The outbreak of the Zika virus across Central and South America, a pandemic that has been linked to birth defects and has triggered warnings in some countries against getting pregnant, has taken a new and more complicated turn.
Public health officials had earlier said certain mosquitos transmit the virus, which can cause fever, rash, joint pain and conjunctivitis. But after local officials in Dallas reported Tuesday that a patient there had contracted Zika — possibly through sexual contact with someone who had traveled to Venezuela, and not from a mosquito bite — the U.S. Centers for Disease Control and Prevention (CDC) said it would reissue guidance within the next few days on how to avoid transmission.
The CDC confirmed that the Texas case was the first in the continental United States to involve an infection contracted by someone who hadn’t traveled abroad. Officials are investigating ways in which the virus may be transmitted through blood or semen, and are warning men and pregnant women who have traveled to affected areas to use condoms when having sex.
“Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others,” Zachary Thompson, director of Dallas County Health and Human Services, said in a news release. “Next to abstinence, condoms are the best prevention method against any sexually transmitted infections.”
The Dallas patient, who has remained anonymous, does not appear to be the first person to have caught Zika virus through sexual transmission.
During a 2013 outbreak of the virus in French Polynesia, the CDC identified a man in Tahiti (PDF) who had detectable levels of Zika virus in his semen and urine but not his blood, signifying sexual transmission.
In another incident, Brian Foy, a microbiology and immunology professor at Colorado State University, contracted Zika in 2008 while conducting field research on malaria in Senegal, as did a fellow scientist. Foy reported that they’d both been bitten by Aedes albopictus mosquitoes, which are known to carry Zika.
Within a week of returning to Colorado, both scientists became ill with extreme fatigue, headache and a rash. Foy’s wife — who had never been to Africa and hadn't left the U.S. in the previous year — developed similar symptoms days later. A colleague tested their blood and confirmed that all three had Zika, Foy and his fellow scientists wrote in a research paper published in the journal Emerging Infections Diseases in 2011.
Though neither scientist had his sperm tested, “circumstantial evidence suggests direct person-to-person, possibly sexual, transmission of the virus,” the authors wrote.
“It is reasonable to suspect that infected semen may have passed from [Foy] to [Foy’s wife]" during sex, the authors wrote. “Another possibility is that direct contact and exchange of other bodily fluids, such as saliva, could have resulted in [Zika] transmission.” None of the couple’s four children developed the same symptoms.
Public health experts say the possibility of sexual transmission of Zika calls for even more caution among pregnant women, who should protect themselves from mosquitoes and be careful about having sex with partners who may have been exposed to the virus.
“Because of rapidly changing knowledge, advice about protecting against Zika may well change over time as more is learned,” Dr. Jesse Goodman, a professor of medicine and infectious diseases at Georgetown University, said in a statement. The Zika case in Dallas, he added, “emphasizes the importance of better understanding the spread and transmission of Zika, and its potential health risks. It also reinforces the critical importance of working in collaboration globally both to control the mosquitos that transmit the disease and, longer term, to develop a safe and effective vaccine.”
The American Red Cross on Wednesday said it would ask blood donors who have traveled to Mexico, the Caribbean or Central or South America within the last month not to donate blood as health officials continue to monitor the spread of Zika. It added the caveat that “the risk of contracting Zika by blood transfusion in the continental U.S. is believed to be extremely low due to the absence of local mosquito transmission.”