Chronic infections

[Ian A. York]

In article <v01510100abb2f542eafb@[199.125.92.21]> deirdre at deeny.MV.COM (Deirdre) writes:

>> strikes me that there are really NOT that many clinically
>> important chronic viral illnesses. HIV, HTLV, EBV, CMV,
>> HSV, VZV, HBV, HCV, just off the top of my head. Out of
>
>How do the other chronic infections work?

Pardon the drastic editing here, some of the text I've deleted included 
some of the questions I'm going to try to answer.  

All the herpesviruses cause chronic or at least recurrent infections.  
There are 7 or 8 in humans: Herpes simplex 1 and 2, varicella zoster 
(chicken pox/shingles), Epestein-Barr virus (EBV) (mononucleosis and more), 
cytomegalovirus (CMV) (various symptoms, including mono-like), human 
herpesviruses 6 and 7 (HHV-6, HHV-7) (clinical importance unclear), and 
possible the newly described kaposi-s sarcoma herpes virus (KSVH, 
awaiting further confirmation).  There are also hundred of animal 
herpesviruses.

Other chronic viruses include adenoviruses (persistence not clear, 
certainly months and possible years) and the retroviruses (including HIV 
and human T-lymphotrophic virus [HTLV] in humans, and I think the 
believed-to-be-nonpathogenic spumaviruses, plus many more in animals), 
hepatitis B, C, delta.  Occasionally and possibly due to defective or 
otherwise aberrant virus there is evidence of persistent infection of 
things like polio and other picornaviruses, measles virus (involved in 
subacute sclerosing panencephalitis?).  There are likely more that I 
haven't thought of here.

In general every human is a walking culture dish for probably 3 - 10 
different latent/persistent viruses all the time.  Give a word of thanks 
to your immune system.  

Something else to think about is that things like the herpesviruses are 
well-adapted to their hosts, to the point that (1) they are very good at 
persistence and (2) they cause fairly low-key symptoms.  This is probably 
a general truth for most persistent viruses, so the statement that 
chronic viral infections are often not clinically important is pushing 
the corners of a tautology.  Further, it seems likely that there are more 
- maybe many more - "chronic" viruses co-existing with us, not causing 
anything in particular so nobody is looking in the right places.  The 
KSVH story may be a case in point.  

As for the question as to how the other chronic infections work: they are 
*all* different.  Even the different herpesviruses differ in their 
mechanisms of latency/persistence, although some are closer than others.  
Different viruses use completely different strategies.  The strategies 
are not well understood.  In some cases the degree of understanding is 
little greater than "mystery rays from outer space", in other cases the 
level is bit closer to "it's magic!" and in a very few cases the strategy 
is understood at a rather vague level.  

Ian
-- 
Ian York   (york at mbcrr.harvard.edu)
Dana-Farber Cancer Institute, 44 Binney St., Boston MA 02115
Phone (617)-632-3921     Fax  (617)-632-2627