- Messages
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My illness first signs was a strange pain after exercise, deep in my right upper nasopharynx area.
This became a cold/flu which settled in the chest for awhile and somewhere along the way ME/CFS began.
My theory is that this is a brain problem in some form.
Either through faulty information from the body
Or a latent brain infection
Or brain infection that no longer exists or never did exist, but the body believes it is ongoing
Or lasting structural damage caused by former infection/toxin---hope not.
The easiest route into the brain with the easiest way to hide seems to me to be the olfactory nerve.
https://hhv-6foundation.org/encepha...rastructural-changes-in-the-olfactory-pathway
http://jvi.asm.org/content/89/5/2731.abstract
https://onlinelibrary.wiley.com/doi/full/10.1002/path.4461
From the above link from wiley.com:
"The olfactory nerve: a shortcut for influenza and other viral diseases into the central nervous system"
Abstract
The olfactory nerve consists mainly of olfactory receptor neurons and directly connects the nasal cavity with the central nervous system (CNS). Each olfactory receptor neuron projects a dendrite into the nasal cavity on the apical side, and on the basal side extends its axon through the cribriform plate into the olfactory bulb of the brain. Viruses that can use the olfactory nerve as a shortcut into the CNS include influenza A virus, herpesviruses, poliovirus, paramyxoviruses, vesicular stomatitis virus, rabies virus, parainfluenza virus, adenoviruses, Japanese encephalitis virus, West Nile virus, chikungunya virus, La Crosse virus, mouse hepatitis virus, and bunyaviruses. However, mechanisms of transport via the olfactory nerve and subsequent spread through the CNS are poorly understood. Proposed mechanisms are either infection of olfactory receptor neurons themselves or diffusion through channels formed by olfactory ensheathing cells. Subsequent virus spread through the CNS could occur by multiple mechanisms, including trans‐synaptic transport and microfusion. Viral infection of the CNS can lead to damage from infection of nerve cells per se, from the immune response, or from a combination of both. Clinical consequences range from nervous dysfunction in the absence of histopathological changes to severe meningoencephalitis and neurodegenerative disease. Copyright © 2014 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.
Introduction
The olfactory nerve connects the nasal cavity directly with the central nervous system (CNS) and might therefore be a fast and easy shortcut for viruses into the CNS (Figures 1A and 1B). Since a whole array of viruses can replicate in the nasal cavity, it is tempting to speculate that the olfactory nerve could function as a shortcut for many viruses. There is substantial evidence from animal models and some human cases that several viruses, including influenza viruses, can use the olfactory nerve to enter the CNS. In this review, we will discuss the anatomy and histology of the olfactory route; evidence for CNS invasion via the olfactory route for different viruses, with a specific focus on influenza; and the mechanism by which viruses spread through the olfactory nerve to the CNS. This review will focus on the initial entry of viruses to the CNS via the olfactory route and not discuss the immune response once viruses have gained entry.
Schematic and histological overview of the olfactory nerve and the presence of influenza virus antigen in olfactory epithelium and olfactory bulb. (A) Schematic overview of the olfactory nerve with the different cell types including the olfactory receptor neurons (ORN) and olfactory ensheathing cells (OEC). Figure 1A adapted from 6. ‘Host strategies against virus entry via the olfactory system’, Virulence, Landes Bioscience, 2011. Copyright and all rights reserved. Material from this publication has been used with the permission of Landes Bioscience. (B) Histological overview of the ferret olfactory nerve. (C) Influenza virus antigen in cells of the olfactory mucosa 1 day post‐intranasal inoculation of ferrets with H5N1 virus. (D) Influenza virus antigen in the glomerular layer of the olfactory bulb in ferrets 3 days post‐intranasal inoculation with H5N1 virus. Figures 1C and 1D are adapted from 26. Copyright © American Society for Microbiology, Journal of Virology, Vol.86, 2012, pgs. 3975–3984, doi: 10.1128/JVI.06828
The olfactory route: histology and cell types
The peripheral nervous system is connected to the outside world by many open nerve‐endings and specialized sensory cells in the skin and mucosal surfaces. However, the shortest connection between the CNS and the external environment is the olfactory nerve. This section describes the histology and cell types encountered en route from the nasal cavity through the cribriform plate to the olfactory bulb and CNS.
The human olfactory mucosa constitutes 1.25% of the nasal mucosa and covers an area of more than 2 cm2 of each nasal cavity. Based on the most extensive study of 17 cadavers, the olfactory epithelium is distributed over the cribriform plate and extends anteriorly to the upper and middle turbinates along a distance of at least 12 mm, to the nasal septum along a distance of at least 15 mm, and posteriorly from the cribriform plate to the nasopharynx 1. Whereas the olfactory epithelium in the human fetus has a uniform distribution 2, the distribution of the olfactory epithelium of adults is variable among people and is thought to change with time, due to conversion to or ingrowth of respiratory epithelium with concomitant loss of olfactory neurons. This may be explained by ageing, toxic damage, and inflammatory disease 2, 3. It is estimated that 10% of the inhaled air passes over the olfactory mucosa, although small changes in the anatomy of the nasal cavity have a large impact on the airflow 4.
The human olfactory mucosa consists of a pseudo‐stratified columnar epithelium resting on a highly cellular lamina propria. The epithelium has five cell types: olfactory receptor neurons (ORNs), sustentacular cells, microvillar cells, duct cells from Bowman's glands, and basal cells 5, 6. The olfactory mucosa is innervated by fibres from the trigeminal nerve, the nervus terminalis 7, autonomic fibres of the cervical ganglion, and most importantly the olfactory nerve. The ORN is a true bipolar neuron, like the photoreceptor cell of the retina, projecting a single dendrite to the surface of the olfactory neuroepithelium and a single axon to the olfactory bulb. The dendrite has a thickened club‐like ending known as the olfactory vesicle, which extends to the epithelial surface and contains non‐motile cilia with membrane receptors, where different molecules bind. The axons of the 10–20 million ORNs cross the basement membrane of the epithelium and pass through the lamina propria, where they join into fila which collectively form the olfactory nerve, and pass through the 15–20 foramina of the cribriform plate of the ethmoid bone to the olfactory bulb, where they form synapses with neurons in the central olfactory nervous system. The cribriform plate, which separates the nasal and cranial cavities, shows an age‐related size reduction in both the number and the diameter of the foramina 8, which may relate to the changes in extent of the olfactory epithelium, as well as the size of the olfactory bulb, with age. The axons of the ORNs are supported in their transition from the olfactory epithelium to the CNS (olfactory bulb) by a unique line of glial, Schwann cell‐like cells: the olfactory ensheathing cells. The olfactory ensheathing cells create continuous, fluid‐filled perineural channels that, interestingly, remain open, regardless of degeneration and regeneration of ORNs 9. The lamina propria also contains mucus‐secreting Bowman's glands, blood vessels, lymphatic vessels, and connective tissue.
