Pregnant women with a male sexual partner who has traveled to, or lives in, an area affected by active Zika virus transmission should refrain from sex or use condoms during sex until the pregnancy is over, the U.S. Centers for Disease Control and Prevention advised on Thursday.
The CDC said the precaution is in place "until we know more" about the dangers of sexual transmission of the mosquito-borne virus, which is linked to thousands of cases of microcephaly in newborns in Brazil.
Speaking at a Friday morning news conference, CDC director Dr. Tom Frieden also said the agency is investigating Brazilian research that detected Zika virus in patients' saliva and urine. At this point, however, the CDC's guidance to pregnant women does not include anything about kissing, he said.
"We're not aware of any prior mosquito-borne disease associated with such a potentially devastating birth outcome on a scale anything like appears to be occurring with Zika in Brazil," Frieden added.
"Because this phenomenon is so new, we are quite literally discovering more about it each and every day," he said.
"Because it's new and can be so severe, it can be scary, especially for women who are pregnant or considering pregnancy. There's no doubt that over the coming months many more travelers will return to the United States with Zika infection. Some of them will be pregnant women," Frieden said.
In the new advisory, the CDC added two new countries, Jamaica and Tonga, to its travel alert list of nations that pregnant women should avoid due to ongoing Zika virus transmission.
While the Zika epidemic first surfaced in Brazil last spring, Zika virus has since spread to 30 countries and territories in South and Central America and the Caribbean. Though a cause-and-effect link has not been proven, many public health experts fear the virus can cause microcephaly, a condition that causes babies to be born with permanent brain damage and very small heads.
On Tuesday, local health officials in Texas confirmed a case of Zika virus infection that was transmitted by sex, and not by the bite of a mosquito.
The Dallas County Health and Human Services Department said that an unidentified patient had become infected with the Zika virus after having sex with an individual who had returned from Venezuela, one of the Latin American countries where Zika is circulating.
Scientists have suspected that Zika could be transmitted sexually, and there have been scattered reports of similar occurrences in recent years.
If research proves that the virus can be spread through sex, it could complicate efforts to contain infections from the virus, which health officials have said is "spreading explosively" across South and Central America.
Ashley Thomas Martino is an assistant professor of pharmaceutical sciences at St. John's University, in New York City, who teaches infectious disease.
"We are dealing with an emerging strain of this virus. Zika is not new -- it has been around since the 1950s -- but this strain is showing that it can be transmitted from the mother to the developing fetus," he said. "So, the occurrence of sexual transmission may be new, but it's not that surprising given that we're dealing with a new strain of this virus."
Martino added that "most cases will be transmitted via mosquito, and this form of sexual transmission is likely to be a rare occurrence of infection."
The blood supply is also being monitored closely. The American Red Cross on Wednesday asked potential blood donors who have traveled to areas where Zika infection is active to wait 28 days before giving blood.
The chances of Zika-infected blood donations remain extremely low in the United States, Dr. Susan Stramer, vice president of scientific affairs at the American Red Cross, said in a statement
"The Red Cross continues to use safety measures to protect the blood supply from Zika and other mosquito-borne viruses," she said.
On Tuesday, the World Health Organization (WHO) declared the Zika virus a global health threat, based on the suspicion that the virus may be to blame for thousands of birth defects in Brazil in the past year.
Dr. Margaret Chan, director general of the WHO, said Monday that the explosive growth of microcephaly cases in Brazil constitutes an "extraordinary event and a public health threat to other parts of the world."
Chan made her remarks during an emergency meeting at the U.N. health agency's headquarters in Geneva, Switzerland, to assess what is known about the Zika virus and its potential relation to the surge of birth defects in Brazil.
The WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year. However, no recommendations were made Monday to restrict travel or trade, the Associated Press reported.
U.S. health officials have said it's unlikely that the Zika virus will cause a widespread threat here, but some infections are likely to occur.
The Zika virus was first identified in Uganda in 1947, and until last year was not thought to pose serious health risks. In fact, approximately 80 percent of people who become infected never experience symptoms.
But the increase of cases and birth defects in Brazil in the past year -- suspected to exceed more than 4,100 -- has prompted health officials to warn pregnant women or those thinking of becoming pregnant to take precautions or consider delaying pregnancy.
"It is important to understand, there are several measures pregnant women can take," Chan said, the AP reported. "If you can delay travel and it does not affect your other family commitments, it is something they can consider.
"If they need to travel, they can get advice from their physician and take personal protective measures, like wearing long sleeves and shirts and pants and use mosquito repellent," she said.
There have been no outbreaks of Zika virus in the United States so far. But, limited U.S. outbreaks are "possible" and "even likely" given that the same sort of aggressive, day-biting mosquito that spreads Zika is present in the southern United States, said Dr. Anne Schuchat, principal deputy director of the CDC.
However, Schuchat emphasized that the main health concern at this time is for pregnant women who are exposed to the virus.
Although health officials view some U.S. cases of Zika infection as likely, particularly in southern states, the United States enjoys certain advantages that should keep such an outbreak limited to a small area, Schuchat said.
Urban areas in the United States are less congested than they are in other countries of the Americas, making it more difficult for mosquitoes to spread disease hopping from one person to the next, she said.
Also, people in the United States are more likely to have their windows shut, thanks to air conditioning, or to have screens on open windows, which keep mosquitoes from invading their homes, she added.