Chickenpox / fetal varicella infection

Chickenpox / fetal varicella infection

Infection:  Maternal chicken pox, maternal primary varicella (VZV), fetal varicella syndrome

Also refer to: How do I diagnose – Chickenpox

Brief description:
  • Chicken pox (primary varicella) is a common childhood illness that rarely causes significant complications, however is important in relation to pregnancy for these reasons:
    • Risk of severe disease in the mother
    • Risk of congenital infection and fetal varicella syndrome
    • Risk of severe infection in neonate if acquired at or around time of birth
  • Secondary varicella infection (shingles/zoster) in the mother does not pose these risks.
  • Fetal varicella syndrome can manifest in several ways, with skin scars and limb abnormalities being commonest.
  • The risk of FVS depends on gestation at time of maternal infection:
    • <1% if prior to 12 weeks
    • 1-2% if between 12-28 weeks
    • Does not occur if  >28 weeks
Did you know?
  • A good history of prior chickenpox in a pregnant woman has been shown to be highly predictive of immunity, and serology to confirm is not required.
  • Another potential indicator of immunity is if the mother has previously been exposed to an older child in the family with chickenpox and not gone on to develop it herself.
Diagnostic approach & test of choice: maternal exposure to varicella during pregnancy
  • Potential routes of exposure:
    • Contact with a person with active shingles (generally requires close exposure to uncovered lesions)
    • The above two conditions can usually be diagnosed based on characteristic clinical appearance. However, due to the implications of exposure in pregnancy, if there are atypical features, confirmation of the diagnosis is recommended (see links above).
  • If exposure has occurred, does mother have a history of past chickenpox or vaccination?
    • Yes – consider immune, testing not required
  • Varicella IgG detected – consider immune, no further action required
  • Varicella IgG not detected – consider susceptible. Discuss urgently with obstetrics regarding requirement for immunoglobulin and/or antiviral.

Diagnostic approach & test of choice: maternal chickenpox during pregnancy
  • The diagnosis can usually be made clinically, especially if there has been a known exposure to chickenpox, however if there are atypical features or diagnostic uncertainty, then testing is recommended.
  • Viral swab of vesicle fluid for VZV PCR (usually requires de-roofing of vesicle to obtain fluid)
    • Good sensitivity – a negative result on an adequately collected sample makes the diagnosis very unlikely
    • Excellent specificity – a positive result confirms the diagnosis
    • Note: swabs run for VZV PCR in our laboratory are run for HSV PCR in parallel, as these viruses can mimic each other.
  • All cases of chickenpox in pregnancy should be discussed urgently with obstetrics, as there is a risk of severe disease in the mother, and further USS follow-up will usually be required.
Diagnostic approach & test of choice: fetal varicella syndrome
  • This should be led by a maternal-fetal medicine specialist.
  • Lab testing is not usually required if USS is normal at least 5 weeks post maternal infection.
  • If testing is required, amniocentesis for VZV PCR is the test of choice
    • Excellent sensitivity – a negative result makes diagnosis extremely unlikely
    • Moderate specificity – a positive result doesn’t necessarily confirm the diagnosis, but if USS also suggests FVS then makes diagnosis likely.
Tests to avoid/specialist tests:
  • Serology for the diagnosis of chickenpox
    • IgM and IgG serology are not recommended for the diagnosis of chicken pox, as IgM is prone to false positives, and IgG seroconversion requires paired sampling, resulting in delayed diagnosis.