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Evolution of HIV

hugh ross haross at students.wisc.edu
Sat Jan 28 11:04:42 EST 1995

        An excellent book on the subject of the evolution of HIV and other 
pathogens is _Evolution of Infectious Disease_ by Paul W. Ewald, Oxford 
Univ. Press (1994).  To me, he makes a convincing argument that pathogens 
only evolve towards a more benign relationship with their host when there is 
a selective advantage to do so. If a pathogen can reporduce more effectively 
by incapacitating or killing it's host it will do so. 
        For example, Plasmodium falciparum (a malarial pathogen) is so 
virulent for two reasons. (1) At a high reproductive rate produces more 
viral and increases the chances of survival of the particularly virulent 
strain as opposed to a strain that reporduces at a slower rate, even though 
it wreaks considerably more havok upon it's host and (2) the incapacitated 
host is more susceptible to the mosquito which acts as the disease vector. 
On the other hand, many respiratory viruses ("the common cold") are passed 
directly from host to host via aeresolized droplets containing copies of the 
virus. An incapacitated host would have less opportunity to cough on a 
potential host and the incapacitating virus would be selcted against. The 
most sucessfull cold viruses that move from host to host via an airborn 
vector would be ones that reproduce at just a high enough rate so that the 
host is mobile and shedding virus rather than at a much higher rate that 
would incapacitate the host. Over a relatively short time (viral 
generations/reporductive cycles being so short) extremely virulent strains 
of airborn pathogens would evolve towards a somewhat more benign form in 
which a balance would be struck between reproduction and transmission and 
the virus could reproduce most efficiently.
        Ewald discusses several viruses (and several methods of 
transmission). He devotes a considerable portion of the book to the 
evolution of the HIV viruses. While I have not  read the book for a while 
and do not have it with me now to refer to I believe he concludes that, (1) 
based upon the evidence, it is not possible to say conclusively whether HIV 
first appeared in non-human primates and mutated to infect humans or visa 
versa; (2) That various strains of HIV have been around for quite some time, 
purhaps even hundreds or thousands of years in a more benign form (the 
benign form being selectively favored-- because it was so difficult for the 
virus to transmit itself to a new host in a predominently monogamous, non-IV 
drug usingpopulation, virulent forms of HIV were likely to kill their host 
before they could be transmitted to a new host ); (3) That the current, very 
virulent strains of HIV have become dominent as transmission vectors 
(unprotected sex with multiple partners, and IV drug use with shared 
needles) have allowed it to spread; and, (4) that there is some evidence 
that in populations that change their behavior so as to make transmission 
more difficult the virus is evolving back to a more benign form.
        My apologizes to Ewald to the the extent I may have misstated his 
position. I suggest that people read his book before commenting on the 
statements I have made herein. 

        Hugh Ross

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