Diarrhea-causing stomach flu viruses such as norovirus and rotavirus are also contagious through saliva. This follows from a test that showed that infected mouse pups could transmit the viruses through their saliva to the mother mouse that nursed them.
Until now, researchers assumed that stomach flu-causing viruses only occurred through fecal contamination, but now there appears to be a second route. This Wednesday appeared in Nature a publication with the new insights.
The discovery of the salivary route follows from a trial in which researcher Nihal Altan-Bonnet of the National Institutes of Health in the United States wanted to follow the course of gastroenteritis in newborn mice. These animals are still so young that they have not yet developed their own immune system, making them dependent on antibodies from the mother, which they ingest through the milk.
Breast milk antibodies
Mice infected with a norovirus or a rotavirus developed the highest peak virus peak in their intestinal tract three to five days after infection, after which the infection had completely disappeared seven to ten days after infection. On day three of the infection, the researchers saw the concentration of antibodies in the boy’s intestines rise sharply, in line with the concentration of antibodies in the breast milk. Further investigation showed that the virus also actively multiplied in the mammary gland of the previously uninfected mother. This infection apparently triggered the antibodies from which the boy subsequently benefited.
But how did the mother get infected? Wasn’t that via contamination through poop through her mouth? To further investigate this, the team deliberately infected a mother mouse that had just given birth through the mouth. That did lead to an intestinal infection, but the mammary glands remained virus-free and there was no detectable increase in antibodies in her milk. A second experiment confirmed this, in which the infected pups of a mother were exchanged with uninfected pups and mother after one day. Two days later, both mothers had demonstrably a lot of virus in the mammary glands and all puppies were also infected. So the virus passed from puppy to mother and vice versa.
Nihal Altan-Bonnet’s team then performed a saliva test with adult mice (the puppies were still too small for that). Two days later, after infection, a large amount of virus could be measured in the saliva, which with some virus strains remained measurable for at least three weeks afterwards. The saliva was found to be contagious to other adult mice and puppies.
The discovery is big news in infectious disease science
That is big news in infectious disease. It was often described in the scientific literature that stomach viruses can sometimes be present in the saliva of a patient. But that has always been regarded as an overflow of the gastrointestinal infection until now. Major outbreaks of gastroenteritis – which are notorious on cruise ships – are therefore always combated by isolation of patients, thorough disinfection of surfaces, especially sanitary facilities, and strict hygiene measures. But those are all aimed at preventing fecal-oral transmission.
If the transmission can also take place via saliva, as Altan-Bonnet showed in the mice, coughing, sneezing and kissing also come into the picture as a risk of infection. Saliva transmission could also play a role in asymptomatic spread of the virus, all the more so because stomach flu viruses were sometimes also detected in the saliva of people who had no other symptoms.
Every year, an estimated one billion people worldwide are infected by stomach viruses, causing abdominal pain, diarrhea and vomiting. It causes a significant burden of disease and, in severe cases, death, especially among very young children and the elderly. Dehydration is the greatest danger.