Even a mild Covid-19 infection can result in "significant" changes to the brain, a UK study has found.
Researchers from the University of Oxford, University College London, Imperial College London and the National Institutes of Health have found the virus reduces grey matter thickness in parts of the brain associated with smell and memory.
A 1.3 to 1.8 per cent loss was recorded.
They also found tissue damage had occurred in parts of the brain associated with the olfactory cortex — an area linked to smell — along with shrinkage of the whole brain itself.
A UK study looking at brain imaging scans found reduced grey matter thickness, tissue damage and brain shrinkage in positive cases. (Source: 1News)
The shrinkage exceeded six per cent on average.
The virus had also caused "cognitive decline" in some cases.
The researchers had looked at brain imaging scans from 785 people aged 51 to 81 who had visited the UK Biobank to reach these findings.
A total of 401 participants had tested positive for Covid-19 between two scans (on average three years apart), including 15 who had been hospitalised. The remaining 384 participants were seen as "controls".
The time between participants testing positive and receiving their second imaging scan was on average 141 days.
Researchers found "significant longitudinal effects" when comparing the participants who had had Covid and those who hadn't.
"Our longitudinal analyses revealed a significant, deleterious impact associated with SARS-CoV-2," they said in a recently published article in journal Nature.
The researchers also performed a control analysis on people who developed pneumonia and seasonal flu not related to Covid-19 to show the changes in the brain were specific to the virus.
They said significant changes to the brain were still seen after excluding the 15 participants who had been hospitalised with the virus.
"Cognitive decline" had been seen between the participants' two scans and was due to "greater atrophy" of a brain region which is linked to cognition, the researchers noted.
The study has been welcomed by experts in Australia and New Zealand who have described it as "remarkable" and "uniquely valuable" to understanding Covid's neurological effects.
University of Auckland professor Maurice Curtis, who is head of department in anatomy and medical imaging, said: "This study demonstrates that there is a long-term consequence to getting Covid-19 and it highlights the importance of taking all measures possible to reduce Covid-19’s impact on the body and especially the brain."
Dr Idranil Basak, a research fellow with the University of Otago's Neurodegenerative and Lysosomal Diseases Laboratory, said the results looked worrying.
"It is a very good example of what Covid-19 can do to the human system long term."
He said their research team was looking at the molecular and cellular level — what's going on inside the brain cells when they are exposed to the virus.
Their preliminary data shows some infection in brain cells, including neurons. They don't know yet if the symptoms inside the cells are due to the virus or not, as there is an immune response when the virus infects the body, which could be causing secondary effects that affect the brain cells as well.
"From this research and our own, it is clear there is an effect on the brain from Covid-19 infection, and this could lead to some long Covid effects. We still don't know how to treat this, because no one has looked at it yet. But we do know that the virus directly, or indirectly, can affect the human brain."
However, the experts in Australia and New Zealand have noted more research is needed on whether these changes remain, reverse or get worse over time and if there are any treatments which could help. This is something the researchers themselves noted.
"Whether this deleterious impact can be partially reversed, or whether these effects will persist in the long term, remains to be investigated with additional follow up."
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