Intestinal germs can cause diarrhea and vomiting – often both at the same time. Up until now it was thought that the pathogens were primarily transmitted through the remains of faeces. Now the focus is also on saliva.

Norovirus and rotavirus, which cause gastrointestinal infections, can also be transmitted through saliva. This was shown in a study with mice and human salivary gland cells.

So far, doctors have generally assumed that infections with these viruses take place almost exclusively via the faecal-oral route: tiny amounts of faeces, for example from contaminated food or drinking water, get into the mouth. A group led by Nihal Altan-Bonnet from the National Institutes of Health in Bethesda (Maryland, USA) describes the newly discovered route of infection in the journal “Nature”.

“Our results draw attention to infection of salivary glands with enteric viruses and saliva as a potentially more significant route of transmission through coughing, sneezing and kissing compared to the accepted route of transmission, faecal contamination,” the researchers write. The results indicate that hygiene measures, in addition to those preventing the spread of faeces, may be needed to prevent community transmission of enteric viruses.

Noro- and rotaviruses multiply in the intestinal wall and cause abdominal pain, nausea, vomiting and diarrhea in those infected. According to the “Global Burden of Disease” study from 2018, there are around 300 million infections with these viruses in children alone every year, and around 200,000 children die. In recent years, the media have repeatedly reported outbreaks of disease in nursing homes or on cruise ships caused by these intestinal viruses. Although virus particles have already been found in the saliva of infected people, they were believed to be a by-product of the intestinal infection and not infectious.

Transmission in mice via suckling

The study by Altan-Bonnet and colleagues now shows that the viruses infect salivary glands and can be passed on via spit.

The scientists infected healthy mouse babies with norovirus and rotavirus and were able to show that they passed the virus on to their mother when they suckled. The mother infected in this way passed the virus on to other healthy mouse offspring with her mother’s milk. When breastfeeding, infection is therefore possible in both directions: from the child to the mother and from the mother to the child. This suggested that infection occurs from child to mother via the child’s saliva.

Evidence of this was provided by an experiment in which the saliva of infected mice was administered to healthy mouse children who became ill. The researchers also investigated whether the viruses can multiply in salivary gland cells. After an infection of mice with different virus strains, the amount of virus in the salivary glands was 10,000 times higher after five days compared to six hours after infection. For norovirus strains MNV-3, MNV-4, and WU23, the extent and duration of viral replication was comparable to that of the central gut. The norovirus CR6, on the other hand, could not multiply in the salivary glands.

Finally, the researchers grew human salivary gland cells in the laboratory and were able to show that noroviruses multiply in large quantities in them.

Elizabeth Kennedy and Megan Baldridge from the Washington University School of Medicine in St. Louis (Missouri, USA) point to an opportunity for medical research in a “Nature” comment: When some norovirus strains multiply in the salivary glands just as well as in the gut, then they could be more easily researched: “3D cultures of human gut cells, so-called mini-guts, have been developed to cultivate human noroviruses, but working with them can be costly and challenging.” Instead, simpler models, “mini salivary glands”, could now be used.