Does loss of smell in COVID patients mean the virus has infected their brain?



It is now well-established that one of the most frequent symptoms of COVID-19 is the失去气味。When the new coronavirus infects the nasal epithelium, does that mean that the virus can travel along the olfactory nerve into the brain? Recent publications make it sound as if this has been proven: “Olfactory transmucosal SARS-CoV-2 invasion as a port of central nervous system entry in individuals with COVID-19“ 和 ”嗅觉路线是SARS-COV-2在恒河猴中侵入中枢神经系统的潜在方式”. Such publications and the attention they have been getting in the media are sufficient grounds to instill fear in COVID-19 patients who have lost their sense of smell. If the virus has impaired their sense of smell, does that mean that the virus is also present in their brains – just waiting to cause brain fog, spread throughout the brain and kill the patient or, if they recover, greatly increase their chance to develop a neurodegenerative disease?

Olfaction – a portal to brain infection?

来自波兰,法国和美国的调查员团队examined the evidence新的冠状病毒感染嗅觉神经元并从鼻子到大脑的嗅觉神经轴颈行进。在嗅觉受体神经元中不存在强制性病毒进入蛋白质的广泛共识。这些进入蛋白在载体细胞中大量表达,从而被感染并死亡减少功能of the adjacent olfactory receptor neurons. This is generally thought to cause the loss of the sense of smellin COVID-19。几天之内,支持细胞regenerate, the neurons resume their function, and smell returns in大多数病人within 1-2 weeks. Finding the new coronavirus in olfactory nerve axons is extremely rare, making this an ineffective route to brain infection.

The evidence just doesn’t add up

So why do many reports advocate an olfactory route to brain infection? One reason is that neurons and their support cells are closely intertwined, and they can easily be confused in confocal microscopy. Another reason is that the virus can indeed reach the brain, especially in genetically engineered mice where the virus entry proteins are abnormally expressed. But a careful analysis shows that the time course and itinerary of virus progression in animal models is inconsistent with a transfer from olfactory neurons in the nose to second- and third-order neurons in the brain. Instead, the data show a rare, but fulminant appearance of the virus in the brain – which suggests a route through blood vessels, the cerebrospinal fluid, or by traveling along ashortcut of nerve fibersthat bypass the olfactory bulb and project directly to targets in the forebrain. We are fortunate that the lack of virus entry proteins in olfactory neurons creates an effective barrier that prevents brain infection through the olfactory pathway.

An illustration of the brain, shown in pink and purple, on a blue background
The new coronavirus can – rarely – be present within the brain, but this seems to have come about through other routes – via blood vessels, cerebrospinal fluid or nerve fibres other than the olfactory nerve.
(jolygon /
An illustration of the brain, shown in pink and purple, on a blue background

Does the virus replicate in the brain?

It should be emphasized that in contrast to some animal models, the virus israrely found在人的大脑中。此外,许多研究依赖于不证明复制病毒的方法,而是检测可以脱落的病毒RNA或病毒蛋白质circulate systemically,不提供脑感染的确凿证据。病毒存在在大脑中的重要性尚不清楚。一些调查人员认为,脑干和呼吸中心的感染可以促进Covid-19的致命结果,而其他调查人员则指出,脑或脑脊液中病毒证据与Covid-19的严重程度之间没有相关性。


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