Over 1.5 million people have been infected with the Zika virus in the past two years and more than 2,200 babies born with Zika-related microcephaly. Numbers like that call for a clear understanding of how Zika virus is spread from person to person - something that we are still not totally sure about.
We are sure that the main route of infection of Zika virus is mosquito bites - the runner up is sexual intercourse. It is known that Zika virus is present in urine, saliva, tears, and breast milk. Zika virus can stay in blood and saliva for about two weeks after infection, longer in breast milk (more weeks) and semen (months). Even though the duration of the virus in each of these different fluids may be known, the relative risk of transmission associated with each one remains unknown.
A group recently focused on one of these fluids - saliva. They published their findings in an article published on Aug. 1, 2017 in the journal Nature Communications that suggests that kissing is a very unlikely way to spread the virus.
Using monkeys (rhesus macaques) they transferred Zika virus through three different routes and looked for a productive infection. In the first group, Zika virus was applied directly to the tonsils of three macaque monkeys. Not surprisingly, all three were infected.
In the next group, saliva was taken from monkeys who had been infected with ZIka virus and swabbed onto the tonsils of five healthy monkeys. None of the five monkeys contracted Zika virus. Lastly, two monkeys had their nostrils or eyelids swabbed with saliva - neither one contracted the virus.
The results suggest that saliva exchange is unlikely to set up an infection which is great news. If Zika were shared through kissing, it would probably be a much bigger problem than it is.
The difference between transmission through saliva and other methods such as oral sex or breastfeeding may have to do with the saliva. Some experiments done with monkey saliva and cells showed that the saliva can make the virus less likely to infect cells.
It is possible that it is the consistency of saliva that is not conducive to infection. Or, the numbers of viruses present in the saliva. Or, the antimicrobial compounds found naturally in saliva. Although transmission of the virus through saliva is theoretically possible, it remains unlikely.
Oropharyngeal mucosal transmission of Zika virus in rhesus macaques. Newman CM et al. Nature Communications 8, 169 (2017) doi:10.1038/s41467-017-00246-8