RARE DISEASE
Herpes simplex virus encephalitis
Herpes simplex virus encephalitis
Herpes simplex virus encephalitis
Synonyms: HSE, HSV encephalitis, HSVE, Herpes simplex meningo-encephalitis, Herpes simplex neuroinvasion, Herpetic encephalitis
Synonyms: HSE, HSV encephalitis, HSVE, Herpes simplex meningo-encephalitis, Herpes simplex neuroinvasion, Herpetic encephalitis
Synonyms: HSE, HSV encephalitis, HSVE, Herpes simplex meningo-encephalitis, Herpes simplex neuroinvasion, Herpetic encephalitis
Drug discovery
2
drugs
With orphan designations
Overview
Herpes simplex virus encephalitis (HSE), primarily caused by HSV-1, is a life-threatening neurologic infection characterized by fever, altered consciousness, seizures, and focal deficits. Prompt IV acyclovir (10 mg/kg every 8 hours for 14–21 days) reduces mortality from 70–80% to 10–20% [2][9]. Adjunct corticosteroids may mitigate inflammation in severe cases [3][13]. Delays in treatment beyond 48 hours increase mortality and neurocognitive sequelae (30–40% survivors) [1][9]. Autoimmune anti-NMDA receptor encephalitis can occur post-HSE [2].
Burden
Mortality: 7.7% in adults, 6.9% in neonates; survivors face epilepsy (38%), cognitive impairment, or behavioral disorders [4][9][12].
Economic burden: Prolonged ICU stays (median 3–14 days) and high rehabilitation needs [1][14].
Global disparities: LMICs face higher morbidity due to delayed diagnostics and treatment barriers [14].
Therapies
First-line: High-dose IV acyclovir (14–21 days), with CSF-PCR monitoring to guide duration [2][11].
Adjunctive therapies: Corticosteroids (e.g., dexamethasone) for fulminant cases to reduce cerebral edema [3][13].
Supportive care: Seizure management, ICP control, and rehabilitation for residual deficits [6][13].
Categories: rare genetic diseases, rare immunological diseases, rare infectious diseases, rare neurological diseases
Research Papers
1,234 drug discovery papers about Herpes simplex virus encephalitis, with 5 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
1,234 drug discovery papers about Herpes simplex virus encephalitis, with 5 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
2026-08-14 | Innate immunity-mediated inflammation and immunopathogenesis in HSV-1-infected encephalitis.
HSV-1 encephalitis (HSE) remains a devastating viral infection of the central nervous system despite effective antiviral therapy. Increasing evidence indicates that disease outcome is not determined by viral replication alone but by how innate immunity is calibrated within the brain. Studies of host susceptibility in childhood HSE have identified brain-intrinsic antiviral mechanisms, referring to central nervous system (CNS)-localized innate immune pathways that enable resident brain cells to sense and restrict HSV-1 infection, particularly TLR-dependent interferon responses, neuron-associated restriction factors, and other CNS-resident innate immune mechanisms. Host innate immune responses initially contribute to viral restriction, whereas excessive or unresolved activation can promote inflammatory amplification and long-term neurological injury. In parallel, HSV-1-mediated immune remodeling may weaken antiviral control and alter the subsequent inflammatory trajectory. Here, HSE is discussed as a dynamic immunopathological disorder in which protective innate immunity can transition into pathogenic neuroinflammation. Building upon these observations, we propose that host-directed therapeutic strategies should shift from uniform immune modulation toward stage-specific immune recalibration on the basis of viral control, antiviral competence, inflammatory persistence, and tissue repair.
2026-08-14 | Immunomodulating treatments for bacterial meningitis and viral encephalitis.
