One of the strangest things about COVID-19 is that quite often the first symptom is a loss of smell. Scientists now think they know why.
Researchers in the US have found the part of the nose responsible for detecting smells - the olfactory epithelium - has enormous amounts of an enzyme known to be used by the virus to enter the body.
"The olfactory epithelium is quite an easy part of the body for a virus to reach, it's not buried away deep in our body," said Mengfei Chen, a research associate at the Johns Hopkins University School of Medicine.
Prior research has found SARS-CoV-2 - the virus which causes COVID-19 - can easily attach itself to the enzyme ACE2, which is found on the surface of cells in the nose, lungs, kidneys, blood vessels and heart, allowing it to attack different parts of the body.
The new research found ACE2 levels between 200 and 700 times higher in the olfactory epithelium than other parts of the nose.
"These results suggest that this area of the nose could be where the coronavirus is gaining entry to the body... the very high levels of ACE2 that we found there might explain why it's so easy to catch COVID-19."
Loss of smell isn't unique to COVID-19, but it's been unclear until now why it was often the first symptom that appeared.
"While other respiratory viruses generally cause loss of the sense of smell through obstruction of airflow due to swelling of the nasal passages, this virus sometimes causes loss of smell in the absence of other nasal symptoms," said Andrew Lane, rhinologist at Johns Hopkins.
"We are now doing more experiments in the lab to see whether the virus is indeed using these cells to access and infect the body. If that's the case, we may be able to tackle the infection with antiviral therapies delivered directly through the nose."
The finding reinforces the need to wear masks when around others, not just to prevent spreading it yourself but reducing the risk of breathing it in through your nose.
The research was published Wednesday in the European Respiratory Journal.