The axons of ORNs pass through the cerebrospinal fluid (CSF) of the subarachnoid space to terminate on mitral and tuft cells in the olfactory bulbs. Axons of the mitral and tuft cells of each olfactory bulb coalesce in the olfactory tracts. These tracts lie in the olfactory sulcus of the basal forebrain and convey information to the ipsilateral primary olfactory cortex. The primary cortical neural projections of the olfactory tracts are the olfactory nucleus, piriform cortex, amygdala, and entorhinal cortex. In the olfactory bulb, many lateral connections between neurons are made by the juxtaglomerular cells 10.
Pathways connecting the CSF‐containing subarachnoid space, perineural spaces encompassing olfactory nerves, and the nasal lymphatics are important for CSF drainage. Pathways between the nasal passages and the CSF are functional in humans, as evidenced by the fact that drugs are delivered directly to the CSF following intranasal delivery, without entering the blood to an appreciable extent 11, 12. The lymphatic circulation would be a preferential pathway of absorption of cerebral interstitial fluid, successively involving perivascular spaces, the arachnoid sheath of the olfactory nerve through the cribriform plate, the nasal submucosa, and cervical lymph nodes.
Evidence for CNS invasion via the olfactory nerve
Influenza viruses
Although influenza virus infections are primarily linked to respiratory tract infections, the most common extra‐respiratory tract complication is CNS disease 13. Evidence for influenza virus invasion along the olfactory nerve is based on studies in experimental animals and a few human cases. This review focuses on those studies in experimental animals in which the anatomical localization of influenza viruses along the olfactory nerve, and the associated histological lesions, is described. Finally, we will discuss the evidence for influenza virus transmission along the olfactory nerve in humans.
Herpesviruses
Herpesviruses can cause disease after a primary infection as well as after reactivation of the virus, which remains in a latent state within the host after primary infection. Evidence for CNS invasion via the olfactory nerve by herpesviruses is mainly based on observations in experimental animals during primary herpesvirus infection. Evidence for CNS invasion via the olfactory nerve in humans includes fatal herpes simplex virus (HSV) encephalitis cases, in which virus antigen was observed in glial cells of the olfactory tract and in cells of the temporal lobes, hippocampus, amygdaloid nuclei, olfactory cortex, insula, and cingulate gyrus. In contrast, virus antigen was not detected in the trigeminal tract, suggesting that virus entered the CNS via the olfactory route 34. Furthermore, HSV‐associated lesions in the olfactory mucosa and olfactory bulb have been observed in HSV encephalitis cases (reviewed in ref 35), and latent HSV has been detected in the olfactory bulb of non‐encephalitis cases 36. Human herpesvirus‐6 has been detected in the olfactory bulb more frequently than in other parts of the CNS, indicating that the olfactory nerve might be a route of entry into the CNS 37.
Experimental herpesvirus infections have shown that several herpesviruses, including HSV‐1, equine herpesvirus (EHV)‐1, EHV‐9, and bovine herpesvirus (BHV)‐5, can enter the CNS of mammals via the olfactory nerve 35. Some of these studies that focused on the olfactory nerve as the route of entry are briefly discussed below. In both rats and mice, intranasal inoculation of HSV‐1 resulted in CNS invasion, with the detection of virus in the olfactory bulb. Interestingly, after the primary infection, virus remained latent in the olfactory mucosa and olfactory bulb 35. In mice inoculated intranasally with EHV‐1, virus antigen could be detected in the olfactory mucosa at 2 dpi and in the olfactory bulb at 3 dpi 38. Virus antigen expression was associated with an influx of lymphocytic and other mononuclear cells in the olfactory bulb and surrounding leptomeninges at 3 dpi 38. After intranasal inoculation of EHV‐9 into suckling hamsters, virus antigen was detected in scattered ORNs at 12 h post‐inoculation (hpi) and in most of them at 48 hpi. Associated histological lesions included necrosis of ORNs and influx of inflammatory cells. Virus antigen was detected in the olfactory bulb from 48 hpi onwards and was associated with lymphocytic meningoencephalitis, characterized by neuronal necrosis, mixed‐cell perivascular aggregates, gliosis, intranuclear inclusion bodies in ORNs, and diffuse lymphocytic infiltrates in the meninges 39.
Other viruses
Poliovirus has been linked to CNS invasion via the olfactory nerve since 1933. In cynomolgus macaques, virus was demonstrated in the olfactory bulb and subsequently the olfactory tract and the rest of the CNS after intranasal inoculation 40, 41. Aerosol infection resulted in CNS invasion via the olfactory nerve in rhesus and cynomolgus macaques. Destruction of the olfactory mucosa using zinc sulphate could prevent virus transmission via the olfactory nerve but in some animals, virus entered the CNS via the trigeminal nerve or peripheral ganglia 42. In contrast to the experimental infections in macaques, the distribution of lesions in fatal human poliomyelitis cases suggested that the trigeminal nerve and the sympathetic nervous system, rather than the olfactory nerve, were the major routes of viral entry into the CNS 43. The possible role of the olfactory nerve as a route of entry into the CNS for other neurotropic picornaviruses (such as enterovirus‐71, Coxsackievirus or echoviruses) has not been investigated to our knowledge.
Members of the Mononegavirales, such as the paramyxoviruses [including canine distemper virus (CDV), Hendra virus, and Nipah virus], have been linked to CNS invasion via the olfactory nerve in experimental infections. Intranasal inoculation of CDV into ferrets resulted in infected ORNs in the olfactory mucosa at 14 dpi but not before, indicating that these cells were infected following systemic replication rather than directly after intranasal inoculation. Subsequently, virus antigen was detected in the olfactory bulbs when other parts of the CNS were not infected. There were no inflammatory lesions observed in the olfactory bulb, which could be due to the severe leukopenia and immunosuppression caused by the CDV infection 44. Intranasal inoculation of Hendra virus into mice resulted in infected cells in the olfactory mucosa at 6 dpi, associated with small foci of necrosis. Within the olfactory bulb, virus was first detected in periglomerular cells, mitral cells, and granule cells, and was associated with inflammation at 8 dpi. Virus antigen and associated lesions were observed in the amygdala and hippocampus at 12 dpi 45. Intranasal Nipah virus inoculation into Syrian hamsters resulted in virus antigen in the olfactory mucosa at 2 dpi, which was associated with inflammation and necrosis. By 8 dpi, virus antigen was detected in the glomerular layer and external plexiform layer, and had spread throughout the olfactory bulb by 12 dpi. Virus antigen was associated with a meningoencephalitis, which became more severe in time 46.
