"Until we know more, if your male sexual partner has traveled to or lives in an area with active Zika virus transmission, you should abstain from sex or use condoms the right way every time you have vaginal, anal, and oral sex for the duration of the pregnancy," the updated guidance says.
The update in recommendations comes one day after Dallas County, Texas, health officials
, announced a case of the virus involving a patient who had sex with someone who had recently returned from Venezuela infected with the mosquito-borne virus. The CDC confirmed this as the first known case of the virus being locally acquired in the continental United States in the current outbreak.
In a statement to CNN, the CDC said it confirmed the test results showing Zika present in the blood of a "nontraveler in the continental United States." The agency stressed that there was no risk to a developing fetus in this instance.
On Tuesday, CDC Director Dr. Tom Frieden told CNN Chief Medical Correspondent Dr. Sanjay Gupta: "There have been isolated cases of spread through blood transfusion or sexual contact and that's not very surprising. The virus is in the blood for about a week. How long it would remain in the semen is something that needs to be studied and we're working on that now."
Frieden said that studies on sexual transmission are not easy studies to do, but the CDC is continuing to explore that avenue of transmission. "What we know is the vast majority of spread is going to be from mosquitoes," Frieden said. "The bottom line is mosquitoes are the real culprit here."
The CDC said it will provide more guidance as more information on sexual transmission is learned, but in the meantime, "Sexual partners can protect each other by using condoms to prevent spreading sexually transmitted infections. People who have Zika virus infection can protect others by preventing additional mosquito bites."
Spokesman Gregory Hartl said the World Health Organization was aware of the Texas case but said, "we understand the case will raise concern but we really need to know a lot more not just about purported sexual transmission, but about any other kinds of transmission other than vector transmission."
History of sexual transmission
Before this case, there have been only two documented cases linking Zika to sex. During the 2013 Zika outbreak in French Polynesia, semen and urine samples
from a 44-year-old Tahitian man tested positive for Zika even when blood samples did not. Five years before that
, in 2008, a Colorado microbiologist named Brian Foy contracted Zika after travel to Senegal; his wife came down with the disease a few days later even though she had not left northern Colorado and was not exposed to any mosquitoes carrying the virus.
In addition, the CDC said there have been documented cases of virus transmission during labor, blood transfusion and laboratory exposure
. While Zika has been found in breast milk, it's not yet confirmed it can be passed to a baby through nursing.
An emergency of international concern
Zika is prompting worldwide concern
because of an alarming connection to a neurological birth disorder and the rapid spread of the virus across the globe.
The Zika virus, transmitted by the aggressive Aedes aegypti mosquito, has now spread to at least 29 countries. The WHO estimates 3 million to 4 million people across the Americas will be infected with the virus in the next year. The Centers for Disease Control and Prevention is warning pregnant women against travel to those areas; health officials in several of those countries are telling female citizens to avoid becoming pregnant, in some cases for up to two years.
The virus is linked to an alarming spike in babies born with abnormally small heads -- a condition called microcephaly -- in Brazil and French Polynesia.
Reports of a serious neurological condition, called Guillain-Barre Syndrome, that can lead to paralysis, have also risen in areas where the virus has been reported. Health officials have specifically seen clusters of this in El Salvador, Brazil and French Polynesia, according to WHO's Dr. Bruce Aylward.
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. You can find all of our Zika coverage on CNN.com/Zika.