the leading cause of recurrent genital herpes but no longer of first episodes in these populations; the other herpesviruses are not significant causes of genital ulceration.

HSV-1 establishes latency chiefly in the trigeminal ganglion, the source of recurrent orolabial herpes, while HSV-2 favours the sacral ganglia.

The other options invert or invent the site: HSV-2 latency is sacral rather than trigeminal or in the olfactory bulb, and HSV does not reside in anterior horn cells as its principal reservoir.

Type-specific serology based on glycoprotein G, with gG1 for HSV-1 and gG2 for HSV-2, distinguishes past infection with the two types and is used for counselling.

IgM is unreliable and does not type; viral culture and the Tzanck smear identify HSV but do not reliably type it (the Tzanck smear cannot even separate HSV from VZV); complement fixation is outdated and not type-specific.

a. Recurrent genital herpes, most likely HSV-2 given the frequent recurrences (HSV-1 is possible but recurs much less at the genital site).

b. A swab of the ulcer base (deroof a fresh vesicle) sent for HSV PCR (polymerase chain reaction) with typing, which is more sensitive than culture. Type-specific serology is for counselling, not for diagnosing the acute lesion.

c. Either episodic antiviral therapy (aciclovir, valaciclovir or famciclovir) started at the first prodrome, or daily suppressive therapy. With frequent, distressing recurrences the treatment of choice is daily suppressive valaciclovir, which reduces both recurrences and transmission to a partner.

• Lifelong latency in sensory neurons that are poorly accessible to immune surveillance, so a vaccine cannot easily prevent or clear established infection.
• Sophisticated immune evasion and the absence of a validated correlate of protective immunity to guide vaccine design.
• No suitable animal model that faithfully reproduces human infection, latency and recurrence; the leading glycoprotein D subunit candidate failed against HSV-2 in trials.

• How and when it is produced: after amplification is complete, the product is heated gradually while fluorescence from a double-stranded-DNA-binding dye (or probe) is monitored. As the duplex denatures, fluorescence falls steeply at the melting temperature (Tm); plotting the negative first derivative of fluorescence against temperature gives a peak at the Tm. It is a post-amplification step.
• Why HSV-1 and HSV-2 differ: their amplicons differ in sequence and guanine-and-cytosine content, so they melt at different temperatures and give distinct peaks.
• Why it is useful: it allows single-tube typing of HSV-1 versus HSV-2 without separate probes or gels, detects mixed infection, and confirms that the signal is the specific amplicon rather than primer-dimer or non-specific product.

Taxonomy. Species Simplexvirus humanalpha1, genus Simplexvirus, subfamily Alphaherpesvirinae, family Orthoherpesviridae (formerly Herpesviridae), order Herpesvirales.

Genome. Linear double-stranded DNA of about 152 kilobases, organised as a long unique (UL) and a short unique (US) region, each flanked by inverted repeats, so the segments invert to give four genome isomers; around 84 genes.

Capsid. Icosahedral, triangulation number 16, built from 162 capsomeres, with a single portal vertex through which the genome is packaged.

Tegument. A protein layer between capsid and envelope that delivers regulatory proteins at entry, including the transactivator VP16 and the host-shutoff RNase (vhs).

Envelope. A lipid bilayer carrying up to twelve glycoproteins; the core entry machinery is glycoprotein B with the glycoprotein H and L pair, triggered by glycoprotein D, the receptor-binding protein (glycoprotein G is the type-specific antigen used to tell HSV-1 from HSV-2).

• Encephalitis or meningoencephalitis: haemorrhagic, necrotising infection of the brain.
• Hepatitis: which can progress to fulminant hepatic failure, classically in pregnancy or immunosuppression.
• Pneumonitis: with, in severe dissemination, adrenal involvement, widespread mucocutaneous lesions and coagulopathy.

• More frequent, severe and prolonged disease: large, persistent, slowly healing or atypical ulcers, including heaped hypertrophic (pseudotumour) lesions in advanced HIV.
• Dissemination and visceral involvement: oesophagitis, hepatitis, pneumonitis and encephalitis, which can be life-threatening.
• Diagnostic challenge: atypical presentations mean reliance on PCR (polymerase chain reaction), with culture kept for resistance testing.
• Antiviral resistance: prolonged high-level replication selects thymidine-kinase-mutant aciclovir-resistant virus, requiring foscarnet or cidofovir despite their nephrotoxicity.
• Management: intravenous aciclovir for severe disease and long-term suppressive therapy during deep immunosuppression; a flare can occur as immune reconstitution inflammatory syndrome when antiretroviral therapy is started.

