Your nose is the window to your brain—and some potentially deadly infections

Your nose is the window to your brain — and some potentially deadly infections

"Don't pick your nose!" you may have been told as a kid. As an adult you wonder, is digging snot out with your finger really that bad?

A recent study suggests you should definitely listen to the advice. The research linked nose-picking with an increased likelihood of being infected with SARS-CoV-2, the virus that causes COVID-19. If you got COVID-19 over the past few years, even if you didn't pick your nose, you may have also lost your sense of smell. The reason for this is your brain is surrounded by several physical and immune barriers to keep it safe from pathogens. But there is a chink in its armour: your nose. This is the shortest distance from the outside world to your brain.

Let's take a closer look and pick apart exactly what this means.

The link from nose to brain

The olfactory system is responsible for your sense of smell. The system's "smelling" structures are situated in the roof of your nose, which is lined by specialised tissue called the olfactory epithelium. It is made up of a layer of epithelial cells — which filter the air you breathe and secrete mucous aka snot — surrounding olfactory neurons, or nerve cells. One end of each neuron is directly exposed to the air you breathe, and the other end extends up the nose towards the brain. As the neurons move further up into the nasal cavity they wrap together in a layer of special cells called glia to form the olfactory nerve.  It eventually fuses with part of the brain called the olfactory bulb. The whole system is only a few centimetres long. While this is fantastic when it comes to detecting odours in the air and relaying this information to your brain, it can be quite deadly when dangerous microbes — which include some bacteria, viruses, parasites and even a brain-eating amoeba — use this same pathway to enter your brain and cause infections.

Are we totally defenceless against microbes?

Not really. We do have a few strategies up our nose — starting with snot. Even though snot feels disgusting, it's rich in antimicrobial enzymes and plays an important role in forming a physical and chemical barrier in the inner lining of your nose, preventing microbes from attaching to it. Damage to snot-producing epithelial cells, which can happen after physical trauma or an injury to the lining of the nose, can, however, make you more susceptible to potentially serious infections by some bacteria (see below). The specialised olfactory glia cells, which wrap around the neurons, can eat and destroy a range of pathogens. And we also have access to a range of immune cells that combat infection in our nose. Finally, within our brain we have a specialised immune cells called microglia that fight off microbes. However, while these layers of immune cells are great at stopping most viruses from entering our brain, this line of defence may not always be as effective in immunocompromised people.

How does the COVID-19 virus compare to other bugs?

While some microbes are a concern, it's important to sniff out the stinkers.  At the start of the COVID-19 pandemic it was feared the SARS-CoV-2 virus could infect the olfactory nerve and travel to the brain. But we now know this virus in fact infects the epithelial cells right next to the olfactory neurons, so — luckily — the infection appears to stay restricted to these cells. However, as epithelial cells support efficient functioning of the olfactory nerve cells, they may partially contribute to the loss of smell experienced during COVID-19 infection.  While SARS-CoV-2 might not enter the brain through the nose, a few viruses such as influenza, herpes simplex (which causes cold sores), Nipah, West Nile and polio can sometimes spread to the brain after infections in the nose.

A few bacteria that cause some respiratory conditions, such as Chlyamydia pneumoniae, and a soil-dwelling bacteria endemic to Northern Australia called Burkholderia pseudomallei have also been reported to travel to the brain, especially after an injury to the lining of the nose. The worst of the lot, however, is a brain-eating amoeba called Naegleria fowleri. This potentially deadly bug is found in warm, unchlorinated water around the world, including Australia.

Can we stop brain infection?

Brain infections are rare, but they are often misdiagnosed and have a high mortality rate. In "The Art of War", ancient Chinese military strategist Sun Tzu famously wrote, "victorious warriors win first and then go to war". To truly prevent brain infections, we need to spend time strategising and understanding our enemies. At CSIRO, we developed models of the olfactory system in a dish. Using these 3D cell models, we are trying to understand how viral infections might start and spread from the olfactory system to the brain.

Through this insight, we hope to develop targeted therapies to fight infection. Another effective approach is the use of nasal vaccines. These vaccines are not only needle-free (and often delivered in the form of a spray), but they are also great at activating a local immune response within the nose — and through this, restrict viral replication and spread. Currently there is a nasal spray flu vaccine available in the US. Similarly, there are a few ongoing clinical trials for nasal vaccines against COVID-19, with India and China approving them for use as boosters. However, as we are still trying to understand their efficacy, we might be a while away before they become readily available here in Australia.

To pick or not to pick? That is the question

So, is nose picking safe? While nose picking doesn't damage the inside of your nose, this act (other than being gross) is also a great way of introducing new and potentially dangerous pathogens into your nose. Given its proximity to the brain, mining green gold might not be the smartest thing to do. But if you are an avid nose picker, and feel like you are losing all hope, fear not! Just thoroughly wash and disinfect your hands prior to embarking on your next nasal excavation.

And if you're tempted to flush snot out with a nasal irrigation device like a neti pot instead of picking your nose, beware.  Make sure you use either previously boiled, sterile, or distilled water to reduce the chances of infectious bacteria or even brain-eating amoeba like Naegleria fowleri from getting in.

Source: ABC News/Health