The kinds of birth defects associated with Zika, including microcephaly and other brain abnormalities, have increased in parts of the United States where mosquitoes were spreading the virus in 2016, including South Florida, according to a new report from the federal Centers for Disease Control and Prevention.
Areas with local spread of Zika — including Florida, a portion of southern Texas, and Puerto Rico — saw a 21 percent spike in these kinds of birth defects during the second half of 2016 compared with births that took place during the first half of that year, CDC officials reported.
But researchers said they do not know if the increase is due to local spread of Zika or other factors because most mothers who delivered babies with birth defects associated with the virus did not have laboratory evidence of infection. Those mothers either were never tested, were not tested at the right time, or were not exposed to Zika, CDC officials said.
Researchers analyzed nearly 1 million births from 2016 in 15 states and territories as part of the report, which provided the first comprehensive data on the prevalence of birth defects potentially linked to Zika.
They identified 2,962 babies and fetuses with birth defects potentially related to Zika, including 1,456 with brain abnormalities or microcephaly — which causes abnormally small heads and incomplete brain development. There was no state breakdown.
An additional 581 had neural tube defects, 262 had eye abnormalities and 662 had some other form of central nervous system dysfunction.
Among the nearly 3,000 birth defects identified, though, a total of 2,821 cases either never received Zika testing or their results were not available for the report.
Because the increase in birth defects was detected during the second half of 2016, CDC researchers said more surveillance is necessary to capture births from 2017, when many pregnant mothers affected by Zika would have delivered their babies.
Zika is spread primarily by the bite of an infected mosquito, most commonly the Aedes aegypti species, but the virus can also be transmitted by sex and through blood transfusions.
Pregnant women are considered to be at the greatest risk from Zika because the virus has been shown to cause microcephaly and other neurological disorders in children born to mothers infected while pregnant.
In 2016, the CDC and Florida Department of Health identified Miami’s Wynwood neighborhood as the first area in the continental United States with local spread of Zika by mosquitoes, triggering a travel advisory that warned pregnant women to avoid travel to Miami. Local spread of the virus also cropped up in areas of Miami Beach and Miami’s Little River neighborhood.
That year, a total of 299 pregnant women in Florida were reported to have laboratory-confirmed Zika, and four babies were born with congenital Zika syndrome, according to the state’s surveillance data.
A total of 1,469 Zika infections, most acquired by people traveling outside the country, were reported in Florida in 2016.
Zika waned in 2017, with 249 cases statewide, including 127 pregnant women and three babies born with congenital Zika syndrome. Only two local cases of Zika were reported by Florida health officials in 2017.
So far in 2018, Florida health officials have reported no Zika cases.