The U.S. Centers for Disease Control and Prevention (CDC) says it is investigating 14 new reports of Zika virus infection in the United States, all suspected to be sexually transmitted.
The Atlanta-based CDC made the announcement Tuesday and said all of the 14 cases are women whose only known risk factor is sexual contact with a male partner who had recently traveled to an area with local Zika transmissions.
In its statement, the CDC said the new reports suggest that sexual transmission may be a more likely means of Zika transmission that previously thought.
Several of the cases involve pregnant women. Zika virus has been linked to a birth defect known as microcephaly, in which babies are born with abnormally small heads and can suffer developmental delays. Scientists have not yet proven a definite cause-and-effect.
CDC is recommending that men who have recently traveled to regions with local Zika transmissions use condoms or refrain from sexual contact with pregnant women or women who could become pregnant.
While sexual transmission of the virus is possible, health authorities note that the primary means of infection by the Zika virus is still from mosquito bites, and they urge people to prevent mosquito bites using mosquito repellent, window screens, and long-sleeved shirts.
On Monday, a study to determine whether the Zika virus is causing babies to be born with the birth defect microcephaly began in Brazil.
A 16-member team from the U.S. Centers for Disease Control and Prevention started work in Joao Pessoa, in northeastern Brazil, that is the epicenter of the country's Zika outbreak.
The World Health Organization (WHO) declared the Zika outbreak an international health emergency February 1.
The WHO cited a "strongly suspected" relationship between Zika infection in pregnancy and microcephaly, although much remains unknown about Zika.