A complete answer covers how resistance arises, how common it is and in whom, and what to use instead.

Mechanism

Aciclovir is a guanosine analogue that must first be activated by the viral thymidine kinase (TK) and then by cellular kinases; aciclovir-triphosphate inhibits the viral DNA polymerase and terminates the growing DNA chain. Resistance therefore arises in two ways:

• Thymidine-kinase mutations (much the commonest): TK-deficient, TK-low-producing, or TK with altered substrate specificity, all of which prevent activation of the drug.
• DNA polymerase mutations (less common), which can also confer cross-resistance to other polymerase inhibitors.

Prevalence

Resistance is rare in immunocompetent people (around 1% or less) but much more common in the profoundly immunocompromised, such as those with advanced HIV or haematopoietic stem-cell transplants, where prolonged high-level replication under drug pressure selects resistant virus.

Alternative treatment

• Foscarnet (a pyrophosphate analogue) and cidofovir (a nucleotide analogue) inhibit the viral DNA polymerase directly and do not need thymidine kinase, so they remain active against TK-mutant virus; both are limited by nephrotoxicity.
• Reducing immunosuppression where possible, topical options such as cidofovir gel, and newer helicase-primase inhibitors are additional considerations.

A complete answer covers the activation pathway of each drug, their shared target, and how the difference explains when each is used.

Aciclovir and its selectivity

Aciclovir is a guanosine analogue. Its first phosphorylation is performed by the viral thymidine kinase, which is present only in infected cells, and cellular kinases then complete the conversion to aciclovir-triphosphate. This triphosphate inhibits the viral DNA polymerase and terminates the DNA chain. Selectivity arises because the activating first step happens essentially only in infected cells and because the drug acts on the viral, not the cellular, polymerase, so uninfected tissue is spared.

Cidofovir

Cidofovir is a nucleotide (monophosphate) analogue of cytosine that already carries a phosphonate group, so it does not need the viral thymidine kinase: cellular kinases alone activate it. It too inhibits the viral DNA polymerase. Being thymidine-kinase-independent, it stays active against TK-mutant virus.

Clinical translation

• Aciclovir (and its prodrug valaciclovir) is first-line for HSV because it is safe and selective.
• Cidofovir is reserved as salvage for TK-deficient aciclovir-resistant HSV in the immunocompromised (and is also used against cytomegalovirus); it is nephrotoxic and needs hydration and probenecid, which keeps it second-line.
• So the activation difference explains the roles: TK-dependence gives aciclovir its safety and front-line place, while TK-independence lets cidofovir work where resistance has emerged, at the cost of greater toxicity.

A complete answer covers what makes one molecular assay better than another, then how that claim is checked first from documentation and then on local samples.

Characteristics of a superior assay

• Higher analytical sensitivity (a lower limit of detection), which matters greatly in cerebrospinal fluid where viral loads are low.
• High specificity with detection and typing of both HSV-1 and HSV-2, ideally as part of a syndromic multiplex panel.
• Built-in controls: an internal extraction and amplification control to flag inhibition or failed extraction.
• Validated across the required specimen types (cerebrospinal fluid, genital and mucocutaneous swabs, blood) and robust to inhibitors such as blood in cerebrospinal fluid.
• Operational advantages: faster turnaround, automation, higher throughput, lower cost per test, and ease of use.

Evaluation on paper

• Review the manufacturer’s validation data, regulatory approvals and independent peer-reviewed evaluations.
• Examine the stated limit of detection, analytical sensitivity and specificity, linearity, reproducibility and cross-reactivity, and compare them against the current assay.

Evaluation in practice

• Method comparison against the current assay or reference standard on local samples, reporting concordance, sensitivity, specificity and an agreement statistic such as Cohen’s kappa.
• Verify the limit of detection with dilution panels, and assess precision within and between runs.
• Run external quality assessment and proficiency panels.
• Assess the practical fit: workflow, turnaround, cost, training needs and likely clinical impact.

Decision

The new assay is adopted if it improves sensitivity, specificity, turnaround or cost without loss of accuracy, and is then kept under ongoing internal quality control and external quality assessment.