HSV1 that causes cold sore is harmless to the unborn child during pregnancy1). Transmission of the virus occurs outside the body not through the blood. The danger of transmission is highest when the mother has an open cold sore on the lips, this for most people is when the infection can be passed to the baby.
The fluid coming from the cold sore is where the virus particles are released2). This fluid should be kept from contacting the baby either directly, such as by kissing or, via the hands by touching the sore and then the baby without proper washing and disinfection. The baby may also be protected from infection by possibly wearing a light face mask over the lips which can help prevent transmission when the baby is being changed, fed or bathed.
For new born babies another member of the Herpes virus family can sometimes present a danger. HSV2 which causes Genital Herpes (not normally found round the mouth) can infect the baby during birth. Studies in the Netherlands and in Denmark have reported between 3 and 5 cases per 100,000 live births so this is quite rare3). Although rare, as the consequences for the baby can be severe, even suspected genital herpes should be reported to the doctor whenever a mother becomes pregnant4). The Herpes virus can also be dangerous for adults in some cases.
At first an infection of a young child will likely show few if any symptoms. Typically the virus moves into the nervous tissues around the face and stays there inactive. Later, and depending on the individual, if the function of the immune system is suppressed, perhaps by another kind of infection, or in adolescence by intensive athletic training and perhaps triggered by exposure to strong sunlight for a long time the virus will start to migrate to the surface to multiply and spread itself. This is when uninfected children and adults need to be guarded from exposure to surface outbreaks.
2) C. A. Daniels, S. G. LeGoff: Shedding of infectious virus/antibody complexes from vesicular lesions of patients with recurrent herpes labialis. In:Lancet Band 20;2, Nr. 7934, 1975, S. 524–528.
3) J. Poeran et al.: The incidence of neonatal herpes in The Netherlands. J. Clin. Virol. (2008) 42(4): S. 321-325
- Fonnest et al.:Neonatal herpes in Denmark 1977-1991. Acta Obstet. Gynecol. Scand. (1997) 76(4): S. 355-358
4) R. F. Jacobs: Neonatal herpes simplex virus infections. Semin. Perinatol. (1998) 22 (1): S. 64–71