The researcher say a viral infection in the nose can trigger bacteria to migrate from there to the ear, causing a middle ear infection.
The researchers, from Wake Forest Baptist Medical Center in Winston-Salem, NC, report their findings in the journal Infection and Immunity.
Lead author W. Edward Swords, professor of microbiology and immunology at Wake Forest Baptist, says we all have bacteria in our noses and most of the time they cause no problems:
"However, under certain conditions those bacteria can migrate to the middle ear and cause an ear infection, and now we have a better understanding of how and why that happens."
In their animal study, he and his colleagues showed that when the nose is infected with both a flu virus and a bacterium that commonly causes ear infections in children, it leads to inflammation of nasal tissue that in turn increases the number of bacteria and the likelihood they will travel down the Eustachian tube to the middle ear.
The bacterium they investigated is Streptococcus pneumoniae, which is known to live in children's noses in two phases: one where it does no harm, and the other where it becomes invasive. In its invasive phase, the bacterium is also often found in the infected ears of children.
Presence of flu virus causes infection by S. pneumoniae even in benign phase
However, the researchers found that when the flu virus is also present in the nose, it causes S. pneumoniae to grow and infect the ear, regardless of whether it is in the harmless, benign phase or the invasive phase.
Prof. Swords says the findings "suggest that a flu infection modifies the response of the immune system to this particular bacterium, enabling even the type that has previously been considered benign to infect the middle ear."
Middle ear infection in children is a big and costly health problem in Western countries.
According to the American Academy of Otolaryngology - Head and Neck Surgery, around 3 in 5 children in Western countries will have their first middle ear infection by the age of 1. By their third birthday, this figure will be over 4 in 5, and all children will have had at least one episode by the age of 5.
In the US alone, middle ear infections account for around 25 million clinical visits a year, costing an estimated $3 billion in treatments. This figure doubles if you take into account lost wages for parents.
Streptococcus pneumoniae bacteria are thought to cause 50-60% of middle ear infections.
The study was sponsored by the National Institutes of Health Institute of Deafness and Other Communication Diseases, and AstraZeneca.
In March 2014, Medical News Today learned about a study published in the journal Science that found the human sense of smell can detect more than 1 trillion odors, far more than previous studies have indicated. The researchers said we have more sensitivity in our sense of smell than we realize and we do not use it in everyday life.