Of the lyssaviruses, vesicular stomatitis virus (VSV) has been studied most extensively. As early as 1937, VSV had been shown to enter the CNS via the olfactory nerve in mice after intranasal inoculation; histological lesions consisted of necrosis of the olfactory mucosa, and necrosis and cell loss in the olfactory bulb 47. Subsequent studies showed VSV virus antigen in ORNs at 12 hpi, after which virus antigen could be detected in the olfactory bulb at 4 dpi 48. Rabies virus normally enters the host via a bite, after which peripheral nerves become infected. However, there is also evidence for CNS invasion via the olfactory nerve in humans and experimental animals. In a human case of airborne rabies virus infection, virions were found by electron microscopy in the glomerular layer of the olfactory bulb 49. Intranasal infection of rabies virus into mice resulted in infection of ORNs within the nasal cavity, after which virus infected neurons in the olfactory bulb and spread along the olfactory tract 50.
Other viruses that have been shown to enter the CNS via the olfactory nerve include parainfluenza virus 51, adenoviruses, Japanese encephalitis virus 52, West Nile virus 53, chikungunya virus 54, La Crosse virus 55, mouse hepatitis virus 56, and bunyaviruses (reviewed in ref 35) but these will not be discussed further here. Together, these studies show that a variety of viruses, causing both acute and chronic infections, are able to use the olfactory nerve as a shortcut into the CNS. However, for many viruses that normally replicate in the nasal cavity, it is still unknown whether they could use the olfactory nerve as a shortcut into the CNS.
Mechanisms
CNS entry via the olfactory nerve
Although many of the studies described above show that viruses can enter the CNS via the olfactory nerve, there are few in‐depth studies on how they do so. We discuss two proposed mechanisms below:
Infection of ORNs
ORNs connect the nasal cavity directly with the CNS. The cilia on the dendrite sample the nasal cavity and the axon passes through the cribriform plate and ends in a glomerulus in the olfactory bulb. As discussed above, ORNs of experimental animals can be infected by different viruses, including different influenza virus subtypes. However, so far, there are no data on the ability of these viruses to infect ORNs of humans. The first step of a virus replication cycle is the attachment of a virus to the host cell. The viruses described above use a variety of receptors to enter host cells, such as sialic acids (eg influenza virus, parainfluenza virus, and adenoviruses) 57-59, heparan sulphate (herpesvirus and Japanese encephalitis virus) 60, 61, nectin‐1 (HSV and pseudorabies virus) 62, CD155 (poliovirus) 63, ephrin B2 and B3 (Hendra virus and Nipah virus) 64, and many others. The expression of these receptors on the apical side of the olfactory mucosa, and specifically on the dendrites of ORNs, has not been studied. Recently we showed that seasonal H3N2 viruses, pandemic H1N1 virus, and HPAI H5N1 viruses are able to attach to the apical side of human and ferret olfactory mucosa (Figure 2) 31, which suggests that multiple influenza virus subtypes are able to infect human ORNs. Interestingly, the attachment pattern of these influenza viruses to the olfactory mucosa differed from the attachment pattern to the respiratory mucosa 65, indicating that the olfactory and respiratory mucosa may differ in their susceptibility to infection by different influenza viruses.
After intranasal inoculation, virus antigen in the CNS is often first detected in the periglomerular cells and mitral cells of the olfactory bulb. This suggests that viruses are transported through the axons of ORNs to the olfactory bulb. Transaxonal transport could occur in two directions: either retrograde (towards the cell body) or anterograde (towards the synapse). Retrograde transport requires internalization of viruses at the synapse, after which they are transported by the dynein motor complex towards the cell body. Viruses that are capable of retrograde transport include alphaherpesvirus (such as HSV and pseudorabies virus), poliovirus, rabies virus, canine adenovirus (reviewed in refs 62 and 66), and HPAI H5N1 virus 22. In contrast, for anterograde transport, viruses or virus proteins located in the cell body first have to pass the axonal hillock, after which kinesins can transport them towards the synapse 67. Herpesviruses can be transported in this direction either as a complete virion, because their envelope is acquired at the trans‐Golgi network, or as a naked capsid (reviewed in refs 62, 66, and 67). Anterograde transport of viruses that are assembled at the cell surface (such as influenza viruses) would thus require the transport of viral components (such as the ribonuclein complex) and viral proteins to the synapse in the glomeruli of the olfactory bulb, where virus assembly could occur. Unfortunately, it is not known whether anterograde transport in neurons is possible for neurotropic viruses, except for herpesviruses and some genetically modified VSVs 64.
A known defence mechanism to prevent anterograde transport of pathogens through ORNs is the fast induction of apoptosis of infected ORNs 15, 68, 69. Influenza virus transmission along the olfactory nerve was inhibited by apoptosis of the infected ORNs after intranasal infection of mice with an H3N1 virus 15, thereby most likely preventing virus spread to the olfactory bulb. Conversely, HSV, which did not induce apoptosis in ORNs, was able to spread to the olfactory bulb. However, a US3‐disrupted mutant of herpesvirus, which did induce apoptosis in ORNs, was not able to spread to the olfactory bulb 70. This suggests that viruses that use the olfactory nerve as a route of entry into the CNS can block or delay apoptosis in ORNs.
Channels formed by the olfactory ensheathing cells
The axons of the ORNs cross the cribriform plate via channels formed by olfactory ensheathing cells. These channels have an open connection with the CSF and therefore create an open route to the olfactory bulb. This route has been investigated as a route for drug delivery into the CSF and CNS (reviewed in refs 71 and 72). Interestingly, particles up to 100 nm could enter the CNS via this route, which suggests that viruses within this size range, such as influenza viruses, picorna viruses, chikungunya virus, West Nile virus, La Crosse virus, Hendra virus, Nipah virus, and bunyaviruses, could enter the CNS via this mechanism 73. In addition, direct infection of the olfactory ensheathing cells could lead to virus release into these channels and subsequent transport to the olfactory bulb. Early histological findings during CNS invasion along the olfactory nerve include meningitis during infection with influenza viruses 23, 26-28, herpesvirus 39, and Nipah virus 46, which might be a result of virus entry into the CNS via the CSF around the olfactory bulb, as suggested by Bodewes et al 23.
