Evolution__3rd_Edition

..
lower
Virulence
higher
1.1
1.0
0.9
0.8 1.0 0.8 0.6 0.4 0.2
Proportion of figs entered by one wasp
Box 22.2
Vaccines and the Virulence of Human Diseases
Vaccines have been developed against
many parasitic diseases of humans
and economically valuable non-human
species. The parasites in turn may
evolve resistance to the vaccine (for
example, HIV a Section 3.2, p. 45).
However, vaccination can have other
effects on parasite evolution too.
Gandon et al. (2001) modeled the effect
of “imperfect vaccines” on the evolution
of virulence. Imperfect vaccines
are partially but not wholly effective
vaccines, where the vaccine works
against the parasites in some but not all
infected members of the host populations.
In practice, almost all vaccines
are imperfect in this sense. Gandon
et al. distinguished two cases.
1. Vaccines that reduce parasitic growth
inside the host. Some vaccines work
against parasites that have successfully
infected a host, reducing the
rate of growth and reproduction of
the parasites. The more virulent
strains of a parasite are those that
grow and reproduce faster in the
host. Virulent parasites “use up” the
host more rapidly than less virulent
parasites. We can assume that the
Figure 22.7
Virulence is higher in nematode species that parasitize fig wasps
in which more individual wasps lay their eggs in a fig. The results
are for 11 species of fig wasps and the 11 species of nematodes
that parasitize them (there is one nematode species per fig wasp
species). Virulence is measured by the average number of
offspring produced by parasitized relative to unparasitized wasps
in each species; virulence is higher down the y-axis. A virulence
of one means parasitized and unparasitized wasps leave the same
number of offspring: these vertically transmitted parasites are so
mild as practically to be commensals. The offspring leave a record
inside the fig and can be counted accurately. The proportion of
figs entered by one or more wasps can also be measured. Redrawn,
by permission, from Herre (1993). © 1993 American Association
for the Advancement of Science.
parasite has, in the absence of vaccination,
some optimal level of virulence,
such that it uses up its host at
the best possible rate a depending
on the number of infections, mode
of transmission to new hosts, and
other factors. Now a growthinhibiting
vaccine is applied. The
effect will be to create a force of natural
selection in favor of more virulent
parasite strains. Suppose the
best amount of time for a parasite to
use a host up in is 10 days. More,
and less, virulent strains of the parasite
that use hosts up in 8 or 12 days
are selected against. Suppose the
vaccine cuts the parasite growth rate
in half. A highly virulent parasitic
strain, that would formerly have
used up the host in only 5 days, will
now be favored, and will use up a
vaccinated host in 10 days. But in
a non-vaccinated host it will show
up its full virulence, and use up the
host in 5 days. Growth-inhibiting
vaccines tend to select for increased
virulence.
2. Vaccines that reduce the chance of
infection by a parasite. Other vaccines
may make it less likely a parasite will
get into a vaccinated host. These
vaccines tend to select for less virulent
parasites. As we saw in the main
text of this chapter, parasites that
typically infect a host only once are
less virulent than equivalent parasites
that infect one host with many
parasitic individuals. A vaccine that
reduces the chance of infection will
reduce the average number of parasites
that infect each host individual,
making it more likely that one parasite
individual has a host individual
to itself. Then the parasites evolve to
become less virulent.
Parasite virulence can be influenced
by other factors beside those in Gandon
et al.’s model. However, the model
illustrates two ways in which parasite
evolution can be influenced, in
humanly important ways, by vaccination
programs. In general, when we
interfere with nature, we set up new
selection pressures, and evolutionary
change is likely to occur in consequence.
The theory of evolution
enables use to work out what those
consequences may be.