It might be harder to contract the Zika virus than previously thought.
Researchers from the University of Wisconsin-Madison have ruled that casual contact, including kissing or sharing an eating utensil is not enough contact for the virus to be passed between hosts.
“If passing the virus by casual contact were easy, I think we would see a lot more of what we would call secondary transmission in a place like the United States,” Tom Friedrich, a virology professor at the UW–Madison School of Veterinary Medicine, said in a statement. “But we’re not seeing clinically apparent spread of Zika throughout the continental U.S. without the presence of the mosquitoes that carry the virus and our study helps to put into context some of the transmission risk.”
After a person is infected with the Zika virus through a mosquito bite the virus remains present in the blood and saliva for up to about two weeks, but remains in bodily fluids like breast milk for weeks and semen for months.
In 2016 the National Institutes of Health conducted a saliva study after officials were concerned by a case of Zika transmission between an elderly man and his caretaker son in Utah that ruled out better-understood routes like mosquito bites or sexual activity.
During the study, rhesus macaque monkeys at the Wisconsin National Primate Research Center were infected with the strain of Zika that has been circulating in North America and South America recently with saliva collected from infected monkeys.
The researchers swabbed the tonsils of five monkeys with saliva and swabbed the tonsils of three monkeys with a concentrated high dose of Zika virus in solution.
Tonsils were selected for testing because they are usually a source of infection for influenza and Epstein-Barr virus.
The saliva-swabbed monkeys avoided infection, as did a pair of monkeys who had infected saliva swabbed in their nostrils or eyelids. They discovered that if you add monkey saliva to the virus you reduce the ability of the virus to infect cells.
The infected monkeys had very little active virus in their saliva, compared to the amounts typically passed into people or monkeys by mosquito bites. The monkeys with the high concentration of Zika had nearly 80,000 times the number of Zika particles as in the saliva from the infected monkeys.
“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,” Christina Newman, co-first author of the study with Dudley and also a scientist with the UW–Madison Zika Experimental Science Team, said in a statement.
Friedrich explained that the study proved the mysterious Utah case was unusual.
“The case in Utah was an outlier — by orders of magnitude — in terms of the amount of virus that was present in his blood,” Friedrich said. “Transmission via saliva is theoretically possible, but it would require extraordinarily high viral loads that just aren’t present in the vast majority of infected people.”
