The eight human herpesviruses each have distinct tissue tropisms, clinical presentations, and diagnostic approaches. Understanding which test to use, which sample to collect, and how to interpret PCR and serological results for each virus is essential knowledge for clinical microbiologists, virologists, infectious disease clinicians, and any healthcare professional managing patients with severe or recurrent viral infections.
The Eight Human Herpesviruses
HSV-1 (Herpes Simplex Virus 1): primary oral herpes, herpes labialis (cold sores), herpetic whitlow, herpes keratitis, and — the most serious manifestation — herpes simplex encephalitis (HSE). HSE is the most common cause of fatal sporadic viral encephalitis in the developed world, with untreated mortality of approximately 70 per cent and treated mortality (aciclovir 10 mg/kg IV 8-hourly for 14 to 21 days) of 20 to 28 per cent. HSE is a diagnostic emergency: CSF PCR for HSV is the gold-standard test and should be ordered on any patient with acute or subacute encephalitis (fever, altered consciousness, behavioural changes, focal neurological signs, temporal lobe involvement on MRI).
HSV-2: primary and recurrent genital herpes, neonatal herpes. Neonatal herpes is a devastating condition occurring when a neonate is exposed to HSV-2 (or HSV-1) during delivery from a mother with primary genital herpes. Disseminated neonatal herpes carries mortality of 30 per cent and severe neurological sequelae in survivors without treatment. Diagnosis of neonatal herpes: surface swabs (eye, mouth, nasopharynx, rectum), blood PCR (viraemia), and CSF PCR if CNS disease suspected. Treatment: high-dose IV aciclovir (60 mg/kg/day in three divided doses for 14 to 21 days).
VZV (Varicella-Zoster Virus): primary infection causes varicella (chickenpox): widespread vesicular rash, fever, and complications including secondary bacterial infection (S. aureus, S. pyogenes), pneumonitis (particularly in adults and immunocompromised), and encephalitis. Reactivation causes herpes zoster (shingles): dermatomal vesicular rash, severe pain, and complications including post-herpetic neuralgia, herpes zoster ophthalmicus (HZO, involving the ophthalmic division of the trigeminal nerve and threatening sight), and disseminated zoster in the immunocompromised.
EBV (Epstein-Barr Virus, HHV-4): primary infection causes infectious mononucleosis (glandular fever): pharyngitis, lymphadenopathy, splenomegaly, fever, and atypical lymphocytosis. EBV establishes latency in B lymphocytes and is associated with Burkitt lymphoma, Hodgkin lymphoma, nasopharyngeal carcinoma, and post-transplant lymphoproliferative disease (PTLD).
CMV (Cytomegalovirus, HHV-5): primary infection in immunocompetent adults is usually subclinical or causes a mononucleosis syndrome (similar to EBV). In immunocompromised patients (transplant recipients, HIV with CD4 below 50) it causes: retinitis (CMV retinitis, a leading cause of blindness in untreated HIV), colitis, oesophagitis, pneumonitis, encephalitis, and systemic CMV disease. CMV is the most clinically significant herpesvirus in the transplant setting.
HHV-6 (Human Herpesvirus 6): primary infection in infancy causes roseola infantum (exanthem subitum): high fever followed by a characteristic spreading rash as fever defervesces. HHV-6 reactivation occurs commonly post-transplant and can cause encephalitis. An important pitfall: chromosomally integrated HHV-6 (ciHHV-6) occurs in approximately 1 per cent of the population (HHV-6 DNA integrated into germline), causing very high HHV-6 DNA levels in blood that do not indicate active infection.
HHV-7 and HHV-8 (KSHV, Kaposi's Sarcoma-associated Herpesvirus): HHV-8 causes Kaposi's sarcoma (in HIV-positive individuals, organ transplant recipients, and elderly Mediterranean men), primary effusion lymphoma, and multicentric Castleman's disease.
Diagnostic Methods for Herpesviruses
PCR (nucleic acid amplification): the primary diagnostic method for most herpesvirus infections. Highly sensitive and specific. Available from many sample types: CSF (encephalitis, meningitis), swabs (vesicle swabs for HSV/VZV), blood (CMV, EBV viral load monitoring), urine (CMV shedding), BAL (CMV pneumonitis), vitreous fluid (CMV retinitis), tissue biopsy.
