Evolutionary theory may contribute to practical solutions
for control of disease by identifying interventions that may
cause pathogens to evolve to reduced virulence. Theory predicts,
for example, that pathogens transmitted by water or arthropod
vectors should evolve to relatively high levels of virulence
because such pathogens can gain the evolutionary benefits of
relatively high levels of host exploitation while paying little
price from host illness. The entrance of Vibrio cholerae
into South America in 1991 has generated a natural experiment
that allows testing of this idea by determining whether
geographic and temporal variations in toxigenicity correspond to
variation in the potential for waterborne transmission.
Preliminary studies show such correspondences: toxigenicity is
negatively associated with access to uncontaminated water in
Brazil; and in Chile, where the potential for waterborne
transmission is particularly low, toxigenicity of strains
declined between 1991 and 1998. In theory vector-proofing of
houses should be similarly associated with benignity of
vectorborne pathogens, such as the agents of dengue, malaria, and
Chagas' disease. These preliminary studies draw attention to the
need for definitive prospective experiments to determine whether
interventions such as provisioning of uncontaminated water and
vector-proofing of houses cause evolutionary reductions in
virulence.