In central nervous system infections outcome is not only determined by pathogen-mediated injury but also by the host inflammatory response. Inhibition of the immune response has been suggested as an important means of reducing morbidity and mortality. In this review, we summarize the current evidence for immunomodulating treatments in bacterial meningitis and viral encephalitis, discuss the biological rationale underlying these interventions, and identify key knowledge gaps and priorities for future research. For this narrative review Pubmed and Medline were searched using Search terms ((bacterial meningitis[Mesh] OR viral encephalitis[mesh]) AND (immunomodulating treatment OR adjunctive treatment OR corticosteroids) NOT (tubercul* OR cryptococc* OR vaccine)). For bacterial meningitis, data from randomized clinical trials and meta-analyses showed a beneficial effect of adjunctive corticosteroids on hearing loss, neurological sequelae and mortality. This led to the introduction of adjunctive dexamethasone in clinical practice, after which observational studies confirmed the beneficial effect. Studies on other immunomodulating treatments in bacterial meningitis showed no benefit on outcome of paracetamol, a harmful effect of therapeutic hypothermia and insufficient evidence for intravenous immunoglobulins. For viral encephalitis, a systematic review and randomized controlled trial showed no clear benefit of corticosteroids. However, as treatment with dexamethasone was generally started late, it remains an open question whether early corticosteroid treatment could be beneficial. For treatment of viral encephalitis with intravenous immunoglobulins insufficient evidence is available to guide clinical practice. Studies on the modulation of the inflammatory response in bacterial meningitis showed adjunctive dexamethasone should be routinely used in patients with community-acquired bacterial meningitis. Other immunomodulating treatments in bacterial meningitis are currently not advised. There is no convincing evidence for immunosuppressive treatment in patients with viral encephalitis. Whether early corticosteroid treatment is beneficial in HSV encephalitis should be evaluated in a new RCT.
2026-08-12 | Adjunctive Intravenous Immunoglobulin in Severe Herpes Simplex Encephalitis: A Case Report with a Favorable Outcome
Both herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) can cause encephalitis; however, HSV-1 is the most common etiological agent. Herpes simplex encephalitis (HSE) presents with a wide spectrum of clinical manifestations from fever and headaches to acute neurological dysfunctions with seizures. The mortality rate is high even with the right treatment. In this case report we present a 36-year-old female with no comorbidities that presented to the emergency department with a headache and reported olfactory hallucinations. The patient experienced rapid cognitive and neurological deterioration, became stuporous, almost entirely unconscious despite intravenous antiviral therapy. The patient was treated as a ‘last resort’ with intravenous immunoglobulins, and was induced into a coma in the Intensive Care Unit. The patient was extubated a few days later and returned to the neurology ward in a very much improved state where she continued her recovery. At the three-month follow-up the patient exceeded all expectations and was slowly returning to her life before the illness.
2026-07-31 | Susceptibility of clinical isolates, laboratory-adapted and drug-resistant herpes simplex virus strains to helicase-primase inhibitor adibelivir (IM-250) and its synergy in combination therapy with acyclovir, penciclovir and foscarnet.
Herpes simplex virus (HSV) infections are widespread and cause a significant global health burden. Clinical manifestations range from herpes labialis and genital herpes to life-threatening herpes simplex encephalitis or neonatal herpes. Current treatment relies on nucleoside analogs such as acyclovir, valacyclovir or famciclovir. The emergence of resistance to these drugs is of concern in transplant recipients and immunocompromised patients. Next-generation helicase-primase inhibitors are under clinical development to provide more efficacious treatments and address resistance to current therapies. Adibelivir (IM-250) is a potent HSV helicase-primase inhibitor (HPI) with nanomolar activity against both HSV1 and HSV2. In the present study, we assessed the susceptibility of more than 50 clinical isolates and laboratory strains against adibelivir. Across all strains tested, adibelivir inhibited HSV replication in cell culture with mean EC50 values of 21 nM (HSV1) and 35 nM (HSV2), compared to up to 42-fold higher EC50 values for acyclovir (883 nM for HSV1 and 1456 nM for HSV2). No preexisting variants with a lack of susceptibility were identified. Furthermore, no cross-resistance was observed between nucleoside analogs and HPIs and finally, the antiviral activity depended on serum concentration and infectious dose. Collectively, these data establish the baseline susceptibility profile of HSV clinical isolates to adibelivir prior to its broader clinical introduction. Adibelivir retains activity against nucleoside-resistant HSV strains and exhibits synergistic antiviral effects when combined with acyclovir, penciclovir and foscarnet in vitro. These results support clinical evaluation of adibelivir as a therapeutic option for HSV infections and as a candidate for combination therapy.
