Zika Resource Center
Zika Resource Center
January 12, 2018
3 min read

LA case a reminder about sexually transmitted Zika

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The first-ever case of a sexually transmitted Zika infection in Los Angeles county was a reminder that sexual transmission of the virus, although rarely reported, is possible.

County health officials said a woman contracted the virus from her partner after he returned from a trip to Mexico, where there remains a risk for Zika infection, according to the CDC.

“This case is a reminder to take precautions during sex or avoid sex if you or your partner [has] traveled to an area with risk of Zika,” Jeffrey Gunzenhauser, MD, MPH, interim health officer for the county, said in a statement.

Scientists once saw Zika as a benign virus but now know it is capable of causing serious fetal brain defects such as microcephaly in children whose mothers were infected, making prevention in women who are pregnant or attempting to become pregnant a top priority. The virus is mostly spread by the bite of infected mosquitoes, but it also can be sexually transmitted by both men and women — even in the absence of symptoms.

“In the U.S., the threat for sexually transmitted Zika is likely to be low,” Samuel V. Scarpino, PhD, assistant professor of marine and environmental science and physics in the Network Science Institute at Northeastern University, told Infectious Disease News.

But Scarpino and colleagues have raised several issues surrounding the risk of sustained sexually transmitted Zika. In a study published last year, they reported findings from an epidemiologic model that suggested the risk of sustained sexual transmission of Zika could be underestimated. According to Scarpino, blind spots in currently available U.S. surveillance data make it difficult to estimate the risk for sexual transmission. He said the people who are primarily being tested for Zika are individuals who are trying to have children and are in monogamous relationships — a group that is unlikely to contribute to sustained transmission.

“It doesn’t mean that they’re at low risk for sexual transmission. It means that they’re at low risk for sustaining sexual transmission,” Scarpino said. “The distinction there is important. The individual in Los Angeles is an example of a sexual transmission event that is maybe unlikely to sustain, meaning more sexual transmission resulting from that initial case. What we’re talking about is sustained transmission where you have ongoing sexual transmission moving through the population. We’re not really capturing surveillance data from the individuals who are most likely to be sustaining infections.”


Scarpino said data from the recent Zika epidemic centered in Latin America and a previous outbreak on Yap Island show that women are infected two or three times more often than men, a finding that has so far gone unexplained. He said most recent studies have attributed it to women being more likely to get tested than men because they are concerned about the risk for birth defects like microcephaly.

“However, we see the same sex bias in the Yap Island outbreak, and that was before anyone knew that there was a risk for birth defects,” Scarpino said. “We still see that women are infected much more frequently than men. This type of sex bias is really only something that you can get from testing bias or from sexual transmission.”

According to officials, there have now been 122 cases of Zika infection reported in Los Angeles county since 2015, and all but one were acquired from a mosquito bite while traveling to a country where the virus was circulating. Officials said vector control agencies routinely test Los Angeles county mosquitoes for Zika.

Scarpino and colleagues said their findings support calls to classify Zika virus as a sexually transmitted infection to raise awareness of the ongoing risk.

“There are far more questions than answers about Zika sexual transmission,” Scarpino said. “But the risk is very real, as we’ve seen from cases of sexual transmission occurring in the United States. And the risk of ongoing transmission is likely, according to the study that we did.” – by Gerard Gallagher


Allard A, et al. PLoS Pathog. 2017;doi:10.1371/journal.ppat.1006633.

Disclosures: Gunzenhauser is the interim health officer for the County of Los Angeles. Scarpino reports no relevant financial disclosures.