HSV PCR from CSF: the gold standard for herpes encephalitis. Sensitivity approximately 98 per cent, specificity approximately 94 per cent. A negative HSV PCR from a CSF taken within the first 24 to 48 hours of encephalitis may be falsely negative due to very early infection: repeat sampling at 3 to 7 days is recommended if clinical suspicion remains high. Aciclovir should not be stopped while awaiting a repeat PCR in a patient with suspected HSE.
CMV quantitative PCR (viral load): used for monitoring CMV replication post-transplant. Rising viral load indicates CMV viraemia: threshold for pre-emptive antiviral therapy varies by centre and organ type (solid organ transplant vs haematological SCT) but typically ranges from 200 to 1,000 IU/mL. Falling viral load on therapy confirms response.
Serology: IgM and IgG antibody testing by EIA or CLIA. IgM indicates recent primary infection (with important caveats: IgM can persist for months, can be produced in reactivation, and false-positive IgM is common, particularly for CMV and EBV). IgG indicates past exposure and immune memory.
EBV heterophile antibody test (Monospot): traditional rapid test for EBV infectious mononucleosis, detecting heterophile antibodies that agglutinate sheep or horse red blood cells. Positive in approximately 85 per cent of EBV mononucleosis in adults, but negative in young children (below 5 years, who typically do not produce heterophile antibodies) and in EBV-negative mononucleosis syndromes (CMV).
EBV-specific serology: VCA-IgM (Viral Capsid Antigen IgM, indicates primary infection), VCA-IgG, EA (Early Antigen, rises in primary infection and reactivation), EBNA-IgG (Epstein-Barr Nuclear Antigen: appears late, 6 to 12 weeks after primary infection; EBNA-IgG positive + VCA-IgM negative indicates past infection, not acute).
Virus culture: now rarely used for herpesviruses in routine clinical practice (PCR has replaced it). Shell vial culture (centrifugation enhanced culture with immunofluorescence at 24 to 48 hours) was used for CMV but has been replaced by CMV PCR.
Antiviral Therapy for Herpesviruses
Aciclovir (acyclovir) and its prodrug valaciclovir: selective for herpes-infected cells through viral thymidine kinase-mediated phosphorylation (HSV TK phosphorylates aciclovir much more efficiently than cellular TK, providing selectivity). Active against HSV-1, HSV-2, and VZV. Ganciclovir-resistant HSV (thymidine kinase-deficient mutants) can occur with prolonged aciclovir therapy in immunocompromised patients.
Ganciclovir and its prodrug valganciclovir: more active against CMV (which lacks TK but activates ganciclovir through the UL97 kinase). First-line for CMV treatment and prophylaxis in transplant patients.
CMV resistance: UL97 mutations (reduced ganciclovir phosphorylation) and UL54 mutations (DNA polymerase mutations) cause ganciclovir resistance in transplant patients on prolonged therapy. Alternatives: foscarnet (pyrophosphate analogue) and cidofovir. CMV resistance testing by genotype is available.
Frequently Asked Questions
What are the eight human herpesviruses?
The eight human herpesviruses are: HSV-1, HSV-2, VZV, EBV (HHV-4), CMV (HHV-5), HHV-6 (A and B), HHV-7, and HHV-8 (KSHV). All share the ability to establish latency and reactivate under immune stress.
What is herpes encephalitis and how is it diagnosed?
Herpes simplex encephalitis (HSE) is the most common cause of fatal sporadic viral encephalitis in high-income countries. It is caused by HSV-1 (occasionally HSV-2) and presents with acute or subacute fever, altered consciousness, behavioural and personality changes, and focal neurological signs. MRI shows characteristic temporal lobe involvement. Diagnosis is by CSF PCR for HSV, which is over 98 per cent sensitive. IV aciclovir must be started empirically in any patient with suspected encephalitis without waiting for PCR confirmation.
What is the significance of CMV in transplant recipients?
