Federal health officials are investigating 14 reports of possible sexual transmission of the Zika virus in the United States, including several involving pregnant women, evidence that is forcing physicians to treat the fast-moving epidemic as more than a mosquito-control problem.

The Centers for Disease Control and Prevention said Friday that two cases have been laboratory confirmed and four more are considered probable among women whose only known exposure was sex with symptomatic male partners who had recently traveled to areas where Zika is spreading. Two reports have been excluded, while six remain under investigation.

The findings do not show that sexual transmission is as common as infection through the bite of an Aedes mosquito. They do demonstrate that travelers can carry risk home after leaving an outbreak region, and that the consequences are most serious when a partner is pregnant because researchers are investigating a possible association between infection and severe fetal brain abnormalities.

Fourteen reports change the clinical conversation

The CDC received the suspected sexual-transmission reports between Feb. 6 and Feb. 22. Its case review describes six confirmed or probable infections in women who had not traveled to active transmission areas and whose male partners had experienced Zika-like illness after returning from abroad.

The pattern is consistent across the cases reported in detail: the men traveled, developed symptoms and had sexual contact without condoms with partners who later became ill. Laboratory confirmation remains difficult because Zika virus is detectable in blood for a limited period and antibody tests can cross-react with related flaviviruses such as dengue.

A CDC health advisory urged clinicians to ask patients with compatible symptoms about both travel and sexual exposure. Health departments were asked to coordinate testing and report suspected sexually acquired infections promptly so investigators can determine how often this route occurs and how long risk persists.

Most Zika infections are believed to cause no symptoms. When illness does occur, it is commonly mild, with fever, rash, joint pain or red eyes lasting several days to a week. That generally reassuring clinical profile contrasts with concern about pregnancy, where the possibility of fetal harm makes even an uncommon transmission route important.

Guidance focuses on pregnancy and barrier protection

The CDC’s initial sexual-transmission guidance, issued Feb. 5 after a confirmed case in Dallas, advises men who live in or travel to Zika-affected areas and have a pregnant partner to abstain from sex or use condoms correctly and consistently for the duration of the pregnancy. The recommendation covers vaginal, anal and oral sex.

For couples who are not pregnant, the agency says men who have been exposed through travel may consider abstinence or condom use. It has not set a definitive duration because scientists do not yet know how long infectious virus can remain in semen. The available reports involve symptomatic men; whether men without symptoms can transmit the virus is unknown.

There have been no documented cases of women transmitting Zika sexually to their partners. Officials caution that absence of reports does not establish impossibility, particularly during an outbreak in which surveillance and testing are still developing.

The recommendations are deliberately more stringent during pregnancy because the potential outcome is so severe. Brazil has reported a sharp rise in infants born with microcephaly, a condition marked by an unusually small head and impaired brain development. Investigators have detected Zika virus in fetal tissue and amniotic fluid in some cases, but health authorities have not yet established the size of the risk or all conditions required for fetal injury.

A mosquito epidemic with additional pathways

Zika remains primarily a vector-borne disease. Aedes aegypti mosquitoes, which also spread dengue and chikungunya, thrive near homes, breed in small collections of standing water and bite during the day. Because the species is established in parts of the southern United States, imported infections raise concern that mosquitoes could acquire the virus from travelers and begin local cycles of transmission.

The World Health Organization declared the clusters of microcephaly and other neurological disorders a public-health emergency on Feb. 1, calling for coordinated surveillance, research, mosquito control and faster development of diagnostics and vaccines. The declaration applies to the suspected neurological consequences rather than proof that Zika alone causes them.

Sexual transmission complicates prevention because mosquito avoidance ends when a traveler leaves the affected location, while virus may persist in semen. A person can therefore return to a place without active mosquito transmission and expose a partner who never traveled. That possibility expands the group clinicians must counsel and makes travel history relevant to reproductive care.

A contemporaneous report said several of the 14 investigations involved pregnant women, increasing pressure on health agencies to communicate uncertainty without implying that every exposed pregnancy will be harmed. Officials continue to emphasize that evidence is evolving and that the number of reports does not represent a population-wide transmission rate.

Blood safety and testing add another layer

Sex is not the only non-mosquito pathway under review. Zika virus has been detected in blood, and transfusion-associated infections have been reported outside the continental United States. Because roughly four of five infected people may have no symptoms, questionnaires based only on illness cannot identify every potentially infectious donor.

The Food and Drug Administration issued recommendations this month that people who have traveled to active transmission areas, experienced compatible symptoms or had sexual contact with an exposed person defer blood donation for four weeks. In areas with active transmission, blood centers should obtain components from unaffected regions or use approved pathogen-reduction methods where possible. The agency said the measures were intended for immediate implementation, according to its guidance summary.

Diagnostic capacity remains a bottleneck. Molecular tests can find viral RNA during the brief period when virus circulates in blood, while antibody tests remain positive longer but may be difficult to interpret in people previously infected with dengue or vaccinated against yellow fever. Testing semen to predict sexual infectiousness is not routinely recommended because the relationship between a positive result and transmission risk is not established.

The University of Minnesota’s Center for Infectious Disease Research and Policy noted in its clinical assessment that Friday’s report narrowed the 14 suspected events into two confirmed, four probable, two excluded and six still being investigated. That breakdown is a reminder that surveillance alerts are designed to capture possible cases broadly before laboratories and epidemiologists sort them.

For clinicians, the practical message is already clear even while the science is incomplete. A patient’s relevant exposure now includes the travel of a sexual partner, not merely her own itinerary or mosquito bites. For public-health agencies, the reports create a dual task: continue aggressive mosquito control while building sexual-health counseling, pregnancy surveillance, laboratory capacity and blood-safety measures into the same response.

Zika’s dominant route remains the mosquito. But the six confirmed or probable U.S. events show that prevention cannot stop at insect repellent and window screens. Until researchers determine how long the virus remains transmissible and how often sexual spread occurs, the safest policy is to treat recent travel as a shared exposure within a couple—especially when a pregnancy is involved.