Georgia State University biology researchers have found that infecting the nasal passages of mice with the virus that causes COVID-19 led to a rapid, escalating attack on the brain that triggered severe illness, even after the lungs were successfully clearing themselves of the virus.
Assistant
professor Mukesh Kumar, the study’s lead researcher, said the findings
have implications for understanding the wide range in symptoms and severity of
illness among humans who are infected by SARS-CoV-2, the virus that causes
COVID-19.
“Our thinking that it’s more
of a respiratory disease is not necessarily true,” Kumar said. “Once it infects
the brain it can affect anything because the brain is controlling your lungs,
the heart, everything. The brain is a very sensitive organ. It’s the central
processor for everything.”
The study, published by the
journal “Viruses,” assessed virus levels in multiple organs of the infected
mice. A control group of mice received a dose of sterile saline solution in
their nasal passages.
Kumar said that early in the
pandemic, studies involving mice focused on the animals’ lungs and did not
assess whether the virus had invaded the brain. Kumars’ team found that virus
levels in the lungs of infected mice peaked three days after infection, then
began to decline. However, very high levels of infectious virus were found in
the brains of all the affected mice on the fifth and sixth days, which is when
symptoms of severe disease became obvious, including labored breathing,
disorientation and weakness.
The study found virus levels
in the brain were about 1,000 times higher than in other parts of the body.
Kumar said the findings could
help explain why some COVID-19 patients seem to be on the road to recovery,
with improved lung function, only to rapidly relapse and die. His research and
other studies suggest the severity of illness and the types of symptoms that
different people experience could depend not only on how much virus a person
was exposed to, but how it entered their body.
The nasal passages, he said,
provide a more direct path to the brain than the mouth. And while the lungs of
mice and humans are designed to fend off infections, the brain is ill equipped
to do so, Kumar said. Once viral infections reach the brain, they trigger an
inflammatory response that can persist indefinitely, causing ongoing damage.
“The brain is one of the
regions where virus likes to hide,” he said, because it cannot mount the kind
of immune response that can clear viruses from other parts of the body.
“That’s why we’re seeing
severe disease and all these multiple symptoms like heart disease, stroke and
all these long-haulers with loss of smell, loss of taste,” Kumar said. “All of
this has to do with the brain rather than with the lungs.”
Kumar said that COVID-19 survivors
whose infections reached their brain are also at increased risk of future
health problems, including auto-immune diseases, Parkinson’s, multiple
sclerosis and general cognitive decline.
“It’s scary,” Kumar said. “A
lot of people think they got COVID and they recovered and now they’re out of
the woods. Now I feel like that’s never going to be true. You may never be out
of the woods.”
Journal article: https://www.mdpi.com/1999-4915/13/1/132
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