2026-07-10 | The cell-mediated adaptive immune response to herpes simplex virus type 1 encephalitis: mechanisms and clinical implications
Herpes simplex virus (HSV) encephalitis is the most frequent cause of sporadic encephalitis globally. Despite the emergence of the antiviral drug aciclovir curtailing mortality, this disease remains a clinical challenge due to its rapid progression and associated neurological sequelae; indicating the urgent requirement for adjunctive neuroprotective treatment options. Recent studies using both human samples and murine models have highlighted how cell mediated immune cells including CD8+, CD4+ and brain tissue-resident memory T cells have the capacity to act as a ‘double-edged sword’; both serving to protect the host against HSV encephalitis, but also increasing neuroglial injury by inflammation. Factors which impair cell-mediated immunity may predispose individuals to herpes simplex encephalitis, including defects in viral immune evasion strategies (infected cell protein 47, UL13 kinase), host genetic pre-disposition (toll-like receptor 3 deficiency, lymphotoxin-α deficiency, Rel mutations) and external factors such as stress. Additionally, there is a particular need for clinical vigilance for patients on immunosuppressive treatments which impair cell-mediated immunity, including azathioprine, hydroxychloroquine and methotrexate. Conversely, understanding these molecular mechanisms may provide new insights into the use of immunomodulatory strategies including anakinra, tocilizumab, and the implementation of targeted vaccination. This review summarises the current state of knowledge of how an impaired cell-mediated immune response could promote herpes simplex virus encephalitis, followed by an exploration of the clinical applications and therapeutic interventions which could viably be implemented to ameliorate immune-mediated tissue injury.
2026-08-14 | Innate immunity-mediated inflammation and immunopathogenesis in HSV-1-infected encephalitis.
HSV-1 encephalitis (HSE) remains a devastating viral infection of the central nervous system despite effective antiviral therapy. Increasing evidence indicates that disease outcome is not determined by viral replication alone but by how innate immunity is calibrated within the brain. Studies of host susceptibility in childhood HSE have identified brain-intrinsic antiviral mechanisms, referring to central nervous system (CNS)-localized innate immune pathways that enable resident brain cells to sense and restrict HSV-1 infection, particularly TLR-dependent interferon responses, neuron-associated restriction factors, and other CNS-resident innate immune mechanisms. Host innate immune responses initially contribute to viral restriction, whereas excessive or unresolved activation can promote inflammatory amplification and long-term neurological injury. In parallel, HSV-1-mediated immune remodeling may weaken antiviral control and alter the subsequent inflammatory trajectory. Here, HSE is discussed as a dynamic immunopathological disorder in which protective innate immunity can transition into pathogenic neuroinflammation. Building upon these observations, we propose that host-directed therapeutic strategies should shift from uniform immune modulation toward stage-specific immune recalibration on the basis of viral control, antiviral competence, inflammatory persistence, and tissue repair.
2026-08-14 | Immunomodulating treatments for bacterial meningitis and viral encephalitis.
In central nervous system infections outcome is not only determined by pathogen-mediated injury but also by the host inflammatory response. Inhibition of the immune response has been suggested as an important means of reducing morbidity and mortality. In this review, we summarize the current evidence for immunomodulating treatments in bacterial meningitis and viral encephalitis, discuss the biological rationale underlying these interventions, and identify key knowledge gaps and priorities for future research. For this narrative review Pubmed and Medline were searched using Search terms ((bacterial meningitis[Mesh] OR viral encephalitis[mesh]) AND (immunomodulating treatment OR adjunctive treatment OR corticosteroids) NOT (tubercul* OR cryptococc* OR vaccine)). For bacterial meningitis, data from randomized clinical trials and meta-analyses showed a beneficial effect of adjunctive corticosteroids on hearing loss, neurological sequelae and mortality. This led to the introduction of adjunctive dexamethasone in clinical practice, after which observational studies confirmed the beneficial effect. Studies on other immunomodulating treatments in bacterial meningitis showed no benefit on outcome of paracetamol, a harmful effect of therapeutic hypothermia and insufficient evidence for intravenous immunoglobulins. For viral encephalitis, a systematic review and randomized controlled trial showed no clear benefit of corticosteroids. However, as treatment with dexamethasone was generally started late, it remains an open question whether early corticosteroid treatment could be beneficial. For treatment of viral encephalitis with intravenous immunoglobulins insufficient evidence is available to guide clinical practice. Studies on the modulation of the inflammatory response in bacterial meningitis showed adjunctive dexamethasone should be routinely used in patients with community-acquired bacterial meningitis. Other immunomodulating treatments in bacterial meningitis are currently not advised. There is no convincing evidence for immunosuppressive treatment in patients with viral encephalitis. Whether early corticosteroid treatment is beneficial in HSV encephalitis should be evaluated in a new RCT.
2026-08-12 | Adjunctive Intravenous Immunoglobulin in Severe Herpes Simplex Encephalitis: A Case Report with a Favorable Outcome
Both herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) can cause encephalitis; however, HSV-1 is the most common etiological agent. Herpes simplex encephalitis (HSE) presents with a wide spectrum of clinical manifestations from fever and headaches to acute neurological dysfunctions with seizures. The mortality rate is high even with the right treatment. In this case report we present a 36-year-old female with no comorbidities that presented to the emergency department with a headache and reported olfactory hallucinations. The patient experienced rapid cognitive and neurological deterioration, became stuporous, almost entirely unconscious despite intravenous antiviral therapy. The patient was treated as a ‘last resort’ with intravenous immunoglobulins, and was induced into a coma in the Intensive Care Unit. The patient was extubated a few days later and returned to the neurology ward in a very much improved state where she continued her recovery. At the three-month follow-up the patient exceeded all expectations and was slowly returning to her life before the illness.
