The nose may do much more than allow us to smell flowers, food, and smoke. Research suggests it may also provide some germs with a possible route toward the brain.
Scientists are studying whether this pathway could help explain some of the changes seen in Alzheimer’s disease.
Researchers at Griffith University investigated a bacterium called Chlamydia pneumoniae. This germ is best known for causing respiratory infections, including infections of the lungs and airways. Earlier research has also detected it in the brain, raising questions about whether infection could influence brain health.
The researchers were particularly interested in the olfactory system, which gives us our sense of smell. Nerves involved in smell connect the nasal area with the brain, creating an unusual link between the outside environment and the nervous system. The brain is normally protected by barriers that make it difficult for many harmful substances and germs to enter.
In experiments with mice, the scientists found that C. pneumoniae could use nerves extending between the nasal cavity and brain as a route into the central nervous system. The bacteria were able to reach the brain relatively quickly. Once there, the infection produced changes in the animals’ brain tissue.
One important change involved amyloid beta, a protein strongly associated with Alzheimer’s disease. The researchers found increased deposits of this protein after infection. In people with Alzheimer’s, amyloid beta can collect between brain cells and form plaques, although scientists are still working to understand exactly how these changes contribute to the disease.
The finding does not mean that this bacterium has been proven to cause Alzheimer’s disease in humans. The experiments were performed in mice, and human Alzheimer’s is a complicated disease influenced by age, genetics, cardiovascular health, lifestyle, and many other factors.
More research is needed before scientists can determine whether this infection pathway plays an important role in people.
The study also examined what happens when the protective lining inside the nose is damaged. Injury to this tissue appeared to make it easier for bacteria to reach nearby nerves. This led the researchers to raise concerns about habits that can damage the nasal lining, such as repeatedly picking the nose or pulling out nose hairs.
Nose hairs have a useful job because they help trap dust, germs, and other particles before they travel farther into the respiratory system. The moist lining of the nose also forms part of the body’s natural defense against infection. Protecting this tissue may therefore be a sensible way to reduce unnecessary exposure to germs.
The connection with smell is especially interesting because changes in the sense of smell can occur in some people before noticeable memory problems develop. The parts of the brain involved in smell can be affected early during several neurological diseases.
Researchers are therefore interested in whether simple smell testing could eventually contribute to identifying people who may need further assessment.
However, losing the sense of smell does not mean a person has Alzheimer’s disease. Smell can decline because of aging, respiratory infections, sinus problems, nasal conditions, medications, or other neurological disorders. Any future use of smell testing would need to be combined with other medical information rather than used as a diagnosis by itself.
The researchers want to determine whether the pathway observed in mice also operates in humans. If future studies confirm that certain infections can travel from the nose to the brain and contribute to harmful changes, the discovery could open new directions for prevention and treatment.
Scientists might eventually investigate ways to block infections or reduce the inflammatory response they trigger.
The research was led by Professor James St John and colleagues at Griffith University in Australia. The study examining how Chlamydia pneumoniae can invade the central nervous system through nerves associated with the nose was published in the peer-reviewed journal Scientific Reports.
For now, the study provides an intriguing clue rather than proof that nose-related infections cause Alzheimer’s disease. It shows how closely the nose, nervous system, immune system, and brain are connected. Understanding these links could eventually help researchers uncover more of the complex biological processes behind Alzheimer’s disease.
If you care about Alzheimer’s, please read studies about the likely cause of Alzheimer’s disease , and new non-drug treatment that could help prevent Alzheimer’s.
For more health information, please see recent studies about diet that may help prevent Alzheimer’s, and results showing some dementia cases could be prevented by changing these 12 things.
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