Zika Virus and Sexual Transmission: New Studies Provide More Insight

Sexual transmission of the Zika virus remains a front-burner concern, even as mosquito season draws to a close in the United States.

Zika infections acquired via bites from “local” mosquitos—as has been documented in the Miami area—as well as bites sustained while traveling to regions where the virus is rampant (ie, Puerto Rico and/or Brazil) can both be passed on to others through sexual contact. Hence, researchers are actively engaged in efforts to better understand how the Zika virus is transmitted sexually, so that such transmission can be reduced or at least controlled.

Two recent studies have added significantly to the growing knowledge base. In one, published online on October 17 by the journal Emerging Infectious Diseases (EID), researchers from Baylor College of Medicine in Texas followed a “previously healthy, nonpregnant, 26-year-old, non-Hispanic white woman” who had travelled to Honduras in May 2016 and presented with Zika virus infection symptoms upon her return. The authors of the EID study longitudinally collected serum, whole blood, urine, saliva, and vaginal mucosa swabs for more than 11 weeks (her symptoms resolved within 17 days), starting on “day 0,” 2 hours after initial symptom onset, and continuing on days 3, 8, 14, 21, 28, 35, 42, 53, 64, and 81. They extracted RNA from the samples and tested it using quantitative reverse transcription quantitative PCR (qRT-PCR).

According to their report, the authors detected Zika virus RNA in serum and saliva samples until day 8 after symptom onset and in body fluids up to day 14; however, whole blood samples remained positive for Zika RNA until day 81, the longest duration of detection in this sample type reported to date. Also of note, vaginal swab specimens were positive for Zika virus until day 14.

“Given recent concerns regarding the ongoing epidemic of Zika virus disease, there is an urgent need to document the natural history of infection and assess transmission risk through nonvector routes,” the EID authors wrote. “With the recent finding of possible female-to-male virus transmission, infectious virus might be present in the vaginal canal and could serve as a risk for sexual or intrapartum transmission… [and w]ith concerns for Zika virus infection during pregnancy, screening of whole blood might be more sensitive in identifying infected patients, particularly if an asymptomatic patient has traveled from an area where exposure is a concern, had high-risk sexual contact, or is convalescing.”

Researchers in the United Kingdom, meanwhile, have mapped the first complete Zika virus genome derived from semen. Working with a sample obtained from a British male recovering from Zika contracted during travel to Guadeloupe (13 days after symptom onset), the scientists were able to isolate viral DNA. Their findings were published on October 13 by the journal Genome Announcements.

Using standard genetic analysis tools, as well as Zika virus PF13/251013-18 genome as a reference, the authors were able to map 2.4% of the genome in their patient sample to the reference, providing 99.3% genome coverage at a minimum depth of 5×. From this, they were able to produce a consensus genome sequence that “aligned well with other sequences of [the virus] from the outbreak.” They believe their findings will lead to better understanding of “possible viral polymorphisms resulting from infection in different cellular environments.”

“Isolating Zika virus from semen has been a major challenge and little information has been published on this specific topic, so the methods we adopted may provide a template for others to follow," study co-author Barry Atkinson, of Public Health England, said in a statement released by Genome Announcements in conjunction with publication of the study. “We have many unanswered questions about how Zika virus is able to be transmitted sexually… It is possible that the answers to these questions lie in the viral genome, but many more sequences from semen are required before scientists can see if there are any changes that shed light on this topic.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.