We learn every day a little more about “long covid”, but its mechanisms remain quite mysterious. A new publication from researchers at the Pasteur Institute confirms certain hypotheses and allows us to see things a little more clearly.
According to them, “several variants of Sars-CoV-2” have a “capacity to infect the central nervous system” – in other words the brain. But not only that: “The study also confirms that Sars-CoV-2 is capable […] to move inside the axons”, these “extensions” of neurons which allow information to circulate. In this study published in Naturethey highlight that “t“All Sars-CoV-2 variants are neuroinvasive”, regardless of the intensity of the infection at the start.
Hence neurological symptoms during infection, such as loss of taste or smell, but also afterwards – such as persistence of breathing difficulties, chronic fatigue, problems with concentration, behavior, etc.
The study recalls that “the neurological symptoms associated with Sars-Cov-2 infection have varied” over the course of the epidemic and the appearance of new variants: loss of smell has become, for example, less frequent . The scientists therefore wanted to check if this development was “the sign of a more or less strong affinity of Sars-CoV-2 for the nervous system”.
Covid, a virus “neurotropic”
They worked on a “panel of variants of interest”: the original strain, then Gamma, Delta and Omicron/BA.1. Result: all affect the brain – in particular the olfactory bulb – from the acute phase of the infection, whatever the variant.
But without necessarily causing loss of smell, which would be mainly linked to the original strain, called “Wuhan”: this has “a genetic sequence” responsible for the loss of smell, and absent from other variants. But these still infect the brain, including the olfactory bulb, note the researchers, who carried out this work on animals.
Therefore, one of the authors, Guilherme Dias de Melo, believes that “it is entirely possible that an infection, even asymptomatic – and therefore clinically benign – is characterized by diffusion of the virus in the nervous system. »
It remained to be seen how the virus could reach the olfactory bulb: thanks to experiments carried out in vitro, they established that once inside the neuron, “the virus is capable of moving in both directions of the axon » – in other words the “roads” which connect all the neurons together. Worse: viruses can travel in all directions. In other words, “the virus seems to effectively exploit the physiological mechanisms of the neuron to move in both directions”.
They therefore conclude that covid-19, much more than a “simple” respiratory infection, is “neurotropic”. They still have to demonstrate what has been strongly suspected so far: how “the presence of the virus can induce persistent inflammation and cause persistent symptoms described in cases of long covid, such as anxiety, depression or even brain fog. » – among the symptoms most often described.