Cell‐to‐cell transmission in the CNS
The ability to spread throughout the CNS depends, among many other factors, on the ability of a virus to spread from cell to cell. The mechanism and efficiency of cell‐to‐cell spread might vary between virus families, strains, and even between isolates. For example, several HPAI H5N1 virus isolates were able to reach the olfactory bulb, but only a portion of these were able to spread further through the CNS 30.
There are many different cell types within the CNS, including neurons, astrocytes, oligodendrocytes, and microglial cells, as well as cells of the meninges, ependymal, and choroid plexus. Many neurotropic viruses, such as herpesviruses and rabies virus, primarily infect neurons in the CNS. However, many, if not all, naturally occurring viruses can also infect other cell types in the CNS. For examples, rabies virus antigen has been detected in astrocytes and oligodendrocytes in some cases 74, and herpesviruses (human herpesvirus 6 and HSV) are able to infect astrocytes and microglial cells 75, 76. The cell tropism of influenza viruses in the CNS varies between subtypes and mammalian species infected. In human cases, HPAI H5N1 virus antigen was detected in neurons 77, while pandemic H1N1 virus antigen was detected in both neurons and glial cells 31. In mice, ferrets, and cats, HPAI H5N1 virus infected both neurons and glial cells in the CNS 22, 26, 78-80, while WSN (H1N1) virus in mice predominantly infected neurons 81.
How viruses spread from an infected neuron to other neurons or to glial cells has not been studied extensively. Different mechanisms for how viruses can spread from cell to cell have been described for neurotropic viruses. The first mechanism, and the best studied, is trans‐synaptic transport. In infections with rabies virus or herpesvirus, virus particles are released from one cell into the synaptic cleft, after which they are internalized 62, 82. The second mechanism is microfusion, as seen with measles virus, in which the F protein supports microfusion of the two cells within the synapse, through which the viral ribonuclein complex can travel 83. A third possible mechanism is non‐trans‐synaptic cell‐to‐cell transmission of viruses in a solid tissue, in which the release of viruses from infected cells could lead to infection of neighbouring cells. For example, rabies virus and herpesvirus particles have been observed by electron microscopy not only in the synaptic cleft but also around the cell body 62, 82. Additional mechanisms by which viruses could transmit to cells in close proximity could include tunnelling nanotubes, virological synapses, actin tails or virally induced varicosities 84, 85 (reviewed in refs 86–88). The exact mechanism of cell‐to‐cell spread for many of the viruses that use the olfactory nerve to enter the CNS, including influenza viruses, has not yet been studied.
Mechanisms of disease
CNS disease caused by virus entry along the olfactory nerve could vary from mild disease, such as temperature changes, to severe or even fatal meningitis or encephalitis 19, 26, 30. Damage to the CNS could be virus‐mediated by infection of cells within the CNS, immune‐mediated or a combination of both. The best example of virus‐mediated dysfunction might be from rabies virus infection. Rabies virus can inhibit the interferon response in infected neurons, thereby preventing an excessive immune response. In addition, histological abnormalities, the presence of Negri bodies, are less prevalent than the expression of virus antigen in neurons 74. Infection of neurons by rabies virus leads to dysfunctional neurons, without the induction of apoptosis. How this dysfunction is linked to the disease manifestations (both furious and paralytic) is not completely understood 89. Interestingly, the Duvenhage variant of rabies virus caused inflammation and cell death in a human fatal case and in experimentally infected mice 90, 91.
In contrast to rabies virus infection of the CNS, most infections of the CNS cause immune‐mediated responses and associated disease. As described above, most of the viruses that enter the CNS via the olfactory nerve cause an inflammatory response characterized by an influx of neutrophils and mononuclear cells, as well as neuronal necrosis. The local production of cytokines in the olfactory bulb or other parts of the CNS might affect the blood–brain barrier by disruption of the tight junctions. For example, the induction of monocyte chemoattractant protein‐1 increases the permeability of the blood–brain barrier 66, 92. Another example is the local production of TNF‐α and IL‐1β after CNS invasion of PR8 influenza virus via the olfactory nerve, which was associated with body temperature, drowsiness, and malaise (discussed in refs 19 and 78). Although there are some associations between immune response and disease manifestation, the exact mechanism remains unknown.
Besides acute CNS disease, pathogens that enter the CNS via the olfactory nerve may also be linked to chronic CNS disease. The invasion of pathogens may cause damage that accumulates over time and eventually contributes to the development of neurodegenerative disease 93-96. Although this is difficult to study, mice that have cleared HPAI H5N1 virus from the CNS have been shown to have an increase in phosphorylation and aggregation of α‐synuclein, which is a hallmark of neurodegenerative diseases 22, 97.
Future perspectives
There is substantial evidence that numerous viruses can use the olfactory nerve as a shortcut into the CNS. However, the majority of these studies are based on samples or tissues available from experimentally inoculated animals. So far, the evidence from human cases remains limited, which is most likely due to the fact the olfactory mucosa, nerve, and bulb are rarely collected during autopsies. Retrospective studies are therefore impossible to perform. The availability of such tissues would not only improve our knowledge of the use of the olfactory route by known neurotropic viruses, but could also shed more light on the ability of common respiratory viruses, such as rhinoviruses, respiratory syncytial virus, human metapneumovirus, and corona viruses, for which the nasal cavity is the primary site of replication, to use the olfactory nerve as a shortcut into the CNS. The fact that these viruses are not regularly linked to severe CNS complications does not mean that these viruses do not enter the CNS via the olfactory nerve, maybe causing subclinical or mild disease.
The mechanism by which viruses use the olfactory nerve as a route of entry into the CNS is also not well understood. First, transaxonal transport of viruses in ORNs requires anterograde transport, which has so far only been shown for herpesviruses. Future in vitrostudies should reveal the ability of other viruses to be transported anterogradely in neurons, preferentially in ORNs. Second, the cell tropism of the different viruses that enter the CNS via the olfactory nerve has not been studied extensively, although these studies might be relatively easy to perform in experimental animals. By comparing cell tropism, associated lesions, and clinical disease manifestations in such virus infections, more information will become available on the role of individual cells during CNS disease. Finally, the subsequent spread from the olfactory bulb to the rest of the CNS is currently not well understood.
Taken together, the olfactory nerve is a relatively neglected route by which influenza virus and many other viruses can gain entry to the CNS. Both those investigators studying experimental animal models and those involved in human clinical cases need to take the olfactory nerve into account in sampling and analysis. This combined approach will help to elucidate the incidence and pathogenesis of CNS disease caused by viruses entering via the olfactory route.
This became a cold/flu which settled in the chest for awhile and somewhere along the way ME/CFS began.
