Discussion

In addition to the present case report, at least five other cases of sexually transmitted Zika virus infection supported by laboratory evidence have now been reported in the published literature; all were male-to-female transmissions involving vaginal sex. All of the male travelers had symptoms consistent with Zika virus infection and could have transmitted infections to their sex partners a few days before or after as well as during the time symptoms appeared (3–5). In this case report, patient B’s potential exposures occurred both before and just after initial appearance of symptoms in the traveler, which is the time when blood viremia appears to be highest (i.e., as clinical signs and symptoms of infection emerge).§

Transmission of Zika virus to patient B by Ae. aegypti or albopictus was unlikely based on environmental conditions. Even if these mosquito species had been present and active, the time from exposure to illness in patient B (i.e., 6–8 days) was shorter than the minimum estimated time required for Aedes to become infectious had a mosquito ingested a Zika virus-infected blood meal from patient A (i.e., Ae. aegypti extrinsic incubation period is a minimum estimated duration of 10 days) (6,7), and for patient B once infected to have then developed illness (i.e., 3–12 days).

Studies investigating seminal shedding of infection-competent Zika virus, including its incidence, pattern (e.g., intermittent shedding or a steady decay), and duration are ongoing. At the time of Patient B’s clinical presentation, there had been only one published report describing testing of semen from a man with Zika virus infection (8); studies of semen from two additional men have since been reported (9,10). Zika virus has been detected by RT-PCR and isolated in culture from the semen of two men at least 2 weeks after onset of illnesses (8,10) and possibly up to 10 weeks after illness in one of these cases (8). One report described Zika virus detectable in semen by RT-PCR 62 days after illness onset; culture was not performed (9). In two men, Zika virus was no longer detectable in their blood by RT-PCR when the semen specimens were analyzed (8,9). None of the three men provided follow-up semen specimens to determine when Zika virus was no longer detectable. Notably, all men in the five case reports and the three semen studies, as well as patient A, experienced symptomatic illness. In the report of the sexual transmission case that occurred in 2008 (1) and of the man with culturable Zika virus in semen in 2013 (8), symptoms also included hematospermia.

Identifying and characterizing cases of sexually transmitted Zika virus infection in areas experiencing intense autochthonous vector-borne Zika virus transmission is challenging. Reports of sexual transmission identified in areas where autochthonous transmission is not occurring offer unique and important opportunities to learn about this emerging mode of transmission and rapidly inform and refine interim prevention recommendations. Such cases highlight the need for clinicians to remain vigilant for and continue reporting any suspected cases of Zika virus infection to their state or local health departments, including suspected infections in symptomatic persons without travel history, but who report unprotected sexual contact with a person who has traveled to an area with active Zika virus transmission.