(CNN) A man who had no idea he had Zika has given the virus to his female partner during sex, the Centers for Disease Control announced today. It's the first documented case of a person with no symptoms sexually transmitting the virus to a partner who had not traveled to an area of active Zika circulation.
The announcement came the same day the Food and Drug Administration revised its guidance to recommend that all blood donations in the United States and its territories be tested for the Zika virus.
The asymptomatic Zika transmission "illustrates the need for careful precautions when visiting an area where Zika is circulating," said Dr. John Brooks, a medical epidemiologist with the CDC. "Be sure to wear insect repellent and appropriate clothing, and use CDC guidance on safe sex when you return. This is especially critical for women who are pregnant or trying to conceive and their partners.
"Pregnant couples need to defer unprotected sexual contact for the entire pregnancy, even if the exposed partner never develops symptoms of Zika. A few months of precautions can prevent devastating lifelong defects for the developing fetus."
Exposure to the Zika virus during the first trimester, including before a woman even realizes she is pregnant, has been linked to a devastating birth defect called microcephaly, in which the infant is born with a small head and underdeveloped brain. Infants with that disorder face overwhelming obstacles such as seizures, eye and hearing problems, failure to thrive and even death. Infants exposed later in pregnancy may escape microcephaly but can continue to have abnormal brain development and learning disabilities.
The historic case came to the attention of the Maryland Department of Health in June. A woman reported typical signs of a Zika infection 16 days after she had unprotected sex with her male partner, who had recently returned from the Dominican Republic, an area of active Zika transmission. The woman had not traveled, had no other sex partners for 14 days before onset of her symptoms and had received no blood products or organ transplants.
Her male partner had been bitten by mosquitoes during his travel but had no signs of Zika infection, which typically include rash, red eyes, fever and malaise, He told officials he had only felt tired, which he attributed to travel. Yet two serum tests confirmed the presence of Zika antibodies in his blood.
"This is not surprising," Brooks said. "Our recommendations had considered the transmission from a person who seems otherwise healthy, because four out of five people who are infected with Zika won't develop symptoms."
Though agreeing that this development could be worrisome to pregnant women, Brooks said it's "reassuring to us at the CDC that all other cases of sexual transmission, with the exception of a possible case in France, have been between people who eventually developed symptoms."
The French case involved a couple who had both traveled to Martinique, where Zika was active, and was discovered through a routine blood test a few months after their return.
"She didn't develop Zika until about 40 days after she got home," said Brooks, "so it's possible she was infected via sex, but it's also possible that it was a very delayed incubation from a mosquito bite."