My theory is that this is a brain problem in some form.
Either through faulty information from the body
Or a latent brain infection
Or brain infection that no longer exists or never did exist, but the body believes it is ongoing
Or lasting structural damage caused by former infection/toxin---hope not.
The easiest route into the brain with the easiest way to hide seems to me to be the olfactory nerve.
https://hhv-6foundation.org/encepha...rastructural-changes-in-the-olfactory-pathway
http://jvi.asm.org/content/89/5/2731.abstract
https://onlinelibrary.wiley.com/doi/full/10.1002/path.4461
From the above link from wiley.com:
"The olfactory nerve: a shortcut for influenza and other viral diseases into the central nervous system"
Abstract
The olfactory nerve consists mainly of olfactory receptor neurons and directly connects the nasal cavity with the central nervous system (CNS). Each olfactory receptor neuron projects a dendrite into the nasal cavity on the apical side, and on the basal side extends its axon through the cribriform plate into the olfactory bulb of the brain. Viruses that can use the olfactory nerve as a shortcut into the CNS include influenza A virus, herpesviruses, poliovirus, paramyxoviruses, vesicular stomatitis virus, rabies virus, parainfluenza virus, adenoviruses, Japanese encephalitis virus, West Nile virus, chikungunya virus, La Crosse virus, mouse hepatitis virus, and bunyaviruses. However, mechanisms of transport via the olfactory nerve and subsequent spread through the CNS are poorly understood. Proposed mechanisms are either infection of olfactory receptor neurons themselves or diffusion through channels formed by olfactory ensheathing cells. Subsequent virus spread through the CNS could occur by multiple mechanisms, including trans‐synaptic transport and microfusion. Viral infection of the CNS can lead to damage from infection of nerve cells per se, from the immune response, or from a combination of both. Clinical consequences range from nervous dysfunction in the absence of histopathological changes to severe meningoencephalitis and neurodegenerative disease. Copyright © 2014 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.
Introduction
The olfactory nerve connects the nasal cavity directly with the central nervous system (CNS) and might therefore be a fast and easy shortcut for viruses into the CNS (Figures 1A and 1B). Since a whole array of viruses can replicate in the nasal cavity, it is tempting to speculate that the olfactory nerve could function as a shortcut for many viruses. There is substantial evidence from animal models and some human cases that several viruses, including influenza viruses, can use the olfactory nerve to enter the CNS. In this review, we will discuss the anatomy and histology of the olfactory route; evidence for CNS invasion via the olfactory route for different viruses, with a specific focus on influenza; and the mechanism by which viruses spread through the olfactory nerve to the CNS. This review will focus on the initial entry of viruses to the CNS via the olfactory route and not discuss the immune response once viruses have gained entry.
Schematic and histological overview of the olfactory nerve and the presence of influenza virus antigen in olfactory epithelium and olfactory bulb. (A) Schematic overview of the olfactory nerve with the different cell types including the olfactory receptor neurons (ORN) and olfactory ensheathing cells (OEC). Figure 1A adapted from 6. ‘Host strategies against virus entry via the olfactory system’, Virulence, Landes Bioscience, 2011. Copyright and all rights reserved. Material from this publication has been used with the permission of Landes Bioscience. (B) Histological overview of the ferret olfactory nerve. (C) Influenza virus antigen in cells of the olfactory mucosa 1 day post‐intranasal inoculation of ferrets with H5N1 virus. (D) Influenza virus antigen in the glomerular layer of the olfactory bulb in ferrets 3 days post‐intranasal inoculation with H5N1 virus. Figures 1C and 1D are adapted from 26. Copyright © American Society for Microbiology, Journal of Virology, Vol.86, 2012, pgs. 3975–3984, doi: 10.1128/JVI.06828
The olfactory route: histology and cell types
The peripheral nervous system is connected to the outside world by many open nerve‐endings and specialized sensory cells in the skin and mucosal surfaces. However, the shortest connection between the CNS and the external environment is the olfactory nerve. This section describes the histology and cell types encountered en route from the nasal cavity through the cribriform plate to the olfactory bulb and CNS.
The human olfactory mucosa constitutes 1.25% of the nasal mucosa and covers an area of more than 2 cm2 of each nasal cavity. Based on the most extensive study of 17 cadavers, the olfactory epithelium is distributed over the cribriform plate and extends anteriorly to the upper and middle turbinates along a distance of at least 12 mm, to the nasal septum along a distance of at least 15 mm, and posteriorly from the cribriform plate to the nasopharynx 1. Whereas the olfactory epithelium in the human fetus has a uniform distribution 2, the distribution of the olfactory epithelium of adults is variable among people and is thought to change with time, due to conversion to or ingrowth of respiratory epithelium with concomitant loss of olfactory neurons. This may be explained by ageing, toxic damage, and inflammatory disease 2, 3. It is estimated that 10% of the inhaled air passes over the olfactory mucosa, although small changes in the anatomy of the nasal cavity have a large impact on the airflow 4.
The human olfactory mucosa consists of a pseudo‐stratified columnar epithelium resting on a highly cellular lamina propria. The epithelium has five cell types: olfactory receptor neurons (ORNs), sustentacular cells, microvillar cells, duct cells from Bowman's glands, and basal cells 5, 6. The olfactory mucosa is innervated by fibres from the trigeminal nerve, the nervus terminalis 7, autonomic fibres of the cervical ganglion, and most importantly the olfactory nerve. The ORN is a true bipolar neuron, like the photoreceptor cell of the retina, projecting a single dendrite to the surface of the olfactory neuroepithelium and a single axon to the olfactory bulb. The dendrite has a thickened club‐like ending known as the olfactory vesicle, which extends to the epithelial surface and contains non‐motile cilia with membrane receptors, where different molecules bind. The axons of the 10–20 million ORNs cross the basement membrane of the epithelium and pass through the lamina propria, where they join into fila which collectively form the olfactory nerve, and pass through the 15–20 foramina of the cribriform plate of the ethmoid bone to the olfactory bulb, where they form synapses with neurons in the central olfactory nervous system. The cribriform plate, which separates the nasal and cranial cavities, shows an age‐related size reduction in both the number and the diameter of the foramina 8, which may relate to the changes in extent of the olfactory epithelium, as well as the size of the olfactory bulb, with age. The axons of the ORNs are supported in their transition from the olfactory epithelium to the CNS (olfactory bulb) by a unique line of glial, Schwann cell‐like cells: the olfactory ensheathing cells. The olfactory ensheathing cells create continuous, fluid‐filled perineural channels that, interestingly, remain open, regardless of degeneration and regeneration of ORNs 9. The lamina propria also contains mucus‐secreting Bowman's glands, blood vessels, lymphatic vessels, and connective tissue.