CMV is the most important opportunistic viral infection in both solid organ transplant and haematopoietic stem cell transplant recipients. CMV infection (primary, reactivation, or superinfection) causes direct end-organ disease (retinitis, colitis, pneumonitis, encephalitis) and indirect effects (increasing rejection risk, immunosuppression, susceptibility to opportunistic infections). CMV prevention strategies: universal antiviral prophylaxis (valganciclovir) or pre-emptive therapy guided by quantitative PCR monitoring.
What is the difference between VZV primary infection and reactivation?
Primary VZV infection causes varicella (chickenpox): widespread vesicular rash involving the scalp, trunk, and extremities, fever, and respiratory spread. VZV then establishes latency in dorsal root ganglia. Reactivation causes herpes zoster (shingles): dermatomal vesicular rash (restricted to one or two dermatomes) with severe burning pain, without systemic spread in immunocompetent individuals. Disseminated zoster (extending beyond two dermatomes) occurs in immunocompromised patients.
What is EBV serology and how do you interpret it?
EBV serology uses VCA-IgM (positive in acute primary infection), VCA-IgG (positive in past or current infection), EA-IgG (Early Antigen, elevated in primary infection and reactivation), and EBNA-IgG (appears 6 to 12 weeks after primary infection, present in past infection). Acute primary EBV: VCA-IgM positive + EBNA-IgG negative. Past resolved EBV: VCA-IgG positive + EBNA-IgG positive + VCA-IgM negative. VCA-IgM positive + EBNA-IgG positive = likely false-positive VCA-IgM (common) or reactivation.
Why does aciclovir not work well against CMV?
Aciclovir requires phosphorylation by HSV thymidine kinase (TK) to its active form. CMV does not have a thymidine kinase and instead uses the UL97 kinase to activate nucleoside analogues. Ganciclovir is a much better substrate for UL97 than for HSV TK, making it the preferred agent for CMV (aciclovir is approximately 3 to 9 per cent as active as ganciclovir against CMV). Valaciclovir at very high doses has some CMV activity but ganciclovir/valganciclovir are the standard of care.
What is chromosomally integrated HHV-6 (ciHHV-6)?
Approximately 1 per cent of the population has HHV-6 DNA integrated into their chromosomes (inherited through the germline). ciHHV-6 individuals have very high HHV-6 DNA levels in all nucleated cells (including blood), which is detectable by PCR. This can be misinterpreted as active HHV-6 reactivation in a transplant patient. Distinction: ciHHV-6 is present in hair follicles and other non-immune cells, has a stable extremely high copy number (approximately 1 copy per cell), and is not associated with clinical disease.
What is post-herpetic neuralgia (PHN)?
PHN is chronic neuropathic pain persisting for more than 3 months after herpes zoster (shingles) rash resolution, caused by VZV-induced damage to dorsal root ganglion sensory neurons. It affects approximately 10 to 15 per cent of herpes zoster cases overall, rising to over 50 per cent in patients aged above 70. PHN is managed with neuropathic pain agents (gabapentin, pregabalin, amitriptyline, topical lidocaine, or capsaicin). The live-attenuated Zostavax vaccine and the recombinant adjuvanted Shingrix vaccine substantially reduce herpes zoster incidence and PHN severity.
What is neonatal herpes and why is it so serious?
Neonatal herpes occurs when HSV is transmitted from mother to neonate during delivery, causing disseminated HSV infection in the neonate's immature immune system. Three forms: localised (skin, eye, mouth), CNS disease, and disseminated (multi-organ failure). Disseminated neonatal herpes has mortality of 25 to 30 per cent even with treatment. Caesarean delivery is offered when a mother has primary genital herpes at term to reduce neonatal exposure risk.
What are the Shingrix and Zostavax vaccines?
Zostavax is a live-attenuated VZV vaccine approved for adults aged 50 or above to reduce herpes zoster incidence and PHN severity. Shingrix is a recombinant glycoprotein E subunit vaccine adjuvanted with AS01B, approved for adults aged 50 or above. Shingrix is significantly more effective than Zostavax (over 90 per cent efficacy vs 51 per cent for Zostavax), maintains efficacy over longer follow-up, and can be given to immunocompromised patients (unlike the live Zostavax). Shingrix requires two doses 2 to 6 months apart.