CMV – a member of the Herpes Virus family – can cause infection in unborn babies. It affects 0.5 to 2 per cent of all pregnancies. Now that we give women information about avoiding listeria and toxoplasma and most pregnant women are immune to rubella (German measles) and varicella (chickenpox), CMV is the most common cause of congenital (acquired within the uterus) infection. It now is the main infectious cause of congenital hearing loss and potentially serious neurological disability although I emphasise it remains a very rare problem.
CMV is a very common infection amongst young children and it is almost ubiquitous in kids that go to day care. Children don’t usually show any symptoms from a CMV infection and most adults suffer a mild flu like illness so the diagnosis of CMV rests on blood tests.
The risk of harm to an unborn baby is greatest if a mother contracts CMV for the first time ( a primary infection) during the first trimester of pregnancy.
A word about CMV and the other members of the Herpes Virus family
If you contract most viral infections (rubella, measles, mumps etc), once you have done so you become immune for life. This is not so for the members of the Herpes family in that having been infected once (primary infection), you can become infected again either through a new exposure to the virus or reactivation of it (secondary infection). This is why they say that genital herpes lasts longer than true love. Primary infections tend to be more severe and carry greater risk to all concerned than secondary infections.
As with most Herpes viruses there is currently no effective vaccine against CMV.
Back to CMV and pregnancy
So as I said most little kids – especially those in day care – have encountered CMV and the problem is most of these shed the virus in their body fluids – saliva, urine and faeces. This is why our advice regarding avoiding CMV rests on avoiding children’s fluids.
If you acquire CMV for the first time (we call this a primary infection) around the first trimester of pregnancy there is a 30% chance the virus will cross the placenta and affect the baby. This risk is 2% if you have had CMV before (this is a secondary infection). Up to a third of babies affected during a primary infection will develop some consequences (hearing loss and/or neurological disability).
Affected babies can be identified by some changes on ultrasound but this is a very nonspecific test for congenital CMV. Blood tests performed on the mother are the most useful investigations for making a diagnosis of primary infection but, given the nonspecific nature of the symptoms of CMV it can be hard to know who to test. (The maternal blood tests are less useful / more confusing in women who have had CMV before (secondary infections) but fortunately in this group of women the risk of the baby being affected by CMV is much much lower – 2%).
If we know that an unborn baby is at risk of acquiring CMV from its infected mother treatment with one of the anti-herpes drugs Valtrex can reduce the likelihood of transmission to the baby from 30ish to 11 percent in primary infections.
Amniocentesis is the best way to confirm whether or not the baby has been infected.
Prevention is better than cure
NSW Health says:
The best way to reduce the risk of congenital CMV is for pregnant women to avoid exposure to CMV by:
- Regular hand washing after contact with young children, especially after changing nappies of being in contact with a child’s face, toys or dummies
- Not sharing food or drinks with young children
- Avoiding contact with saliva when kissing a child (good luck with that!) and
- Washing children’s toys and other surfaces that come into contact with their body fluids
Blood tests for everyone?
Routine testing for all pregnant women is currently not recommended by NSW Health (or other authorities). I don’t disagree with this approach but do perform CMV testing:
- If it’s me organising your pregnancy (or pre pregnancy) blood tests – this is on the grounds that it is at least useful to know if you have had CMV before (especially given the very great difference in transmission rates between primary and secondary infections).
- If you work with (e.g. childcare, paediatric services or teaching) or are regularly exposed to small kids – so this means that if you have a small child (and more than half of my patients do) I test you for CMV at around 13 weeks gestation.
- If there are any suspicious features on any of your ultrasound scans including simply being small for gestational age (SGA).
Notwithstanding all of the above congenital CMV is bloody rare – in Australia one baby in every thousand born will have CMV related permanent disability albeit of a varying degree. So it is possible to do a lot of tests and generate a great deal of anxiety on the basis of an extremely low statistical risk – but then again, risk is in the eye of the beholder…
NSW Health: Cytomegalovirus (CMV) and Pregnancy
Screening for cytomegalovirus in pregnancy (Editorial). Kilby MD et al, BMJ 2019; 367: 16507
– Rob Buist