The axons of ORNs pass through the cerebrospinal fluid (CSF) of the subarachnoid space to terminate on mitral and tuft cells in the olfactory bulbs. Axons of the mitral and tuft cells of each olfactory bulb coalesce in the olfactory tracts. These tracts lie in the olfactory sulcus of the basal forebrain and convey information to the ipsilateral primary olfactory cortex. The primary cortical neural projections of the olfactory tracts are the olfactory nucleus, piriform cortex, amygdala, and entorhinal cortex. In the olfactory bulb, many lateral connections between neurons are made by the juxtaglomerular cells 10.
Pathways connecting the CSF‐containing subarachnoid space, perineural spaces encompassing olfactory nerves, and the nasal lymphatics are important for CSF drainage. Pathways between the nasal passages and the CSF are functional in humans, as evidenced by the fact that drugs are delivered directly to the CSF following intranasal delivery, without entering the blood to an appreciable extent 11, 12. The lymphatic circulation would be a preferential pathway of absorption of cerebral interstitial fluid, successively involving perivascular spaces, the arachnoid sheath of the olfactory nerve through the cribriform plate, the nasal submucosa, and cervical lymph nodes.
Evidence for CNS invasion via the olfactory nerve
Influenza viruses
Although influenza virus infections are primarily linked to respiratory tract infections, the most common extra‐respiratory tract complication is CNS disease 13. Evidence for influenza virus invasion along the olfactory nerve is based on studies in experimental animals and a few human cases. This review focuses on those studies in experimental animals in which the anatomical localization of influenza viruses along the olfactory nerve, and the associated histological lesions, is described. Finally, we will discuss the evidence for influenza virus transmission along the olfactory nerve in humans.
Herpesviruses
Herpesviruses can cause disease after a primary infection as well as after reactivation of the virus, which remains in a latent state within the host after primary infection. Evidence for CNS invasion via the olfactory nerve by herpesviruses is mainly based on observations in experimental animals during primary herpesvirus infection. Evidence for CNS invasion via the olfactory nerve in humans includes fatal herpes simplex virus (HSV) encephalitis cases, in which virus antigen was observed in glial cells of the olfactory tract and in cells of the temporal lobes, hippocampus, amygdaloid nuclei, olfactory cortex, insula, and cingulate gyrus. In contrast, virus antigen was not detected in the trigeminal tract, suggesting that virus entered the CNS via the olfactory route 34. Furthermore, HSV‐associated lesions in the olfactory mucosa and olfactory bulb have been observed in HSV encephalitis cases (reviewed in ref 35), and latent HSV has been detected in the olfactory bulb of non‐encephalitis cases 36. Human herpesvirus‐6 has been detected in the olfactory bulb more frequently than in other parts of the CNS, indicating that the olfactory nerve might be a route of entry into the CNS 37.
Experimental herpesvirus infections have shown that several herpesviruses, including HSV‐1, equine herpesvirus (EHV)‐1, EHV‐9, and bovine herpesvirus (BHV)‐5, can enter the CNS of mammals via the olfactory nerve 35. Some of these studies that focused on the olfactory nerve as the route of entry are briefly discussed below. In both rats and mice, intranasal inoculation of HSV‐1 resulted in CNS invasion, with the detection of virus in the olfactory bulb. Interestingly, after the primary infection, virus remained latent in the olfactory mucosa and olfactory bulb 35. In mice inoculated intranasally with EHV‐1, virus antigen could be detected in the olfactory mucosa at 2 dpi and in the olfactory bulb at 3 dpi 38. Virus antigen expression was associated with an influx of lymphocytic and other mononuclear cells in the olfactory bulb and surrounding leptomeninges at 3 dpi 38. After intranasal inoculation of EHV‐9 into suckling hamsters, virus antigen was detected in scattered ORNs at 12 h post‐inoculation (hpi) and in most of them at 48 hpi. Associated histological lesions included necrosis of ORNs and influx of inflammatory cells. Virus antigen was detected in the olfactory bulb from 48 hpi onwards and was associated with lymphocytic meningoencephalitis, characterized by neuronal necrosis, mixed‐cell perivascular aggregates, gliosis, intranuclear inclusion bodies in ORNs, and diffuse lymphocytic infiltrates in the meninges 39.
Other viruses
Poliovirus has been linked to CNS invasion via the olfactory nerve since 1933. In cynomolgus macaques, virus was demonstrated in the olfactory bulb and subsequently the olfactory tract and the rest of the CNS after intranasal inoculation 40, 41. Aerosol infection resulted in CNS invasion via the olfactory nerve in rhesus and cynomolgus macaques. Destruction of the olfactory mucosa using zinc sulphate could prevent virus transmission via the olfactory nerve but in some animals, virus entered the CNS via the trigeminal nerve or peripheral ganglia 42. In contrast to the experimental infections in macaques, the distribution of lesions in fatal human poliomyelitis cases suggested that the trigeminal nerve and the sympathetic nervous system, rather than the olfactory nerve, were the major routes of viral entry into the CNS 43. The possible role of the olfactory nerve as a route of entry into the CNS for other neurotropic picornaviruses (such as enterovirus‐71, Coxsackievirus or echoviruses) has not been investigated to our knowledge.
Members of the Mononegavirales, such as the paramyxoviruses [including canine distemper virus (CDV), Hendra virus, and Nipah virus], have been linked to CNS invasion via the olfactory nerve in experimental infections. Intranasal inoculation of CDV into ferrets resulted in infected ORNs in the olfactory mucosa at 14 dpi but not before, indicating that these cells were infected following systemic replication rather than directly after intranasal inoculation. Subsequently, virus antigen was detected in the olfactory bulbs when other parts of the CNS were not infected. There were no inflammatory lesions observed in the olfactory bulb, which could be due to the severe leukopenia and immunosuppression caused by the CDV infection 44. Intranasal inoculation of Hendra virus into mice resulted in infected cells in the olfactory mucosa at 6 dpi, associated with small foci of necrosis. Within the olfactory bulb, virus was first detected in periglomerular cells, mitral cells, and granule cells, and was associated with inflammation at 8 dpi. Virus antigen and associated lesions were observed in the amygdala and hippocampus at 12 dpi 45. Intranasal Nipah virus inoculation into Syrian hamsters resulted in virus antigen in the olfactory mucosa at 2 dpi, which was associated with inflammation and necrosis. By 8 dpi, virus antigen was detected in the glomerular layer and external plexiform layer, and had spread throughout the olfactory bulb by 12 dpi. Virus antigen was associated with a meningoencephalitis, which became more severe in time 46.
