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   When Viruses Invade the Brain    
   Neurodegenerative diseases may result from a nasal infection    
   By Stephani Sutherland    
      
      
   Image: Patrick McDonogh/Getty Images    
   Neurodegenerative diseases were once considered disorders of the mind, rooted   
   in psychology. Now viruses rank among the environmental factors thought to   
   trigger brain-ravaging diseases such as multiple sclerosis (MS) and   
   Alzheimer's disease. Human    
   herpesvirus-6 (HHV-6), in particular, has been linked to MS in past studies.   
   Neuroscientist Steven Jacobson and his colleagues at the National Institute of   
   Neurological Disorders and Stroke have determined that the virus makes its   
   entry to the human    
   brain through the olfactory pathway, right along with the odors wafting into   
   our nose.    
      
   The researchers tested samples of brain cells from people with MS and healthy   
   control subjects and found evidence of the virus in the olfactory bulb in both   
   groups. Infection via the nasal passage is probably quite common, as is   
   harboring a dormant    
   reservoir of HHV-6, but in people with MS, the virus is active. Genetics and   
   other unknown environmental factors probably determine the likelihood of the   
   virus reactivating once inside the brain, which can cause the disease to   
   progress.    
      
   The virus appears to invade the brain by infecting a type of glial cell called   
   olfactory ensheathing cells (OECs), which nourish smell-sensing neurons and   
   guide them from the olfactory bulb to their targets in the nervous system.   
   These targets include    
   the limbic system, a group of evolutionarily old structures deep in the brain,   
   "which is where viruses like to reactivate," Jacobson explains. He points out   
   that olfactory neurons and their OECs are among the few brain cells known to   
   regenerate    
   throughout our life. This neurogenesis may keep our sense of smell sharp, but   
   at the cost of providing the virus the opportunity to spread.    
      
      
   This article was originally published with the title When Viruses Invade the   
   Brain.    
      
      
      
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   http://www.scientificamerican.com/article.cfm?id=when-viruses-in   
   ade-the-brain&WT.mc_id=SA_CAT_HLTH_20120103   
      
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