Herpes Simplex Virus (HSV) Serology

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) cause orolabial, genital and neonatal infection, keratitis and encephalitis. Type specific serology detects IgG to glycoprotein G1 (gG-1) and glycoprotein G2 (gG-2) by enzyme immunoassay, chemiluminescence, immunoblot or point-of-care assay.

The type specific tests distinguish HSV-1 from HSV-2 antibody, unlike older assays based on shared glycoproteins. IgM tests are unreliable and are not used. Diagnosis of active lesions is by PCR or culture of swabs of the lesion, and CSF PCR for encephalitis and meningitis.

HSV-1 seroprevalence is 60 to 80% in adults worldwide, and HSV-2 about 11% globally (13% in people aged 15 to 49 years), with the highest rates in sub-Saharan Africa (30 to 60% in some regions) and higher rates in women and people with HIV. HSV-2 infection is mostly unrecognised (70 to 80%). Genital HSV-1 is rising as a cause of first episode genital herpes.

HSV-2 increases HIV acquisition risk 2 to 3 times. Neonatal herpes occurs in 1 in 3000 to 1 in 20 000 live births with high mortality if untreated, and HSV encephalitis is the commonest sporadic viral encephalitis.

Indications. Partners of people with genital herpes (discordant couples). Recurrent genital ulcers with negative swabs (to document HSV-2). Pregnancy where the partner has genital herpes or history of lesions. Counselling after first episode genital herpes (to separate primary from non-primary first episode). Not a routine screening test in asymptomatic people, because of false positives and psychosocial harm. Not for diagnosing active lesions or encephalitis (PCR is used).

Specimen. Serum 1 mL, no fasting. Stable for days at 2 to 8 °C. For active disease: lesion swab in viral transport medium (unroofed vesicle) for PCR, CSF 0.5 to 1 mL for PCR, and ocular swabs or aqueous humour for keratitis or retinitis.

Reference values (assay dependent). Type specific IgG index: negative under 0.9, equivocal 0.9 to 1.1, positive over 1.1 (enzyme immunoassay). HSV PCR: not detected.

Interpretation.

• HSV-2 IgG positive (index over 3.5): HSV-2 infection (anogenital in most), with high specificity. Specificity is 94 to 98% in high prevalence settings but falls to under 50% for low positive indices (1.1 to 3.5) in low prevalence populations, so confirmation by immunoblot or a second assay is needed.
• HSV-1 IgG positive alone: orolabial infection in most, and genital HSV-1 cannot be excluded.
• HSV-2 IgG negative and HSV-1 IgG positive in a person with genital lesions: genital HSV-1 or HSV-2 seroconversion not yet complete (repeat in 12 weeks).
• Seroconversion in a person with recent genital lesions: primary infection (antibodies appear within 2 to 12 weeks, typically 3 to 6 weeks).
• Negative serology in a recently exposed or newly symptomatic person: repeat after 12 weeks.
• Neonatal herpes: serology has no role, and PCR of blood, CSF, skin, mouth, eye and rectal swabs is used.
• Herpes simplex encephalitis: CSF PCR is positive in over 95% (sensitivity may be low in the first 72 hours, and repeat PCR after 3 to 7 days is done if negative with high suspicion).
• Serological status in pregnancy: women seronegative for both types with a seropositive partner are at highest risk of primary infection in late pregnancy and neonatal transmission (30 to 50% risk with primary infection near delivery).

Critical values and findings needing immediate action.

• Fever, headache, behavioural change, seizures or focal signs: suspect herpes simplex encephalitis, start intravenous aciclovir immediately without waiting for PCR (mortality over 70% untreated, falling to about 10 to 20% with early treatment), with CSF PCR, MRI and EEG.
• Neonate with vesicles, lethargy, fever, poor feeding or seizures in the first 4 weeks: neonatal herpes, urgent intravenous aciclovir and full evaluation.
• First episode genital herpes in late pregnancy: obstetric review about antiviral treatment and delivery mode (caesarean section if active lesions at labour).
• Painful red eye with dendritic corneal ulcer: herpes keratitis, same day ophthalmology.
• Severe or disseminated herpes in an immunocompromised patient (oesophagitis, hepatitis, pneumonitis, eczema herpeticum): intravenous aciclovir and consider resistance.

Interferences and pitfalls.

• Serology should not be used for diagnosing acute lesions, since antibody may not yet be present. Lesion PCR is the most sensitive test and is preferred to culture.
• Low positive HSV-2 results (index under 3.5) have a high false positive rate and need confirmation. A false diagnosis has serious psychosocial consequences.
• Many HSV-1 and HSV-2 infections are asymptomatic, so a positive result does not date the infection or prove the partner source.
• IgM tests are non-specific and may be positive in recurrences, so they are not used.
• Cross-reactivity between HSV-1 and HSV-2 and with varicella zoster virus is small with gG-based tests but not absent.
• Neonatal blood contains maternal IgG for up to 12 months, so infant serology is uninterpretable.
• HSV-2 positive status in HIV increases viral shedding and infectivity.
• Suppressive antiviral therapy reduces genital shedding and transmission to partners by about 50%, but does not eliminate it, and condoms add protection.
• Negative HSV PCR from a healing lesion does not exclude herpes.
• CSF PCR can be negative early, and antiviral treatment continues until the diagnosis is excluded.

Special groups.

• Children: orolabial HSV-1 gingivostomatitis is common. Genital lesions in a child require assessment for sexual abuse.
• Pregnancy: maternal primary infection near term is the highest risk. Antiviral suppression from 36 weeks is given to women with a history of genital herpes.
• Older adults: reactivation and encephalitis (a second peak after 50 years).
• HIV and immunosuppression: more severe, prolonged lesions, and resistance to aciclovir is seen.
• Regions: very high HSV-2 prevalence in sub-Saharan Africa, with synergy with HIV transmission, and delayed access to PCR.

Follow up tests. HSV PCR of lesion swab (typing), CSF HSV PCR with cell count, protein, glucose, MRI brain and EEG in encephalitis, type specific confirmatory immunoblot, HIV test, syphilis serology, chlamydia and gonorrhoea NAAT, hepatitis B screening, renal function before intravenous aciclovir, varicella zoster PCR if the diagnosis is uncertain, and partner counselling.