Of the lyssaviruses, vesicular stomatitis virus (VSV) has been studied most extensively. As early as 1937, VSV had been shown to enter the CNS via the olfactory nerve in mice after intranasal inoculation; histological lesions consisted of necrosis of the olfactory mucosa, and necrosis and cell loss in the olfactory bulb 47. Subsequent studies showed VSV virus antigen in ORNs at 12 hpi, after which virus antigen could be detected in the olfactory bulb at 4 dpi 48. Rabies virus normally enters the host via a bite, after which peripheral nerves become infected. However, there is also evidence for CNS invasion via the olfactory nerve in humans and experimental animals. In a human case of airborne rabies virus infection, virions were found by electron microscopy in the glomerular layer of the olfactory bulb 49. Intranasal infection of rabies virus into mice resulted in infection of ORNs within the nasal cavity, after which virus infected neurons in the olfactory bulb and spread along the olfactory tract 50.
Other viruses that have been shown to enter the CNS via the olfactory nerve include parainfluenza virus 51, adenoviruses, Japanese encephalitis virus 52, West Nile virus 53, chikungunya virus 54, La Crosse virus 55, mouse hepatitis virus 56, and bunyaviruses (reviewed in ref 35) but these will not be discussed further here. Together, these studies show that a variety of viruses, causing both acute and chronic infections, are able to use the olfactory nerve as a shortcut into the CNS. However, for many viruses that normally replicate in the nasal cavity, it is still unknown whether they could use the olfactory nerve as a shortcut into the CNS.
Mechanisms
CNS entry via the olfactory nerve
Although many of the studies described above show that viruses can enter the CNS via the olfactory nerve, there are few in‐depth studies on how they do so. We discuss two proposed mechanisms below:
- infection of ORNs in the olfactory mucosa, after which virus is transported anterogradely to the olfactory bulb;
- via diffusion through the channels formed by olfactory ensheathing cells, which form an open connection to the CNS.
Infection of ORNs
ORNs connect the nasal cavity directly with the CNS. The cilia on the dendrite sample the nasal cavity and the axon passes through the cribriform plate and ends in a glomerulus in the olfactory bulb. As discussed above, ORNs of experimental animals can be infected by different viruses, including different influenza virus subtypes. However, so far, there are no data on the ability of these viruses to infect ORNs of humans. The first step of a virus replication cycle is the attachment of a virus to the host cell. The viruses described above use a variety of receptors to enter host cells, such as sialic acids (eg influenza virus, parainfluenza virus, and adenoviruses) 57-59, heparan sulphate (herpesvirus and Japanese encephalitis virus) 60, 61, nectin‐1 (HSV and pseudorabies virus) 62, CD155 (poliovirus) 63, ephrin B2 and B3 (Hendra virus and Nipah virus) 64, and many others. The expression of these receptors on the apical side of the olfactory mucosa, and specifically on the dendrites of ORNs, has not been studied. Recently we showed that seasonal H3N2 viruses, pandemic H1N1 virus, and HPAI H5N1 viruses are able to attach to the apical side of human and ferret olfactory mucosa (Figure 2) 31, which suggests that multiple influenza virus subtypes are able to infect human ORNs. Interestingly, the attachment pattern of these influenza viruses to the olfactory mucosa differed from the attachment pattern to the respiratory mucosa 65, indicating that the olfactory and respiratory mucosa may differ in their susceptibility to infection by different influenza viruses.
After intranasal inoculation, virus antigen in the CNS is often first detected in the periglomerular cells and mitral cells of the olfactory bulb. This suggests that viruses are transported through the axons of ORNs to the olfactory bulb. Transaxonal transport could occur in two directions: either retrograde (towards the cell body) or anterograde (towards the synapse). Retrograde transport requires internalization of viruses at the synapse, after which they are transported by the dynein motor complex towards the cell body. Viruses that are capable of retrograde transport include alphaherpesvirus (such as HSV and pseudorabies virus), poliovirus, rabies virus, canine adenovirus (reviewed in refs 62 and 66), and HPAI H5N1 virus 22. In contrast, for anterograde transport, viruses or virus proteins located in the cell body first have to pass the axonal hillock, after which kinesins can transport them towards the synapse 67. Herpesviruses can be transported in this direction either as a complete virion, because their envelope is acquired at the trans‐Golgi network, or as a naked capsid (reviewed in refs 62, 66, and 67). Anterograde transport of viruses that are assembled at the cell surface (such as influenza viruses) would thus require the transport of viral components (such as the ribonuclein complex) and viral proteins to the synapse in the glomeruli of the olfactory bulb, where virus assembly could occur. Unfortunately, it is not known whether anterograde transport in neurons is possible for neurotropic viruses, except for herpesviruses and some genetically modified VSVs 64.
A known defence mechanism to prevent anterograde transport of pathogens through ORNs is the fast induction of apoptosis of infected ORNs 15, 68, 69. Influenza virus transmission along the olfactory nerve was inhibited by apoptosis of the infected ORNs after intranasal infection of mice with an H3N1 virus 15, thereby most likely preventing virus spread to the olfactory bulb. Conversely, HSV, which did not induce apoptosis in ORNs, was able to spread to the olfactory bulb. However, a US3‐disrupted mutant of herpesvirus, which did induce apoptosis in ORNs, was not able to spread to the olfactory bulb 70. This suggests that viruses that use the olfactory nerve as a route of entry into the CNS can block or delay apoptosis in ORNs.
Channels formed by the olfactory ensheathing cells
The axons of the ORNs cross the cribriform plate via channels formed by olfactory ensheathing cells. These channels have an open connection with the CSF and therefore create an open route to the olfactory bulb. This route has been investigated as a route for drug delivery into the CSF and CNS (reviewed in refs 71 and 72). Interestingly, particles up to 100 nm could enter the CNS via this route, which suggests that viruses within this size range, such as influenza viruses, picorna viruses, chikungunya virus, West Nile virus, La Crosse virus, Hendra virus, Nipah virus, and bunyaviruses, could enter the CNS via this mechanism 73. In addition, direct infection of the olfactory ensheathing cells could lead to virus release into these channels and subsequent transport to the olfactory bulb. Early histological findings during CNS invasion along the olfactory nerve include meningitis during infection with influenza viruses 23, 26-28, herpesvirus 39, and Nipah virus 46, which might be a result of virus entry into the CNS via the CSF around the olfactory bulb, as suggested by Bodewes et al 23.
