The number of babies born in Puerto Rico with microcephaly and other birth defects caused by the Zika virus appears to be unexpectedly low – so low that experts are beginning to question whether the actual count is being significantly underreported by authorities on the island.
As Zika surged across the Americas last year, US health authorities warned that Puerto Rico was facing a perfect storm – and braced for a large number of pregnancies affected by the virus.
But, to date, Puerto Rico has reported only 16 cases of congenital defects associated with Zika, even though more than 3,300 pregnant women are known to have contracted the virus and several times that number are believed to have been infected.
By contrast, US states and the District of Columbia, where the threat posed by Zika was thought to be much lower overall, have registered congenital defects in 63 fetuses or newborns among 1,300 pregnant women who have contracted the Zika virus.
Some observers believe Puerto Rico, which is heavily dependent on tourism, is downplaying the scale of its Zika problem.
Last October, without fanfare, the Centers for Disease Control and Prevention stopped reporting the outcomes of pregnancies in US territories in which women had been infected with Zika. Without providing details, the agency simply said that Puerto Rico wasn’t counting cases the same way.
Last August researchers from the Puerto Rico health department and the CDC published a study predicting that Zika’s first wave of activity would strike a large number of pregnancies there, based on analyses factoring in the percentage of the population thought to be infected and the number of pregnant women on the island.
The study, published in the journal JAMA Pediatrics, projected that between 100 and 270 babies with Zika-induced microcephaly would be born between mid-2016 and the same time in 2017.
That study didn’t forecast figures for a host of other birth defects that Zika is known to cause but that are not always readily apparent shortly after birth, including destruction of brain tissue, damage to newborns’ optical nerves, and partial or total hearing loss.
A more recent study from the CDC mined the US Zika pregnancy registry to try to get a clearer picture of how often infections in pregnancy lead to birth defects in infants. According to that study, 5 percent of babies born to women with confirmed or suspected Zika infection during pregnancy had Zika-related birth defects; when the researchers only included women with confirmed Zika infection, the rate was 10 percent.
And when confirmed infection occurred in the first trimester of pregnancy – when the risk Zika poses to a developing fetal brain is highest – 15 percent babies born or fetuses lost had Zika-related defects.
It’s not possible to perform the same calculations for Puerto Rico. The health department’s weekly report does not indicate how many of the 3,356 pregnancies with confirmed Zika infections have been completed.