Recent surveillance data from Colombia suggest that infection with Zika virus during the third trimester of pregnancy is not linked to obvious birth defects, according to a study published online June 15 in the New England Journal of Medicine.

The findings, however, are preliminary. Further monitoring of these children will be needed to assess long-term repercussions of maternal infection with Zika virus on offspring. The authors urge continued caution and vigilance.

"The ongoing outbreak of [Zika virus disease] in the Americas poses a major public health threat, particularly for fetuses of infected pregnant women," write Oscar Pacheco, MD, from the Instituto Nacional de Salud (INS), Bogota, Colombia, and colleagues.

Maternal infection with Zika virus has been linked to microcephaly, hearing loss, vision problems, and other serious brain abnormalities in offspring.

The most recent outbreak of Zika virus began in Brazil in May 2015, and official monitoring for Zika cases began in Colombia in August 2015. By October 2015, the outbreak had reached Colombia.

In the study, the researchers used Colombia's national surveillance system to analyze patients with clinical symptoms of Zika virus reported from August 9, 2015, to April 2, 2016. They also looked at cases confirmed with reverse-transcriptase polymerase chain reaction (RT-PCR).

Results showed widespread distribution of Zika virus cases across Colombia. By April 2, 2015, 65,726 cases of Zika virus had been reported, of which 2485 (4%) were confirmed with RT-PCR.

Pregnant women positive for Zika virus numbered 11,944, of which 1484 (12%) were confirmed on RT-PCR assay.

Among a subgroup of 1850 pregnant women who had data on gestational age at the time when symptoms began, 616 were infected during the third trimester, of whom 90% gave birth. Researchers have not yet identified any apparent abnormalities among infants born to these women.

Most of the women infected with Zika in the first or second trimester were still pregnant at the time of the study, so the full extent of the effects of Zika on their infants could not be reported.

However, 50 infants were born with microcephaly between January 2016 and April 2016, four of whom tested positive for congenital Zika virus infection. The mothers of all four children had not shown clinical symptoms of illness and would not have been included in Colombia's Zika virus surveillance system.

The authors point out that the number of cases reported in this study likely underrepresents the true extent of the problem, because the surveillance system does not capture asymptomatic infections or unreported illnesses.

They advised providers to educate patients on mosquito bite prevention, especially women who may become pregnant. Because Zika virus can be transmitted sexually, they also advised providers to educate pregnant women about condom use or abstinence.

The INS, together with the Centers for Disease Control and Prevention, is continuing to monitor for Zika cases and other illnesses with similar symptoms, such as dengue and chikungunya virus.

"The INS is implementing intensified surveillance of pregnant women with ZVD in selected sites to better understand the spectrum of adverse pregnancy and infant outcomes associated with Zika virus infection and to identify the effect of factors such as the timing of infection during pregnancy," the authors conclude.

The study was supported by the INS and the Centers for Disease Control and Prevention. The authors have disclosed no relevant financial relationships.

N Engl J Med. Published online June 15, 2016. Full text

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