The Antenatal Serology Programme

The microbiology laboratory has begun a programme of routine testing for VZ antibodies in pregnancy as part of the booking blood serology. The presence of IgG in this specimen is indicative of immunity, acquired from previous exposure, and thus the pregnant patient and her foetus are protected, as reinfection is not seen in the inunune patient. It will take a few months for the programme to catch up with all pregnancies, and in the meantime, if a pregnant patient is in contact with chickenpox, the problem should be discussed in the first instance with the consultant microbiologist.

In pregnancy, there are several aspects to consider: · The impact on the mother. · The impact on the developing foetus. · The consequences of infection late in the pregnancy.

Among the obvious measures to minimise the spread of infection to pregnant women are the following: Pregnant women should be discouraged from visiting or helping at playgroups or nurseries when chickenpox is prevalent in the community, and children with chickenpox should not attend antenatal clinics with their mothers.

The impact on the mother - primary chickenpox infection

Based on serological studies, 80-85% of adults have had chickenpox in childhood, but the remaining 15% are more likely to develop complications if they have chickenpox as an adult, and infection in pregnancy is particularly likely to be complicated. Complications include: pneumonitis, haemorrhagic chickenpox and encephalitis. Mortality is significant, and morbidity high. This is particularly true of pneumonitis. If a pregnant woman reports contact with chickenpox, an attempt should be made to ascertain a history of previous infection. Check, in the first instance, whether her antibody status is recorded on the laboratory computer. If she has antibodies, there is nothing to be done; simply reassure the patient. If she has no recorded result, and the patient cannot remember having chickenpox or shingles, her mother or other relatives may be able to confirm the history. Appropriate skin scars may be taken as evidence of infection in the past. If the history remains uncertain, the antibody status can be confirmed serologically. Contact should be made with the Consultant microbiologist to discuss the problem and to arrange for serology if it is agreed that that is appropriate.

The impact on the foetus - congenital varicella syndrome

This can occur up to 20 weeks gestation, and in form it ranges from multisystem involvement and neonatal death to limb hypoplasia, or minimal skin scarring.

With a maternal primary infection, the risk of this is about 1%, being maximal at 2% at 13-20 weeks. There is no method at present for prenatal diagnosis, though ultrasound may detect limb deformities, but this may be too late to intervene.

The rationale for offering VZIg is the potential protection offered to the mother, who has a high risk of severe complicated disease, and is not primarily to protect the foetus.

The consequences of infection late in the pregnancy

Foetal infection rate is around 50% if the mother has chickenpox in the last 4 weeks of the pregnancy, and one third of these babies will develop clinical varicella in utero (NOT congenital varicella syndrome).

If a non-immune pregnant woman has contact with chickenpox in the period one week before to one week following delivery, the baby should have VZIg, as it will have no passive maternal antibodies, and the risk of life-threatening encephalitis is high at this time.

Laboratory Arrangements Out of Hours

VZ antibodies can be measured any weekday, though the antenatal routine tests are done in a batch once a week. The test may be done as an emergency at weekends, when to wait till Monday might be too late for treatment to be effective. This MUST be agreed by the consultant microbiologist on call, and will rarely be necessary. Depending on the result, VZIg will be issued to you from the Area Laboratory if necessary.