Zika-Related Birth Defects Increasing in United States

The results of a new CDC Morbidity and Mortality Weekly Report revealed that in US states and territories with local Zika virus transmission, the prevalence of Zika-related birth defects is increasing.

The United States is seeing an increase in birth defects linked with Zika virus infection as a result of the spread of the virus into the country, according to an article in the Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report (MMWR).

The Zika virus, which was once seen as a mild infection when it was discovered in 1947, has since become directly linked with devastating microcephaly in newborns and Guillain-Barré Syndrome (GBS) in adults. The initial wave of birth defects related to Zika was seen in Latin America, while the United States only saw incidents of travel-related cases of infection. However, since 2016, the country has seen an increase in sexually-transmitted cases and its first locally-acquired cases in southern Florida, south Texas, and Puerto Rico. According to the CDC, these areas, “saw a 21% increase in births with outcomes most strongly linked to Zika virus in the last half of 2016 compared with births in the first half of that year.”

It is important to note that investigators cannot state with certainty whether or not the increase is because of local transmission or other factors. They state that “most of the mothers who had babies with the Zika virus-linked birth defects did not have laboratory evidence of Zika virus infection—either because they were not tested, were not tested at the right time, or were not exposed to Zika virus.”

Furthermore, after analyzing approximately 1 million births in 15 US states and territories in 2016, investigators determined that 3 of every 1000 live births in these states in that year had a birth defect that could be associated with a Zika virus infection in the mother. These defects are broken down as follows:

  • 49% were born with brain abnormalities and/or microcephaly
  • 20% had neural tube defects and other early brain abnormalities
  • 9% had eye abnormalities without brain abnormalities
  • 22% had nervous system damage, including joint problems and deafness, without brain or eye abnormalities

The CDC acknowledged that because many of the pregnant women who were exposed to the virus in 2016 gave birth in 2017, another increase in birth defects could be seen when those data are analyzed. Moreover, because birth defects surveillance data are not final until approximately 24 months after the end of the birth year, the final 2016 numbers could also change.

Readers may note that the prevalence of birth defects seen in the 2016 data is similar to that seen in the 2013-2014 data, which was a baseline prevalence of Zika-related birth defects reported in every 2.9 out of 1000 live births. According to the MMWR, “The findings presented here included data from an additional 12 jurisdictions, which covers a larger birth cohort totaling nearly 1 million live births, representing approximately one-fourth of the total live births in the US states and territories.”

Still, the authors do note some limitations to their report. The 3 jurisdictions with local Zika virus transmission (southern Florida, south Texas, and Puerto Rico) differed in terms of the “scope and timing of identified local transmission of Zika virus,” according to the report. Puerto Rico’s widespread outbreak began in early 2016, while Texas did not have confirmed local transmissions until the latter end of 2016. A high prevalence of travel-acquired cases of infection may have also contributed to “the observed increased prevalence of birth defects.”

Additionally, an increased awareness of Zika-related birth defects might have led to the increased identification of them; however, the authors note that this is unlikely as a significant increase in early brain malformations was not observed. Finally, the authors acknowledge that “jurisdictions in this analysis might differ in population demographics and systematic case-finding methodology, contributing to differences in observed prevalences.”

When speaking about the overall ramifications of these findings, CDC Director Brenda Fitzgerald, MD, is quoted in the press release as saying, “Babies with Zika-related birth defects need all the help they can get, as soon as possible and for as long as they need it. This report highlights the critical importance of documenting birth defects possibly related to Zika and our need to maintain vigilance.”

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