By Julie Steenhuysen

CHICAGO (Reuters) – Adult women in Puerto Rico were significantly more likely to develop Zika than men, researchers said on Thursday, raising new questions about the potential role of sexual transmission of the virus from males to females.

The study, published in the U.S. Centers for Disease Control and Prevention’s weekly report on death and disease, evaluated more than 29,000 laboratory-confirmed cases of Zika since the outbreak began in Puerto Rico in November 2015.

The data show that of all Zika cases with laboratory evidence of infection, 62 percent were female. The results pattern similar observations from Brazil and El Salvador, the authors said.

One obvious explanation might be that pregnant women are more likely than men to seek treatment for Zika because of the potential risk of birth defects.

To account for that, the researchers excluded all pregnant women who tested positive for the virus. Of the remaining 28,219 non-pregnant women and men testing positive for Zika, 61 percent of these cases occurred in women over the age of 20.

The Zika findings differ from prior outbreaks in Puerto Rico of arboviruses transmitted by the same mosquitoes as Zika. For example, in the 2010 dengue outbreak and the 2014 chikungunya outbreak, infections were equally distributed among men and women.

“It is possible that male-to-female sexual transmission is a contributing factor to this skewing of the burden of disease toward women,” the CDC said in a statement summarizing the findings.

However, the contribution of sexual transmission to overall Zika rates is just beginning to be explored, the CDC said. It could be that women are more likely than men to seek care if they are sick, or that women are more likely to develop Zika symptoms if they become infected.

The CDC is conducting blood tests of individuals living near people with confirmed Zika to try to answer some of these questions.

Zika infections in pregnant women have been shown to cause microcephaly – a severe birth defect in which the head and brain are undersized – as well as other brain abnormalities. The connection between Zika and microcephaly first came to light last year in Brazil, which has since confirmed more than 2,000 cases of microcephaly.

(Reporting by Julie Steenhuysen; Editing by Andrew Hay)