periods of ‘impossible transmission’, when the infected host leaves the
area where vectors are present (Gill and Mock, 1985). This parasite should
reduce its reproduction and virulence, perhaps rebounding when cues
indicate that the transmission period is approaching.
For vector-borne parasites,Ntowill often be high when vectors are
abundant in the environment, but vector abundance may not always be a
good predictor of selection for high parasitaemia. Variation in vector
competence among sites (such as differences in host-seeking behaviour or
the physiological environment presented to the parasite) could obscure
any pattern between vector abundance and opportunities for trans-
mission. Also, if hosts behaviourally avoid sites where transmission
is likely or if they flick off alighting vectors (Hart, 1994),Ntocould drop
and select for low parasitaemia and consequently low virulence. Thus,
host behaviour can apply selective pressure on the parasite to reduce
virulence.
Overall, in environments or times with lowNto, infections of prudent,
slow-growing genotypes would always be prone to invasion by mutation
to rapidly reproducing genotypes of parasites, but such infections would
fail to yield many (if any) daughter infections. There is thus a trade-off
betweenPtandD, which is driven byNto.Mobility hypothesis
Parasites that require their host to be mobile for successful transmission
cannot cause disabling morbidity and still enjoy successful transmission.
Transmission opportunities would be few for directly transmitted para-
sites that disable their host (Ewald, 1995), but this may not be the case for
vector-borne parasites (indeed, vectors may prefer non-ambulatory hosts).
Parasites should always be avirulent for vectors that must remain mobile
to allow transmission (Ewald and Schubert, 1989). Parasites using non-
living vectors, such as flowing water, should be among the most virulent
pathogens (Ewald, 1988). Parasites can also increaseNtoby developing
long-lived, highly durable transmission stages, reducing the need for a
mobile host. This is termed the ‘curse of the pharaoh’, from the folk-tales
that ancient corpses may harbour viable and exotic pathogens (Bonhoeffer
et al., 1996).Host-demography hypothesis
Ebert and Mangin (1997) suggest that selection favours a high repro-
ductive rate (and thus high virulence) for parasites exploiting hosts with
naturally short lifespans – that is, ifDis naturally short (a short-lived host
could support an infection only of short duration), selection would favour
high parasite replication to increase the probability of transmission.290 J.J. Schall