- What are the risks to the patient and her unborn child from chicken pox?
- What is the nature of the exposure?
- Is the patient immune to chicken pox?
- What can be done to prevent the patient from developing chicken pox?
- What should be done if the patient does develop chicken pox?
What are the risks to the patient and her unborn child from chicken pox?
Chicken pox (Varicella Zoster) in pregnancy is a serious infection and potentially has severe consequences for both the mother and her unborn child. It is estimated that 10-20% of pregnant women with varicella develop varicella pneumonia, and 15-30% of these will die. This means that between 3% and 6% of pregnant women with varicella will die from varicella pneumonia. That would grab headlines...”pregnant mum dies of chicken pox!”
Infection of the fetus can also occur if virus from the mother’s blood crosses the placenta and infects the fetus, known as transplacental transmission. In utero infection of the fetus can result in congenital varicella syndrome (CVS). If infection of the mother occurs before the 12th week of pregnancy there is a 0.4% risk of the fetus developing CVS however if the infection occurs between 13 and 20 weeks then the risk is 2%. After 20 weeks CVS is very rare.
CVS occurs when the Varicella virus reactivates in the developing nerve cells causing permanent damage; this is like the fetus having shingles. The result can be anything from cutaneous scarring in a dermatomal distribution, limb hypoplasia, brain damage, visual damage, bowel abnormalities and low birth weight. CVS does not appear to cause spontaneous abortion or prematurity but 30% of affected babies die in the first few months of life and 15% of the survivors have shingles “again” before they are 4 years old!
The other way that varicella can affect babies is when they are exposed to the infection around the time of delivery. The highest risk is if the mother develops chicken pox 5 days before to 2 days after delivery. These babies develop severe chicken pox with an associated 30% mortality. The same applies to babies who develop chicken pox within the first 2 weeks of life whoever was the source of exposure. Babies and chicken pox just do not mix!
However the risk to the fetus if the mother develops shingles in pregnancy is very low with only a single case of CVS being described in the last 30 years. This is because shingles is reactivation of past infection rather than new infection therefore the mother’s antibodies against the virus helps to protect them both and as the viral load in the mother is low, cross placental infection is very unlikely.
When is a chicken pox patient considered infectious?
An infected person is someone displaying the chicken pox rash who remains infectious until all of their skin lesions have dried and crusted over PLUS they were infectious 2 days before they developed the rash. Don’t forget that these 2 pre-rash days are just as infectious!
What is the nature of the exposure?
Varicella is spread by droplets from the oropharynx; it is essentially a “respiratory” disease even though the most well-known clinical feature is the rash. Significant exposure is considered as:
- Being a household member of an infected person
- Having a face-to-face conversation with an infected person
- Being in the same room for 15 minutes with an infected person
Is the patient immune to chicken pox?
More than 95% of women of child bearing age in the UK have had chicken pox. Most people are able to say whether they have had the infection or not but some don’t know. It is also possible to have asymptomatic chicken pox and therefore be immune even with no history of the rash.
If the pregnant patient has a definite history of chicken pox they are considered immune and nothing more needs to be done.
If there is no history of chicken pox then the pregnant patient should have their blood tested for immunity asking the laboratory to look for VZIgG. If the patient has detectable VZIgG then they are immune irrespective of whether they have a history of chicken pox or not. If you give good clinical information this test can be done the same day; standard testing without clinical priority can take 3 days.
What can be done to prevent a pregnant patient from developing chicken pox?
Pregnant patients with significant exposure to chicken pox who have no evidence of immunity should be given passive immunity with injectable VZIgG (VZIG) within 10 days of exposure. This prevents the patient developing chicken pox in about 50% of cases and attenuates chicken pox in most of the others and therefore reduces the risk of CVS. However VZIG is not 100% effective at preventing either maternal chicken pox or CVS and severe infection can still occur.
I am often asked why we don’t just give pregnant patients VZIG if they are exposed to chicken pox and my answers are:
- 90% of adults with no history of chicken pox actually have detectable VZIgG and are therefore immune to chicken pox and do not need VZIG
- VZIG is a blood product collected from multiple blood donations and therefore there is a small risk of blood borne virus exposure to the recipient
- The supply of VZIG is variable and there can be shortages therefore its use should be restricted to those who are definitely not immune
- VZIG is expensive; the cost of an adult dose of 1000mg VZIG is £1400 (BNF)
Your local microbiology service will be able to advise how to obtain VZIG.
What about VZIG for new born babies exposed to chicken pox?
If a normal birth-weight term baby is exposed to chicken pox because their mother develops chicken pox from 7 days before to 7 days after delivery they should be given 250mg IM of VZIG. The same dosage applies if a baby born to a non-immune mother is significantly exposed to chicken pox from another source (e.g. friend’s child) within 7 days of delivery.
For premature or low birth weight baby’s specialist advice should be sort from a Microbiologist, Virologist, Infectious Diseases Physician or a Neonatologist as each individual situation can be very complicated.
What should be done if the pregnant patient does develop chicken pox?
If even after the patient has been tested and given VZIG up to 50% may still develop chicken pox. Any non-immune pregnant patient who has been exposed to chicken pox should be advised to call their GP on the day they develop the rash and they should be started on PO Aciclovir 800mg 5 times a day for 7 days. If you diagnose chicken pox and the patient is unwell or develops respiratory symptoms or signs they should be admitted to hospital urgently for IV Aciclovir 10mg/kg TDS and supportive care as appropriate.
Any baby who develops chicken pox within 2 weeks of birth should also be admitted for supportive care and started on IV Aciclovir 10mg/kg TDS.
Please remember to isolate any patient with chicken pox who is admitted to hospital… otherwise you will be managing many more exposures and infections...especially if you admit them to a maternity ward...yep it’s been done before!
So our 20 week pregnant patient had blood taken for VZIgG which was sent to the laboratory labelled “chicken pox exposure at 20 weeks of pregnancy, no history of chicken pox” (great clinical information!) and fortunately she was shown to be VZIgG positive and could therefore be reassured that she was immune and nothing further needed to be done… of course she could go on to develop shingles in the future but that’s another painful story…see the last blog!
There are a couple of good resources for the management of varicella in pregnancy in the UK:
The Department of Health Green Book Chapter 34
The Royal College of Obstetricians and Gynaecologists Green-top guideline No. 13