The Science of Herpes

There are nine herpesviruses known to infect humans. All share the same hallmark — lifelong latency — but infect different cell types and cause very different diseases:

• HSV-1: Oral herpes (cold sores), increasingly genital herpes. Latency in trigeminal ganglion.
• HSV-2: Primarily genital herpes. Latency in sacral dorsal root ganglia.
• Varicella-zoster virus (VZV): Causes chickenpox (primary infection) and shingles (reactivation from dorsal root ganglia).
• Epstein-Barr virus (EBV): Causes infectious mononucleosis. Latency in B lymphocytes. Associated with certain lymphomas and nasopharyngeal carcinoma.
• Cytomegalovirus (CMV): Usually asymptomatic in healthy adults. Serious pathogen in immunocompromised individuals and congenital infection. Latency in myeloid cells.
• HHV-6A and HHV-6B: HHV-6B causes roseola infantum. Both latent in T lymphocytes.
• HHV-7: Similar to HHV-6. Ubiquitous, usually asymptomatic.
• HHV-8 (KSHV): Kaposi's sarcoma-associated herpesvirus. Causes Kaposi's sarcoma, particularly in HIV-infected individuals. Latency in B cells.

Importantly, HPV (Human Papillomavirus) is not a herpesvirus. Despite the shared name suffix, HPV belongs to a completely different viral family — Papillomaviridae — with a different genome structure, replication strategy, and mechanism of persistence. The confusion is understandable but the two viral groups are biologically unrelated.

HSV is an enveloped virus — its genetic material is enclosed in a protein capsid, surrounded by a layer of proteins called the tegument, and then wrapped in a lipid membrane studded with glycoprotein spikes. These glycoproteins (gB, gC, gD, gH, gL) mediate cell entry through a multi-step fusion process:

• gC initially tethers the virus to heparan sulphate proteoglycans on the cell surface — ubiquitous molecules that act as a docking point.
• gD then binds to one of several specific entry receptors: HVEM (herpesvirus entry mediator), nectin-1, or 3-O-sulphated heparan sulphate.
• gD binding triggers conformational changes in the gH/gL heterodimer, which in turn activates gB — the core fusion protein.
• gB fuses the viral envelope with the cell membrane (or an endosomal membrane after endocytosis), releasing the capsid into the cytoplasm.

Once inside, the DNA-laden capsid travels along the cytoskeleton to the nuclear pore, where the viral DNA is injected into the nucleus and immediately begins transcribing viral genes. The productive infection cycle takes approximately 18–20 hours, after which new virions bud from the nuclear membrane and are exported through the secretory pathway to the cell surface.

After the initial mucosal infection, HSV does something that sets it apart from most pathogens: it travels to the peripheral nervous system. Sensory nerve terminals innervate the skin and mucosae, and HSV infects these terminals, then travels — inside axons, inside the capsid — by retrograde axonal transport (moving from the nerve ending back toward the cell body). This journey can cover tens of centimetres. In genital HSV, the destination is the sacral dorsal root ganglia at the base of the spine; in oral HSV, it is the trigeminal ganglion behind the cheekbone. Here, in the neuron cell bodies, HSV establishes latency. Reactivation involves anterograde transport back down the axon to the original mucosal site — explaining why outbreaks recur in the same location.

Inside the latently infected neuron, the HSV genome circularises and is maintained as an episome — a closed circular DNA molecule sitting in the nucleus, not integrated into the chromosome. This is different from HIV, which must integrate. The viral DNA is coated in nucleosomes (the same protein structures that package cellular DNA) and is maintained in a largely repressed, chromatinised state.

During latency, only one set of viral transcripts is actively produced: the Latency-Associated Transcripts (LATs). These are non-coding RNAs that do not encode viral proteins but serve several functions — they stabilise the latent state, inhibit apoptosis of the neuron (keeping the cell alive to protect the viral genome), and block the viral lytic gene expression programme.

Reactivation occurs when cellular stress signals — UV light, physical trauma, fever, immunosuppression, hormonal fluctuations — trigger epigenetic changes that shift the viral chromatin from a repressed to an active state. Lytic gene expression resumes, new virions are produced, and the virus travels back to the periphery.

The neuronal latency reservoir is the fundamental barrier to curing herpes. Neurons are among the most long-lived cells in the body — most neurons in the dorsal root ganglia persist for a human lifetime. The HSV episome is stably maintained within them, invisible to immune surveillance (neurons express very low levels of MHC molecules, making immune recognition extremely difficult), and inaccessible to current antiviral drugs, which only act on actively replicating virus.

Antivirals like aciclovir and valaciclovir work by being activated by viral thymidine kinase into a form that specifically inhibits the viral DNA polymerase. They are highly effective at stopping replication — but there is nothing to inhibit in a latently infected neuron that is not replicating. The dormant episome simply waits.

Several curative approaches are in research:

• Gene editing (CRISPR): CRISPR-Cas9 and related tools have been used to excise or disrupt latent HSV genomes in ganglionic neurons in mouse models, achieving partial reduction of the latent reservoir. Delivery to peripheral ganglia in humans remains a formidable challenge.
• Immunological approaches: Therapeutic vaccines aim to train cytotoxic T cells to recognise and kill latently infected neurons — but the immune privilege of neurons makes this difficult.
• Latency-disrupting compounds: Analogous to HIV's "shock and kill", forcing latent HSV into a replication cycle where antivirals can then suppress it — the challenge is that reactivated virus causes tissue damage.