Scientists increasingly believe that the Zika virus may cause birth defects in infants. Here’s why.
The World Health Organization said on Thursday that there is “strong scientific consensus” that Zika virus is a cause of microcephaly, unusually small heads with brain damage in infants, as well as other neurological disorders.
Yet a surge in microcephaly has been reported only in Brazil; a small increase was reported in French Polynesia, and a cluster of 32 cases is now under investigation in Colombia.
For proof of the connection between infection with the virus and birth defects, scientists are waiting for the results of a large study of 5,000 pregnant women, most of them in Colombia. Women with past Zika infections will be compared with similar women without infections to see if they have more microcephalic children.
The epidemic peaked in Colombia in early February, according to the W.H.O. Most of the women in the study are due to give birth in May and June.
Virtually all public health agencies already believe the virus is to blame for these birth defects and are giving medical advice based on that assumption. Here are the lines of evidence they cite.
1. Cases of microcephaly have surged in Brazil.
As early as last August, hospitals in northeast Brazil realized that something unheard of was happening: Neonatal wards that normally saw one or two microcephalic babies a year were seeing five or more at the same time. Doctors learned from the mothers that many of them had had Zika symptoms months earlier.
In October, Brazil declared a state of emergency and asked all doctors to report suspected cases. Eventually, nearly 7,000 were reported. It became clear that not all were real cases of microcephaly. But now that thousands of reports have been investigated, the numbers remain overwhelming.
According to the latest W.H.O. report, Brazil normally has an average of 163 cases of microcephaly each year, and only about 40 in the less-populated northeast. Since October, officials have confirmed 944 cases, 876 of them in the northeast.
2.On a smaller scale, it seems to have happened in French Polynesia, too.
French Polynesia’s population is only 270,000, compared with Brazil’s 200 million, but the island nation also had a surge in microcephaly during its Zika outbreak. That outbreak began in October of 2013 and ended seven months later, infecting an estimated 66 percent of the population.
French Polynesia normally has no more than two cases of microcephaly each year. In a four-month period between March and mid-July 2014, it had seven; an eighth case was later reported. Most fetuses identified as microcephalic were aborted.
A study in the Lancet concluded that first-trimester infections were the most dangerous. Based on data from French Polynesia, scientists have estimated that the risk of microcephaly among pregnant women infected in the first trimester is 95 per 10,000.
3. The virus crosses the placenta and can reach the fetus.
Zika virus has been found in amniotic fluid surrounding microcephalic babies, and autopsies have found it in the brains of fetuses that died in the womb. The virus was discovered in the brain of a microcephalic baby aborted at 32 weeks, six months after the mother recovered from the disease.
A study released March 29 described the fetus of a pregnant woman who appeared to have contracted the Zika virus on a trip to Guatemala. While the fetal brain appeared normal in three ultrasounds, it later stopped growing. After the woman aborted the pregnancy, Zika virus was found in her blood and in the fetal brain.
4. It also attacks brain cells.
Studies in cell cultures and in mice have shown that the virus is “neurotropic” — that is, it targets nerve cells, including the ones that eventually become the baby’s brain.
In a study in Cell Stem Cell, researchers exposed fetal stem cells to the virus and found that it particularly attacked cortical neural progenitor cells, which ultimately form the brain’s cortex, the region responsible for many higher functions.
A study in immune-deficient mice found that the virus entered brain cells and caused tremors; it killed very young mice but did not kill any older ones. That echoed the observations in humans: The infection causes grievous damage in newborns but not in adults.
Another study released March 29 found that the same receptor that the Zika virus uses to enter skin cells when causing rashes — a protein called AXL — lies on the surfaces of three types of fetal brain cells.
5. A small study found a big difference in birth defects.
A small study of 42 pregnant Brazilian women published March 4 found that 29 percent of those who had been infected with Zika virus experienced “grave outcomes,” while none of the uninfected women did.
Those outcomes included fetal death, tiny heads, shrunken placentas and fetal nerve damage that suggested blindness. The most shocking finding was that two babies that had been developing normally died suddenly when their mothers were infected late in pregnancy.
Health authorities still await the results of the much larger Colombian study, but they consider the evidence that has already piled up to be very persuasive.