The number of babies born with microcephaly in Colombia during a Zika outbreak this year more than quadrupled from a year ago, dispelling earlier suggestions that the nation with the second-largest number of infections had somehow escaped the dreaded wave of fetal deformities witnessed in Brazil, the epicenter of the outbreak.

In a report released Friday that provides the most detailed information about microcephaly prevalence to date in Colombia, health officials in that country and the Centers for Disease Control and Prevention said 476 cases of microcephaly were identified between February and mid-November, compared with 110 cases reported during the same period in 2015. Of those, 432 were in live births and 44 were stillbirths and pregnancy losses, including abortions and miscarriages.

The Zika outbreak began in Brazil in May 2015; Colombia confirmed local transmission of the Zika virus about five months later, in October 2015.

But for months, some health officials and experts noted that the number of Colombia’s microcephaly cases has been relatively small despite the 105,000 suspected cases of Zika virus disease that have been reported, including in nearly 20,000 pregnant women. Microcephaly is a severe and rare condition characterized by abnormally small heads and often underdeveloped brains.

Other experts have cautioned against making premature predictions about microcephaly cases in Colombia. If a pregnant woman gets infected early in pregnancy, when the risks for microcephaly are greatest, it can take up to eight months for serious birth defects to be evident.

Data collection in Colombia is also labor-intensive; information is collected at the local level before being transmitted to state and national offices.

The findings in Friday’s report provide the first month-by-month analysis of reported cases of microcephaly in Colombia. The data show the peak month for reported cases of microcephaly occurred in July of this year, about 24 weeks, or six months after the peak of the Zika outbreak. In July, there were 94 reported cases of microcephaly, about nine times as many as took place in July 2015. The report confirms earlier findings that Zika poses the greatest risk for fetuses during the first and early second trimesters.

“The evidence was compelling,” said Margaret Honein, chief of the birth defects branch at the CDC and one of the authors of Friday’s report. She added: “We expect that every country with a major Zika outbreak will see an increase in microcephaly and severe brain abnormalities.”

Even with mosquito season winding down in the United States, officials said pregnant women and their partners need to remain vigilant.

In Florida, which had the first local spread of Zika in the summer, Gov. Rick Scott declared an end to the Zika transmission zone in Miami’s South Beach area on Friday. But in Texas, state health officials said Friday they have identified four additional cases of suspected locally transmitted Zika virus in Cameron County, on the border with Mexico.

The Texas cases were identified as part of the follow-up to the state’s first case of likely local Zika spread, announced Nov. 28. The additional patients live close to the first patient, a Brownsville resident. They reported getting sick with Zika-like symptoms between Nov. 29 and Dec. 1 and were likely infected several days earlier, before mosquito control efforts intensified in that part of Brownsville. None is a pregnant woman.

As the winter tourism season gets underway in other warm-weather destinations, Honein said it would be dangerous to downplay the risk of Zika-related microcephaly across many of the countries in the Caribbean, and South and Central America where the virus has spread. According to the latest report from the World Health Organization, 29 countries or territories have recorded cases of babies with Zika-related birth defects. Nicaragua is the latest to report two cases of microcephaly.

Zika is primarily transmitted through the bite of an Aedes aegypti mosquito, but it can also be passed through sex from a person who has Zika to his or her sex partners, even if the person does not have symptoms.

One limitation of Friday's report is that the majority of microcephaly cases did not have laboratory confirmation of maternal Zika virus infection. Honein said it’s possible that blood and tissue specimens were not always collected for infants with microcephaly, or they might not have been submitted in time for accurate testing. Nevertheless, because the data were collected across Colombia, the increase in microcephaly prevalence from the previous year “indicates the magnitude of the increase,” the report said.

Differences between the Zika outbreaks in Brazil and Colombia could account for some of the difference in prevalence of microcephaly in the two countries. In Colombia, 50 to 75 percent of the population lives at altitudes about a mile above sea level, where Zika transmission by mosquitoes is unlikely, the report said. Microcephaly is also a difficult birth defect to register because of inconsistent definitions and terminology.

Researchers said the frightening accounts of birth defects and the link to Zika reported in Brazil might also have served as an early warning to Colombia. The Colombian Ministry of Health urged women in February to consider delaying pregnancy for six months, which might have affected subsequent birthrates. Abortion is legal in Colombia in cases of fetal abnormality, but it is not clear whether the abortion rate increased during the Zika outbreak.

Live births in Colombia dropped by about 18,000 between early February and mid-November compared with the same period from the previous year, the report said.

This post has been updated.

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