Kissing Unlikely to Cause Zika Infection

A new study released in Nature Communications indicates that casual contact, such as kissing, is not a major threat in the transmission of Zika virus. According to the Centers for Disease Prevention and Controls (CDC) Zika virus is most likely to be transmitted through mosquito bites, sexual conduct, or from mother to child during pregnancy.

Even though Zika has been known by scientists since the 1950s, there is still a large gap in knowledge about the virus, including all of the ways in which it is transmitted. The virus can persist in bodily fluids following the acute infection period, and can survive in semen for months, resulting in sexual transmission of the virus. The virus is also detectable in breast milk for several weeks; however, there is no conclusive evidence that Zika virus can be passed through nursing. Until the advent of this study, researchers were unable to form a conclusion about whether or not saliva posed a serious threat as well.

For this study, researchers at the University of Wisconsin-Madison looked at rhesus macaque monkeys to learn more about the possible connection between saliva and Zika transmission. The study was funded by the National Institutes of Health with the purpose of determining alternative ways that Zika is transmitted person-to-person after an elderly man and his son contracted the virus in Utah.

Three monkeys at the Wisconsin National Primate Research Center were infected by researchers with a strain of Zika virus that has been found in North and South America. The researchers collected saliva samples from the infected monkeys and instigated casual contact by swabbing the tonsils of 5 uninfected monkeys. The researchers also swabbed the tonsils of 3 uninfected monkeys with highly concentrated doses of Zika virus in solution. Tonsils were selected as the area to be swabbed because infections like the flu can prevail in the tonsils.

The researchers found that the monkeys that received saliva swabs avoided the infection entirely; however, the 3 monkeys that were swabbed with high-dose virus were all infected with Zika. The amount of virus in the saliva was far less viral than the amount that mosquito bites transmit. The monkeys infected directly by the virus, as opposed to saliva containing the virus, were exposed to 80,000 times more Zika particles, suggesting that saliva may have an effect on the virus particles by preventing the virus from making contact with cells that it could infect.

"The viral loads in the saliva in general are low, but there are also anti-microbial components in saliva making that low level of virus even less infectious than it might be in another medium," said Christina Newman, co-first author of the study from University of Wisconsin Madison Zika Experimental Science Team, in a press release.

From the evidence, the scientists determined that kissing and other casual contact do not pose a serious threat in Zika transmission. It is possible for Zika to be transmitted through saliva, but it is highly unlikely because infected individuals are unable to produce the high number of particles necessary to infect another person.