2026-07-31 | Susceptibility of clinical isolates, laboratory-adapted and drug-resistant herpes simplex virus strains to helicase-primase inhibitor adibelivir (IM-250) and its synergy in combination therapy with acyclovir, penciclovir and foscarnet.
Herpes simplex virus (HSV) infections are widespread and cause a significant global health burden. Clinical manifestations range from herpes labialis and genital herpes to life-threatening herpes simplex encephalitis or neonatal herpes. Current treatment relies on nucleoside analogs such as acyclovir, valacyclovir or famciclovir. The emergence of resistance to these drugs is of concern in transplant recipients and immunocompromised patients. Next-generation helicase-primase inhibitors are under clinical development to provide more efficacious treatments and address resistance to current therapies. Adibelivir (IM-250) is a potent HSV helicase-primase inhibitor (HPI) with nanomolar activity against both HSV1 and HSV2. In the present study, we assessed the susceptibility of more than 50 clinical isolates and laboratory strains against adibelivir. Across all strains tested, adibelivir inhibited HSV replication in cell culture with mean EC50 values of 21 nM (HSV1) and 35 nM (HSV2), compared to up to 42-fold higher EC50 values for acyclovir (883 nM for HSV1 and 1456 nM for HSV2). No preexisting variants with a lack of susceptibility were identified. Furthermore, no cross-resistance was observed between nucleoside analogs and HPIs and finally, the antiviral activity depended on serum concentration and infectious dose. Collectively, these data establish the baseline susceptibility profile of HSV clinical isolates to adibelivir prior to its broader clinical introduction. Adibelivir retains activity against nucleoside-resistant HSV strains and exhibits synergistic antiviral effects when combined with acyclovir, penciclovir and foscarnet in vitro. These results support clinical evaluation of adibelivir as a therapeutic option for HSV infections and as a candidate for combination therapy.
2026-07-10 | The cell-mediated adaptive immune response to herpes simplex virus type 1 encephalitis: mechanisms and clinical implications
Herpes simplex virus (HSV) encephalitis is the most frequent cause of sporadic encephalitis globally. Despite the emergence of the antiviral drug aciclovir curtailing mortality, this disease remains a clinical challenge due to its rapid progression and associated neurological sequelae; indicating the urgent requirement for adjunctive neuroprotective treatment options. Recent studies using both human samples and murine models have highlighted how cell mediated immune cells including CD8+, CD4+ and brain tissue-resident memory T cells have the capacity to act as a ‘double-edged sword’; both serving to protect the host against HSV encephalitis, but also increasing neuroglial injury by inflammation. Factors which impair cell-mediated immunity may predispose individuals to herpes simplex encephalitis, including defects in viral immune evasion strategies (infected cell protein 47, UL13 kinase), host genetic pre-disposition (toll-like receptor 3 deficiency, lymphotoxin-α deficiency, Rel mutations) and external factors such as stress. Additionally, there is a particular need for clinical vigilance for patients on immunosuppressive treatments which impair cell-mediated immunity, including azathioprine, hydroxychloroquine and methotrexate. Conversely, understanding these molecular mechanisms may provide new insights into the use of immunomodulatory strategies including anakinra, tocilizumab, and the implementation of targeted vaccination. This review summarises the current state of knowledge of how an impaired cell-mediated immune response could promote herpes simplex virus encephalitis, followed by an exploration of the clinical applications and therapeutic interventions which could viably be implemented to ameliorate immune-mediated tissue injury.
Access all drug discovery papers and probability of success in trials forecasts:
Access all drug discovery papers and probability of success in trials forecasts:
Drug Discovery Landscape
2 orphan drug designations for Herpes simplex virus encephalitis.
2 orphan drug designations for Herpes simplex virus encephalitis.
Drug | Therapy type | Regulator | Orphan designation | Approval | Sponsor |
|---|---|---|---|---|---|
granzyme B associated peptide | peptides | FDA | 2025-06-25 | — | BioJENC, LLC. |
PR-225 (redox-acyclovir) | small molecules | FDA | 1990-05-29 | — | Pharmos |
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