Norovirus is the most common cause of acute gastroenteritis, causing an estimated 685 million cases each year. The virus can lead to severe illness and contributes to an estimated 50,000 child deaths on an annual basis, mostly in developing countries. Until now, medical researchers assumed that norovirus was only spread via the fecal-oral route – however, new research in mice suggests that salivary transmission may be much more common than previously thought.
How Is Norovirus Spread?
As mentioned above, researchers have known for some time that noroviruses can spread via the fecal-oral route. The fecal-oral route refers to the act of consuming food or liquids that are contaminated with fecal matter – fecal matter which contains the norovirus. Until now, experts assumed that norovirus bypassed the salivary gland and targeted the intestines. However, new research from scientists at the National Institutes of Health (NIH) suggests that norovirus can grow in the salivary glands of mice, spreading entirely via the salivary route.
Exploring Salivary Transmission in Mice
To explore norovirus transmission, NIH scientists fed a group of newborn mice food contaminated with either norovirus or rotavirus, the latter being another virus that transmits in a similar way. The mouse pups were then allowed to suckle their mothers, who were entirely virus-free. Surprisingly, the mouse pups showed a surge of disease-fighting antibodies in their guts just a day after their initial viral exposure. The mouse pups were too young to produce their own antibodies, which suggested that they had received the antibodies via their mothers’ mammary glands. The scientists also observed a surge of antibodies in the mothers’ milk duct cells, which accompanied rapid viral replication in the milk ducts. In other words, the pups had clearly transmitted the virus to their mothers during suckling. This suggested that norovirus can, in fact, be transmitted via saliva.
The Implications of Salivary Norovirus Transmission
The scientists conducted a number of additional experiments, all of which confirmed the salivary theory. Of course, the findings need to be confirmed in human subjects. Still, the possibilities are striking. “This is completely new territory because these viruses were thought to only grow in the intestines,” said study senior author Nihal Altan-Bonnet, PhD. “Salivary transmission of enteric viruses is another layer of transmission we didn’t know about. It is an entirely new way of thinking about how these viruses can transmit, how they can be diagnosed, and, most importantly, how their spread might be mitigated.”
As a next step, researchers will need to confirm that salivary transmission of norovirus is possible in humans. If that theory is proven, researchers may realize that salivary transmission is a common mode of infection – knowledge that could be employed to help improve public health outcomes in areas with high rates of norovirus.
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