This is the second of a three-part series on Zika’s effects during pregnancy.
In the first part, I introduced ‘congenital Zika syndrome’ and described how Zika is responsible for a lot more than just microcephaly.
In this post I will explain why we may be under-estimating the full extent of the problem.
Zika infection in pregnancy causes birth defects – but what’s the level of the risk for each pregnancy?
It’s now well established that Zika causes birth defects, but for any given woman with a Zika infection during pregnancy, what are the chances that her baby will have a birth defect?
The first study to address this came from the outbreak in French Polynesia, which occured slightly earlier than the outbreak in the rest of Latin America. Data from French Polynesia suggested that if a mother gets a Zika virus infection in the first 12 weeks of her pregnancy, the risk of her baby being born with microcephaly was around 1/100. However, this estimate was based on very low numbers – only 8 microcephaly cases occurred in total – and so it may not be that accurate.
Mathematical models based on the number of microcephaly cases reported and the incidence of Zika virus infection suggest the level of risk to be between 1 and 4.5%. The variation in the estimate depends on what you assume to be the background rate of infection across the population and whether you assume that the number of microcephaly cases has been over-reported.
If mothers are actually symptomatic with Zika infection during pregnancy, the likelihood of fetal malformations seems to be higher, at around 42%. Remember that 80% of Zika infections are asymptomatic, so those mothers who get symptoms of Zika during pregnancy (e.g. a rash, fever or conjunctivitis) probably have a particularly high ‘viral load’ in their blood.
The best data yet comes from the USA’s Centre for Disease Control and Prevention (CDC), which estimates that around 5% of women affected by Zika virus during pregnancy will have a baby with a brain defect. The highest risk is if the woman has the Zika virus during the first 12 weeks of her pregnancy at around 13-15%. The risk drops to around 5% if the viral illness was in the second trimester of pregnancy and 4% for the third trimester.
However, the CDC’s data has probably under-estimated the number of babies born with problems because the gold-standard test for detecting brain defects in infants (cranial ultrasound) was only used in around a quarter of cases.
However, it is likely that ALL these studies so far have under-estimated the full effects of the Zika virus
There are two reasons that the above studies have most likely under-estimated the number of babies affected by Zika virus:
1) Microcephaly may not be evident at birth, but develop later on
More than one study has now shown that some infants may be born with a normal-sized brain and a normal-sized head, but then develop microcephaly later on, because the brain stops growing after birth (as a result of damage from the virus).
2) Babies may be born without microcephaly but develop other, more subtle, problems later on
As I mentioned briefly in a previous blog post, Zika is responsible for more than just microcephaly. It is likely that many of the babies in the above studies who were born without microcephaly will have other, more subtle, brain problems that will only become apparent as these children become older.
Indeed, this sort of thing is already being seen in Brazil. Children born to mothers who had Zika during pregnancy and did not have microcephaly or any other obvious problems at the time of birth are now returning to clinics with milder problems such as learning difficulties and epilepsy.
Update: A very recent study shows that when pregnant monkeys are infected with Zika, the virus gets through to various different tissues of the developing fetus and also causes inflammation in the placenta, the crucial interface between the mother and her developing baby.
Importantly, these effects were observed in 100% of the pregnancies, even though only a small number of the offspring had microcephaly. This would suggest potential for microcephaly to have wide-ranging impacts on the developing child, beyond the microcephaly and brain-damage which has thus far been the focus of most scientific research.
To round-off this three-part series on congenital Zika syndrome I will be talking about the future: the future for research on congenital Zika syndrome, whether we can expect more children to be born with the syndrome and what we can expect for the children already born with the syndrome.
Related blog posts:
- An introduction to the Zika virus
- The ‘story’ of the Zika virus outbreak: where it came from, where it is now and where it appears to be going next
- An introduction to congenital Zika syndrome
- Zika and its neurological complications: Guillain-Barré syndrome