Varicella zoster virus (VZV) is a DNA virus of the herpes family that causes chickenpox, a common childhood infection. Following the primary infection, the virus remains dormant in sensory nerve root ganglia but can be reactivated to cause herpes zoster (HZ), or shingles. 

Chickenpox is highly contagious and transmitted by respiratory droplets and by direct personal contact with vesicle fluid or indirectly via fomites (eg. skin cells, hair, clothing and bedding). It is infectious for two days before the appearance of the rash and for the duration of the illness while the skin lesions are active. It ceases to be infectious when the lesions have crusted over. The incubation period is one to three weeks. 

The risk of infection following contact with herpes zoster (shingles) that is not in an exposed area (eg. thoracolumbar shingles) is remote. Herpes zoster (shingles) poses a risk if it is disseminated, or if it occurs in an exposed area of the body (eg. ophthalmic shingles) or in an immunocompromised individual where viral shedding may be greater.

Increased risk of complications 

Adults, especially pregnant women, who develop chickenpox have a 25-fold increased risk of complications. Pneumonia is reported to complicate 10-14% of chickenpox cases in pregnancy, especially in the third trimester, and chickenpox has a mortality rate of 2% in pregnancy. 

Chickenpox infection in pregnancy between three and 28 weeks gestation can lead to foetal varicella syndrome, which is characterised by skin scarring on limbs, hypoplasia and paresis of limb and eye, and neurological anomalies. The risk of this is very low, with an overall incidence of 0.91% < 20 weeks and 0.55% between 13-20 weeks. Abnormalities can be detected on ultrasound approximately five weeks after the primary infection. Varicella infection of the newborn can occur if maternal infection occurs in the last four weeks of a woman’s pregnancy. 

Over 90% of the antenatal population in the UK and Ireland are seropositive for VZV IgG antibody. For this reason, although contact with chickenpox is common in pregnancy, especially in women with young children, primary VZV infection in pregnancy is uncommon; it is estimated to complicate three in every 1,000 pregnancies. 

Women from tropical and subtropical areas are more likely to be seronegative for VZV IgG and are therefore more susceptible to the development of chickenpox. 

Vaccination

Varicella vaccination should be offered to women who are not immune to chickenpox and are planning a pregnancy or receiving infertility treatment. The varicella immune status of women can be determined by obtaining a past history of chickenpox or shingles or by testing the serum for varicella antibodies in those who have no history or an uncertain history of previous infection. 

If a woman of reproductive age is vaccinated, she should be advised to avoid pregnancy for one month after the last dose and to avoid contact with other susceptible pregnant women should a post-vaccination rash occur (live attenuated vaccine).

Many maternity units currently test for immunity to VZV at booking. If VZIG is not detected in the booking serum, the woman should be advised to avoid contact with chickenpox and shingles during pregnancy and should inform healthcare workers of potential exposure without delay. If a woman is identified as being non-immune to chickenpox during pregnancy, she can be offered vaccination following delivery. 

Small studies have not detected the varicella vaccine in the breast milk of women who have been vaccinated postpartum and therefore, women who receive vaccination postnatally can be reassured that it is safe to breastfeed. Postpartum vaccination of women who are seronegative for chickenpox has been shown to be cost-effective. 

The HSE Clinical Programmes and the Institute of Obstetricians and Gynaecologists developed a clinical practice guideline: Chickenpox in Pregnancy and this document states: “The vaccine should be administered in the postnatal period. Every effort should be made to ensure that the process is user-friendly, so as to increase chances of the woman receiving the vaccine. Where possible, clear local arrangements should be in place between the maternity hospital service and local GPs to ensure high vaccination rates”.¹ However, I am not aware of any organised vaccination programme in Irish maternity hospitals for women identified as non-immune during pregnancy. 

Exposure to chickenpox in pregnancy 

When a pregnant woman has contact with chickenpox or shingles, a history should be taken to confirm the significance of the contact and the susceptibility (or immunity) of the patient. A significant exposure is considered to be the following:

• Living in the same household as a varicella case 

• Face-to-face contact with a varicella case for five minutes 

• Same room as a varicella case for 15 minutes

• Direct contact with extensive/disseminated shingles. 

Susceptibility (or immunity) to chickenpox can be assessed by checking if the woman had immunity done on booking bloods and if this wasn’t done then ask about past history. If there is a definite past history of chickenpox or shingles or two varicella vaccines, it is reasonable to assume that she is immune to varicella infection. 

If there is no history of these, then serum should be tested for varicella IgG. This can usually be performed on stored serum from her booking bloods if you check with your local lab. 

If the pregnant woman is not immune to VZV and she has had a significant exposure to chickenpox or shingles, she should be offered varicella zoster immunoglobulin (VZIG) as soon as possible. Ideally, it should be administered within 96 hours, but it can have effect up to 10 days after exposure. 

If the immune status of the woman is unknown, the administration of VZIG can be delayed until serology results are available. VZIG should be given in a maternity hospital setting and can be given at any time during pregnancy. VZIG prevents or attenuates maternal/neonatal infection and reduces the risk of transplacental spread. 

Adverse effects of VZIG include pain and erythema at the injection site. Anaphylaxis occurs in less than 1% of recipients. Non-immune pregnant women who have been exposed to chickenpox should be managed as potentially infectious from eight to 21 days after exposure but this period is prolonged to eight to 28 days after exposure if they receive VZIG. They should avoid contact with susceptible individuals. 

Clinical practice guideline

If a pregnant woman develops the chickenpox rash during pregnancy, the diagnosis should be confirmed by sending VZV PCR on a swab from the base of a lesion. The clinical practice guideline Chickenpox in Pregnancy states that oral aciclovir should be prescribed for pregnant women with chickenpox if they present within 24 hours of the onset of the rash and if they are more than 20 weeks of gestation. The recommended dose is 800mg five times daily for one week. The guidelines state that aciclovir should be used with caution in women less than 20 gestational weeks and the risks and benefits should be discussed with them. 

In severe or complicated infection, intravenous aciclovir is indicated. VZIG has no therapeutic benefit once chickenpox has developed and should therefore not be used in pregnant women who have developed chickenpox vesicles. The guidelines states that a hospital assessment should be considered in women at high risk of severe or complicated chickenpox. Risk factors for severe disease include smoking, chronic lung disease, immunosuppression and the second half of pregnancy. 

Women who develop chickenpox in pregnancy should be referred to a foetal medicine specialist at 16 to 20 weeks or five weeks after infection for discussion and detailed ultrasound examination. VZV DNA can be detected in amniotic fluid by polymerase chain reaction (PCR). 

VZV DNA has a high sensitivity but a low specificity for the development of foetal varicella syndrome. Women need to be counselled about the risks versus benefits of amniocentesis to detect varicella DNA by polymerase chain reaction (PCR).

Shingles in pregnancy

If a woman develops shingles in pregnancy, there is no risk to the foetus or infant. Dr Susan Knowles, microbiologist in the National Maternity Hospital, recommends giving acyclovir to pregnant women who develop shingles after 20 weeks gestation. She does not recommend aciclovir in the first trimester unless the woman is immunocompromised or has severe infection.

To read more about chickenpox in pregnancy see the Clinical Practice Guideline: Chickenpox in Pregnancy available on the RCPI website www.rcpi.ie 

Reference

1. https://rcpi-live-cdn.s3.amazonaws.com/wp-content/uploads/2016/05/35.-Chicken-Pox-in-Pregnancy.pdf