virulence

derek a. zelmer zelmeda4 at WFU.EDU
Tue Jan 3 08:05:48 EST 1995


I would like first to clear up an apparent misunderstanding. I understand 
that, as C. Graham Clark puts it, "natural selection lacks foresight", 
and that is not what I meant when I said that evolution towards increased 
pathogenicity will eventually use up host resources resulting in 
selection for more benign strains. In terms of evolution and natural 
selection we are most often left with an end product that has us 
guessing about selective forces. Because selection for increased 
virulence will rapidly use up the parasites resource, i.e. susceptible 
hosts, the conditions favoring this type of selection will be short 
lived. In other words, selection for host mortality is not evolutionarily 
stable, and the endpoint we are most likely to see is a more benign 
relationship. In a recent post C.T. Faulkner makes a few points that I 
feel could be misleading. In stating that "virulence is a side effect or 
cost of high reproduction rates" the association between reproductive 
ratios and virulence is again brought up. As I posted earlier, in studies 
that have looked at host mortality and parasite reproductive ratios 
simultaneously no correlation other than a negative one has been found 
between the two. This means that host pathology is not merely the by 
product of a parasite maximizing its fitness, but is the result of some 
other selective force(s), such as, perhaps, competition. Also mentioned 
by C.T. Faulkner was Ewald's view that parasite induced behavioral 
changes equate with pathology, and that in these instances host death by 
predation is the equivalent of parasite induced host mortality. This is 
not the case. Normally parasite induced host death prevents further 
transmission, whereas with behavior modification the death of the host 
*ensures* transmission. In the case of behavior modification, it is also 
desirable for the host to stay alive until eaten, so parasite induce 
death is still not favored. Take for example the classical example of 
Dicrocelium dendriticum, where infected ants are induced to take hold of 
a blade of grass, increasing their probability of being eaten by sheep. 
This behavior only occurs when it is cool enough outside for the ant to 
survive. During the heat of the day the ant is "allowed" by the parasite 
to let go of the grass, and return to the cool depths of its burrow. The 
evolutionary pathway leading to such associations must surely differ from 
that which leads to host death due to disease. High "virulence" is only 
favored under conditions where transmission rates are high. Mechanisms of 
host manipulation would seem to have evolved under conditions where 
transmission needed a little help. - Derek



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