Cell‐to‐cell transmission in the CNS
The ability to spread throughout the CNS depends, among many other factors, on the ability of a virus to spread from cell to cell. The mechanism and efficiency of cell‐to‐cell spread might vary between virus families, strains, and even between isolates. For example, several HPAI H5N1 virus isolates were able to reach the olfactory bulb, but only a portion of these were able to spread further through the CNS 30.
There are many different cell types within the CNS, including neurons, astrocytes, oligodendrocytes, and microglial cells, as well as cells of the meninges, ependymal, and choroid plexus. Many neurotropic viruses, such as herpesviruses and rabies virus, primarily infect neurons in the CNS. However, many, if not all, naturally occurring viruses can also infect other cell types in the CNS. For examples, rabies virus antigen has been detected in astrocytes and oligodendrocytes in some cases 74, and herpesviruses (human herpesvirus 6 and HSV) are able to infect astrocytes and microglial cells 75, 76. The cell tropism of influenza viruses in the CNS varies between subtypes and mammalian species infected. In human cases, HPAI H5N1 virus antigen was detected in neurons 77, while pandemic H1N1 virus antigen was detected in both neurons and glial cells 31. In mice, ferrets, and cats, HPAI H5N1 virus infected both neurons and glial cells in the CNS 22, 26, 78-80, while WSN (H1N1) virus in mice predominantly infected neurons 81.
How viruses spread from an infected neuron to other neurons or to glial cells has not been studied extensively. Different mechanisms for how viruses can spread from cell to cell have been described for neurotropic viruses. The first mechanism, and the best studied, is trans‐synaptic transport. In infections with rabies virus or herpesvirus, virus particles are released from one cell into the synaptic cleft, after which they are internalized 62, 82. The second mechanism is microfusion, as seen with measles virus, in which the F protein supports microfusion of the two cells within the synapse, through which the viral ribonuclein complex can travel 83. A third possible mechanism is non‐trans‐synaptic cell‐to‐cell transmission of viruses in a solid tissue, in which the release of viruses from infected cells could lead to infection of neighbouring cells. For example, rabies virus and herpesvirus particles have been observed by electron microscopy not only in the synaptic cleft but also around the cell body 62, 82. Additional mechanisms by which viruses could transmit to cells in close proximity could include tunnelling nanotubes, virological synapses, actin tails or virally induced varicosities 84, 85 (reviewed in refs 86–88). The exact mechanism of cell‐to‐cell spread for many of the viruses that use the olfactory nerve to enter the CNS, including influenza viruses, has not yet been studied.
Mechanisms of disease
CNS disease caused by virus entry along the olfactory nerve could vary from mild disease, such as temperature changes, to severe or even fatal meningitis or encephalitis 19, 26, 30. Damage to the CNS could be virus‐mediated by infection of cells within the CNS, immune‐mediated or a combination of both. The best example of virus‐mediated dysfunction might be from rabies virus infection. Rabies virus can inhibit the interferon response in infected neurons, thereby preventing an excessive immune response. In addition, histological abnormalities, the presence of Negri bodies, are less prevalent than the expression of virus antigen in neurons 74. Infection of neurons by rabies virus leads to dysfunctional neurons, without the induction of apoptosis. How this dysfunction is linked to the disease manifestations (both furious and paralytic) is not completely understood 89. Interestingly, the Duvenhage variant of rabies virus caused inflammation and cell death in a human fatal case and in experimentally infected mice 90, 91.
In contrast to rabies virus infection of the CNS, most infections of the CNS cause immune‐mediated responses and associated disease. As described above, most of the viruses that enter the CNS via the olfactory nerve cause an inflammatory response characterized by an influx of neutrophils and mononuclear cells, as well as neuronal necrosis. The local production of cytokines in the olfactory bulb or other parts of the CNS might affect the blood–brain barrier by disruption of the tight junctions. For example, the induction of monocyte chemoattractant protein‐1 increases the permeability of the blood–brain barrier 66, 92. Another example is the local production of TNF‐α and IL‐1β after CNS invasion of PR8 influenza virus via the olfactory nerve, which was associated with body temperature, drowsiness, and malaise (discussed in refs 19 and 78). Although there are some associations between immune response and disease manifestation, the exact mechanism remains unknown.
Besides acute CNS disease, pathogens that enter the CNS via the olfactory nerve may also be linked to chronic CNS disease. The invasion of pathogens may cause damage that accumulates over time and eventually contributes to the development of neurodegenerative disease 93-96. Although this is difficult to study, mice that have cleared HPAI H5N1 virus from the CNS have been shown to have an increase in phosphorylation and aggregation of α‐synuclein, which is a hallmark of neurodegenerative diseases 22, 97.
Future perspectives
There is substantial evidence that numerous viruses can use the olfactory nerve as a shortcut into the CNS. However, the majority of these studies are based on samples or tissues available from experimentally inoculated animals. So far, the evidence from human cases remains limited, which is most likely due to the fact the olfactory mucosa, nerve, and bulb are rarely collected during autopsies. Retrospective studies are therefore impossible to perform. The availability of such tissues would not only improve our knowledge of the use of the olfactory route by known neurotropic viruses, but could also shed more light on the ability of common respiratory viruses, such as rhinoviruses, respiratory syncytial virus, human metapneumovirus, and corona viruses, for which the nasal cavity is the primary site of replication, to use the olfactory nerve as a shortcut into the CNS. The fact that these viruses are not regularly linked to severe CNS complications does not mean that these viruses do not enter the CNS via the olfactory nerve, maybe causing subclinical or mild disease.
The mechanism by which viruses use the olfactory nerve as a route of entry into the CNS is also not well understood. First, transaxonal transport of viruses in ORNs requires anterograde transport, which has so far only been shown for herpesviruses. Future in vitrostudies should reveal the ability of other viruses to be transported anterogradely in neurons, preferentially in ORNs. Second, the cell tropism of the different viruses that enter the CNS via the olfactory nerve has not been studied extensively, although these studies might be relatively easy to perform in experimental animals. By comparing cell tropism, associated lesions, and clinical disease manifestations in such virus infections, more information will become available on the role of individual cells during CNS disease. Finally, the subsequent spread from the olfactory bulb to the rest of the CNS is currently not well understood.
Taken together, the olfactory nerve is a relatively neglected route by which influenza virus and many other viruses can gain entry to the CNS. Both those investigators studying experimental animal models and those involved in human clinical cases need to take the olfactory nerve into account in sampling and analysis. This combined approach will help to elucidate the incidence and pathogenesis of CNS disease caused by viruses entering via the olfactory route.
