Common Mode of Transmission of Herpes Simplex Virus
Herpes simplex virus is transmitted primarily through direct contact with infected oral or genital secretions or lesions, with the majority of transmissions occurring during asymptomatic viral shedding rather than during clinically apparent outbreaks. 1, 2
Primary Transmission Routes
Person-to-Person Transmission in Older Children and Adults
- Direct mucosal or cutaneo-mucosal contact is the classic transmission mode for HSV among older children and adults, occurring through contact with infected oral secretions or genital fluids 1, 3
- HSV-1 predominantly causes orofacial herpes through oral contact, while HSV-2 nearly exclusively causes genital disease through sexual contact 4, 3
- Asymptomatic viral shedding is responsible for most horizontal transmissions to sexual partners, occurring on approximately 28% of days when measured by sensitive PCR testing, compared to only 2-6% when measured by viral culture 5, 6
Neonatal Transmission
- Neonatal HSV transmission occurs primarily through exposure to HSV-infected maternal genital fluids during passage through the birth canal, representing the most common mode of vertical transmission 1, 7
- This perinatal transmission occurs at a rate of 1 case per 2,000-5,000 deliveries 1
- Additional neonatal transmission routes include ascending infection from the cervix and through invasive procedures like fetal scalp monitoring that disrupt fetal skin integrity during labor 1
- Congenital (in utero) HSV acquisition is rare but can result in devastating cutaneous, ocular, and CNS damage 1
Critical Transmission Risk Factors
Maternal Infection Status
- The risk for neonatal HSV infection is highest (30-50%) when an infant is born to a woman with primary HSV infection during pregnancy 1
- The transmission risk is much lower (0-5%) for infants born to women shedding HSV from reactivated infection 1
- Genital shedding of HSV at the time of delivery is associated with increased transmission risk 1
- Prolonged rupture of membranes (>6 hours) increases transmission risk, likely due to ascending HSV infection from the cervix 1
Special Populations
- Women infected with HIV, particularly those with low CD4+ cell counts, shed HSV from the vulva and cervix more commonly than HIV-uninfected women, with the majority of this shedding being asymptomatic 1
- An estimated 10% of women coinfected with HIV and HSV have cervical shedding on the day of delivery, compared to 2-3% in HIV-uninfected women 1
Viral Load and Transmission Dynamics
- Transmission is unlikely at viral loads less than 10⁴ HSV DNA copies, and most transmissions occur during prolonged episodes with high viral copy numbers 8
- Many shedding episodes that result in transmission do not reach the threshold of clinical detection because ulcers remain very small, explaining why HSV-2 spreads so effectively within populations 8
- Viral expansion, decay, and re-expansion kinetics are extremely rapid during shedding episodes, making it impossible to directly measure genital viral load at the time of sexual activity 8
Important Clinical Caveats
- Most HSV-infected persons are unaware of their infection—only 13% of HSV-2-seropositive persons have been diagnosed with genital herpes 4
- A positive maternal history of HSV is not needed to support neonatal diagnosis; typically the mother is asymptomatic at delivery 7
- Cesarean delivery substantially lowers the risk for neonatal transmission and is recommended for women with genital herpes prodrome or visible HSV genital lesions at the onset of labor 1, 9