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Could Nose Picking Increase Alzheimer’s Risk?

A man wiping his nose.
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A new study is the first to show that the bacterium Chlamydia pneumoniae can enter the brain via the olfactory nerve in mice. The findings may have implications for neurodegenerative conditions such as Alzheimer’s disease (AD).

Bacterium associated with Alzheimer’s disease

In a number of studies, the respiratory tract pathogen Chlamydia pneumoniae has been identified in post-mortem brain samples from AD patients. Within the brain, antigens of the bacterium have been localized with amyloid beta (Aβ) deposits, the classical “hallmark” symptom of AD.

What is Chlamydia pneumoniae?

A respiratory tract pathogen, C. pneumoniae can cause infections such as pneumonia. Growing research shows that the bacterium is also present in diseases outside of the respiratory tract, such as arthritis, multiple sclerosis and late-onset dementia.

Whether C. pneumoniae is a “contributing factor” to neurodegeneration is not yet clear, but its association with chronic CNS pathologies warrants further investigation. A new study by Griffith University researchers has sought to understand how the respiratory pathogen enters the brain. Their work is published in the journal Scientific Reports.

C. pneumonia infects the olfactory and trigeminal nerves

Led by Dr. Jenny Ekberg, associate professor of Neurophysiology and a member of the Griffith Institute for Drug Discovery, the researchers isolated injected female mice with phosphate buffer saline or C. pneumonia via an intranasal route of administration.

Immunohistochemical analysis of the mice models showed that C. pneumonia can infect both the olfactory and trigeminal nerves, in addition to the olfactory bulb and the brain, within approximately 72 hours post-injection. The bacterium was found to cause deregulation of key pathways known to be involved in the pathogenesis of AD both at 7- and 28-days post-inoculation. In the olfactory system, deposits of Aβ were co-localized with C. pneumonia.

What are the olfactory and trigeminal nerves?

  • The olfactory nerve is responsible for the sense of smell. Its receptors can be found in the mucosa beneath the roof of the nasal cavity.

  • The trigeminal nerve is the largest cranial nerve, and it carries sensory input from the face region and is involved in motor actions such as biting down or chewing.

“We’re the first to show that C. pneumoniae can go directly up the nose and into the brain where it can set off pathologies that look like Alzheimer’s disease,” said Professor James St John, head of the Clem Jones Centre for Neurobiology and Stem Cell Research at Griffith and co-author of the study. “We saw this happen in a mouse model, and the evidence is potentially scary for humans as well.”

The team also used a “nasal epithelium injury model” as part of their study, in which methimazole was injected three days prior to C. pneumonia inoculation. The logic behind this protocol was to explore whether injury to the epithelium would affect the risk of C. pneumonia infection in the CNS. “Injury to the nasal epithelium resulted in increased peripheral nerve and olfactory bulb infection, but did not alter general CNS infection,” the authors write in the publication.

Nose picking not advised

Ekberg and colleagues acknowledge that this study needs to be conducted in humans to understand how translatable data is on the pathway of infection. Based on the results obtained from the animal research, St John proposes several ways in which you can protect the lining of your nose – as you might expect, nose picking isn’t a great idea, and neither is plucking hairs away from the nasal cavity. “We don’t want to damage the inside of our nose and picking and plucking can do that,” St John said. “If you damage the lining of the nose, you can increase how many bacteria can go up into your brain.”

As a next step, the researchers plan to extend their research into humans. They are also interested in the implications their data may have for AD diagnostics. Currently, loss of sense of smell is classed as an early diagnostic marker of the disease. St John proposes that smell tests could be introduced for populations above a certain age to help detect the disease as early as possible: “Once you get over 65 years old, your risk factor goes right up, but we’re looking at other causes as well, because it’s not just age – it is environmental exposure as well. And we think that bacteria and viruses are critical,” he concluded.

Reference: Chacko A, Delbaz A, Walkden H, et al. Chlamydia pneumoniae can infect the central nervous system via the olfactory and trigeminal nerves and contributes to Alzheimer’s disease risk. Sci Rep. 2022;12(1):2759. doi: 10.1038/s41598-022-06749-9.

This article is a rework of a press release issued by Griffith University. Material has been edited for length and content.