Microcephaly Risk Highest With Early-Pregnancy Zika Infection

December 09, 2016

A preliminary report on the Zika outbreak in Colombia buttresses the long-held belief that infection in the first months of a woman's pregnancy poses the highest risk of causing microcephaly in her offspring, the Centers for Disease Control and Prevention (CDC) announced today.

In an article published in the agency's Morbidity and Mortality Weekly Report (MMWR), CDC scientists and their counterparts in Colombia reported 476 microcephaly cases in the South American country from January 31 through mid-November this year, more than a fourfold increase compared with the same period in 2015. Of these 476 cases, 432 were live-born infants and 44 were lost pregnancies. Likewise, the prevalence of microcephaly more than quadrupled, going from 2.1 per 10,000 live births to 9.6.

On a monthly basis, the number of cases (94) and prevalence (17.7 cases per 10,000 live births) of microcephaly in 2016 peaked in July. This high-water mark for infants born with abnormally small heads and associated brain defects came roughly 24 weeks after the peak period for Zika infections in Colombia back in January. Study authors view that finding as evidence that an infected pregnant woman is most likely to give birth to a child with microcephaly if the infection occurred in the first trimester or early in the second trimester.

The authors noted, however, that they based their case count of microcephaly on an assessment of fetuses and newborns at delivery. Left uncounted were infants whose microcephaly developed several months after birth. A study by Brazilian researchers in MMWR last month reported that seemingly normal infants with a congenital Zika infection may experience slow postnatal head growth that can morph into microcephaly.

In a news release, the CDC said that the MMWR report "demonstrates that an increase in microcephaly is not specific to Brazil," the epicenter of the Zika outbreak. Rather, any country experiencing Zika infections is likely to see large increases in microcephaly and other birth defects.

The United States does not have to look far to see confirmation of the CDC's claim. On December 7, the New York City health department reported four more cases of congenital Zika virus syndrome, which includes microcephaly, other brain anomalies, ocular defects, congenital contractures such as clubfoot, and neurological impairments such as motor disabilities. The four new cases of the syndrome bring the total number to five this year, according to the New York City Department of Health and Mental Hygiene.

As of December 2, 962 New Yorkers, including 325 pregnant women, have tested positive for the Zika virus. More than 200 infants in New York City have been born to women with Zika infections since January.

No More Active Transmission in South Miami Beach

In other Zika news, the CDC announced today that it has determined that mosquitos are no longer actively spreading the virus in a 1.5 square-mile area of South Miami Beach in Florida. "There have been no new cases of local Zika virus transmission identified in South Miami Beach for more than 45 days, suggesting that the risk of Zika virus infection is no longer greater than in the rest of Miami-Dade County," the agency said in a news release.

In August, designation of the area as a Zika transmission zone resulted in the CDC advising pregnant women to avoid traveling there. The CDC now has downgraded it to a Zika cautionary area, like the rest of Miami-Dade County. The agency's travel advice to pregnant women is downgraded as well — consider postponing a trip to the county.

The South Miami Beach area is the last of four locales in Miami-Dade County that the CDC no longer considers beset by mosquito-borne transmission of the Zika virus. One area, North Miami Beach, was at one time lumped together with its southern neighbor.

Meanwhile, another Zika zone may have emerged in Texas. On November 28, the Texas Department of State Health Services (DSHS) announced the first case of a person infected with the Zika virus likely spread by a bite from a local mosquito. The case was discovered in Brownsville in Cameron County, which borders Mexico. Mosquitos are still spreading the virus in that country.

Today, DSHS said it has identified four more people in Brownsville with the virus who were probably bitten by an infected mosquito. These patients, none of whom are pregnant women, live near the first patient, the agency said in a news release. They were probably infected several days before Texas authorities stepped up mosquito control efforts in response to the first reported case.

Testing in the eight-block area around the homes of the infected patients has yet to turn up more evidence of Zika transmission, according to DSHS.

MMWR. Published online December 9, 2016. Full text

Follow Robert Lowes on Twitter @LowesRobert

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