What Is the Difference Between Congenital and Neonatal HSV?

When parents read about herpes simplex virus (HSV) in newborns, the terminology can be confusing. The terms “congenital” and “neonatal” are often used to describe when and how the baby was exposed to the virus.

Neonatal HSV is the broad medical term used for any HSV infection in a newborn ^[1]. Within this broad category, the infection is classified by how it was acquired. Studies estimate that approximately 85% of cases are acquired during delivery (perinatally), about 10% are acquired after birth (postnatally), and roughly 5% are acquired in utero before birth ^{[1] [2]}.

When an infection is acquired before birth, it is specifically referred to as congenital HSV ^[3]. Understanding this distinction helps explain why symptoms appear at different times and affect the baby differently.

Congenital HSV: Infection Before Birth

True congenital HSV is rare, making up only about 5% of neonatal herpes cases ^[1]. In these instances, the virus is transmitted to the fetus during pregnancy ^{[3] [4]}. This can happen even if the birth parent never had any visible herpes symptoms ^{[5] [3]}.

Because the infection happens while the baby is still developing in the womb, the virus can cause structural malformations ^{[6] [7]}. These symptoms are often present and recognized on the baby’s first day of life, though sometimes they are identified later ^[6].

The classic “triad” (group of three) signs for congenital HSV includes:

• Skin abnormalities: Blisters (vesicles) or distinct scarring ^{[6] [7]}.
• Eye damage: Issues like chorioretinitis (inflammation of the back of the eye) or keratitis (inflammation of the cornea) ^{[6] [7]}.
• Neurological issues: Brain damage formed in the womb, often showing up as microcephaly (an abnormally small head and brain) or visible calcifications on a brain scan ^{[7] [6]}.

While this triad is classic, it is only completely present in about 29% of congenital cases, meaning a baby can have congenital HSV without showing all these signs ^[7].

Acquired Neonatal HSV: Infection During or After Birth

The vast majority of newborn HSV infections are acquired perinatally (during labor and delivery) or postnatally (after birth) ^{[1] [2]}.

• Perinatal transmission: The baby contracts the virus while passing through the birth canal ^[1].
• Postnatal transmission: The baby contracts the virus after birth, usually from contact with a caregiver or family member who has the virus ^{[8] [9]}.

Postnatal infections do not require contact with an “active cold sore.” The virus can spread through contact with oral or skin lesions, or even through asymptomatic shedding where the person has no visible sores at all ^{[8] [10]}. To prevent postnatal transmission, anyone with a cold sore or active HSV lesion should cover the lesion, practice meticulous hand hygiene, and avoid kissing the newborn or allowing the lesion to touch the baby ^{[8] [9]}.

Unlike congenital HSV, babies with acquired neonatal HSV generally do not have structural birth defects, and they may appear completely healthy when they are born ^{[11] [6]}. The virus may be present before symptoms appear (an incubation period). Symptoms most often start between the first and third week of life, but they can appear in the first few days or as late as 4 to 6 weeks after birth ^{[11] [12]}.

Clinicians group neonatal HSV into three main clinical patterns, though these categories can overlap and progress from one to another ^[11]:

1. Skin, Eyes, and Mouth (SEM) disease: The infection is localized to surface blisters, eye inflammation, or mouth sores ^[13].
2. Central Nervous System (CNS) disease: The virus infects the brain (encephalitis), causing lethargy, irritability, or seizures ^[14].
3. Disseminated disease: The most dangerous form, where the virus causes a dangerous whole-body illness (acting like severe sepsis) that spreads to multiple internal organs such as the liver, lungs, and heart ^{[13] [15]}.

A crucial warning: In studies of disseminated neonatal HSV, roughly 74% of babies never developed a single skin blister ^[13]. Instead, they appeared generally ill, sometimes without even a fever ^{[13] [15]}.

Structural Birth Defects vs. Acquired Brain Injury

It is important to clarify how these infections affect the brain. Infection acquired during or after birth generally does not produce the congenital malformations that are formed in utero (like being born with an abnormally small head) ^[6]. However, CNS neonatal HSV is still incredibly dangerous. It can cause severe brain inflammation, permanent brain injury, developmental disabilities, and sometimes an acquired loss of head growth later in infancy ^{[14] [16]}.

If you are a parent reading this because your baby is sick, please know that determining the exact timing and route of infection is complex. Prolonged rupture of membranes is a risk factor, not proof that a parent caused the infection ^{[17] [18]}. A newborn can contract HSV despite appropriate medical care and without anyone having visible sores ^{[10] [18]}.

When to Seek Emergency Care

Because neonatal HSV can cause severe organ damage or brain injury rapidly, prompt antiviral evaluation and treatment are essential to improve outcomes ^{[14] [15]}. Do not wait for a blister to appear. Seek immediate emergency medical assessment if a newborn or young infant shows any of the following signs ^{[11] [14] [13]}:

• Unusual sleepiness, lethargy, or difficulty waking up
• Poor feeding
• Marked irritability
• Fever, or an unusually low body temperature (hypothermia)
• Breathing difficulties
• Seizures or abnormal movements
• Jaundice (yellowing of the skin or eyes)
• Any unexplained vesicles (blisters), unusual rashes, or eye redness
• A generally ill or “sepsis-like” appearance

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