Data from: Host age modulates parasite infectivity, virulence and reproduction

1. Host age is one of the most striking differences among hosts within
most populations, but there is very little data on how age-dependent
effects impact ecological and evolutionary dynamics of both the host and
the parasite. 2. Here, we examined the influence of host age (juveniles,
young and old adults) at parasite exposure on host susceptibility,
fecundity and survival as well as parasite transmission, using two clones
of the water flea Daphnia magna and two clones of its bacterial parasite
Pasteuria ramosa. 3. Younger D. magna were more susceptible to infection
than older ones, regardless of host or parasite clone. Also,
younger-infected D. magna became castrated faster than older hosts, but
host and parasite clone effects contributed to this trait as well.
Furthermore, the early-infected D. magna produced considerably more
parasite transmission stages than late-infected ones, while host age at
exposure did not affect virulence as it is defined in models (host
mortality). 4. When virulence is defined more broadly as the negative
effects of infection on host fitness, by integrating the parasitic effects
on host fecundity and mortality, then host age at exposure seems to slide
along a negative relationship between host and parasite fitness. Thus, the
virulence–transmission trade-off differs strongly among age classes, which
in turn affects predictions of optimal virulence. 5. Age-dependent effects
on host susceptibility, virulence and parasite transmission could pose an
important challenge for experimental and theoretical studies of infectious
disease dynamics and disease ecology. Our results present a call for a
more explicit stage-structured theory for disease, which will incorporate
age-dependent epidemiological parameters.