Only a few months ago, sexual transmission of the Zika virus through infected semen was considered possible, but highly unlikely. Since then, we've seen a growing number of cases, and earlier this week the WHO summed up the risk by saying:
The primary transmission route of Zika virus is via the Aedes mosquito. However, mounting evidence has shown that sexual transmission of Zika virus is possible and more common than previously assumed.
Complicating matters is that we've seen both male-to-female and male-to-male sexual transmission, and parsing out exactly how the virus was exchanged between sexual partners in private encounters is exceedingly difficult.
Since the virus had been detected in male semen, that has been the prime suspect, but traces of Zika virus RNA were also detected in urine and saliva back in 2014 (see Detection of Zika virus in saliva).
What hadn't been demonstrated until earlier this year was the presence of live Zika virus in these bodily fluids (see FIOCRUZ: Statement On Detection Of Zika Virus In Saliva and Urine).
Since then, there has been speculation that saliva might be a plausible route for human-to-human transmission as well. The March ECDC Risk Assessment had this to say:
Risk of sexual transmission
Zika virus genome has also been detected in saliva during the acute phase of the disease . Data about the presence of viable virus, viral load or kinetics are lacking, and at this point in time the risk of transmission via saliva cannot be further assessed.
About the same time, in Eurosurveillance: Prolonged Shedding Of Zika Virus RNA In Saliva - Italy, the authors suggested their findings posed `questions on the potential risk of human-to-human transmission of the virus through saliva'.
Since then, we've revisited the subject a number of times (see here and here) and while intriguing, we've yet to see anything resembling hard evidence.
While far from conclusive, we've a correspondence to the NEJM - published late yesterday - which adds to the speculation that Zika might be transmissible via the oral route and possibly even through saliva.
Specifically, this letter chronicles the sexual transmission of the Zika virus between a man recently returned from Brazil and a Parisian woman during a time when the man was still shedding the virus.
They detail the couple's sexual activity - which included oral sex with ejaculation and vaginal sex without ejaculation - and conclude that the data supports `sexual transmission (either oral or vaginal) of ZIKV'.
They also stated they cannot `. . . rule out the possibility that transmission occurred not through semen but through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing.'
Despite some of the hyperbolic headlines this morning, not being able to `rule out' a possibility is a far cry from proving it.
Nonetheless, this will probably raise new questions over the type and scope of warnings that public health agencies may need to consider going forward.
I've only included a brief excerpt, so follow the link below to read the full correspondence.
Evidence of Sexual Transmission of Zika Virus
N Engl J Med 2016; 374:2195-2198DOI: 10.1056/NEJMc1604449
To the Editor:
These data support the hypothesis of sexual transmission (either oral or vaginal) of ZIKV from Patient 2 to Patient 1. We cannot rule out the possibility that transmission occurred not through semen but through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing.
The saliva of Patient 2 tested negative on day 10 after the onset of his symptoms, but it was not tested earlier. ZIKV has been detected in saliva,5 but, to our knowledge, no cases of transmission through saliva have been documented.
The current outbreaks of ZIKV infection should be an opportunity to conduct studies to understand the natural history of ZIKV. We need to better define recommendations to prevent transmission of the virus. In particular, guidelines regarding how long men who are returning from an area where active ZIKV transmission is occurring should continue to use condoms during sexual contact with pregnant women and those of child-bearing age are lacking. In addition, recommendations regarding the possibility of oral transmission of the virus through semen are needed.