Age-prevalence curves in a multi-species parasite community

The relationship between infection prevalence and host age is informative
because it can reveal processes underlying disease dynamics. Most prior
work has assumed that age-prevalence curves are shaped by infection rates,
host immunity, and/or infection-induced mortality. Interactions between
parasites within a host have largely been overlooked as a source of
variation in age-prevalence curves. We used field survey data and models
to examine the role of interspecific interactions between parasites in
shaping age-prevalence curves. The empirical dataset included
quantification of parasite infection prevalence for eight co-occurring
trematodes in over 15,000 snail hosts. We characterized age-prevalence
curves for each taxon, examined how they changed over space in relation to
co-occurring trematodes, and tested whether the shape of the curves
aligned with expectations for the frequencies of coinfections by two taxa
in the same host. The models explored scenarios that included negative
interspecific interactions between parasites, variation in the
force-of-infection, and infection-induced mortality that varied with host
age, which were mechanisms hypothesized to be important in the empirical
dataset. In the empirical dataset, four trematode parasites had monotonic
increasing age-prevalence curves and four had unimodal age-prevalence
curves. Some of the curves remained consistent in shape in relation to the
prevalence of other potentially interacting trematodes, while some shifted
from unimodal to monotonic increasing, suggesting release from negative
interspecific interactions. The most common taxa with monotonic increasing
curves had lower co-infection frequencies than expected, suggesting they
were competitively dominant. Taxa with unimodal curves had coinfection
frequencies that were closer to those expected by chance. The model showed
that negative interspecific interactions between parasites can cause a
unimodal age-prevalence curve in the subordinate taxon. Increases in the
force-of-infection and/or infection-induced mortality of the dominant
taxon cause shifts in the peak prevalence of the subordinate taxon to a
younger host age. Infection-induced mortality that increased with host age
was the only scenario that caused a unimodal curve in the dominant taxon.
Results indicated that negative interspecific interactions between
parasites contributed to variation in the shape of age-prevalence curves
across parasite taxa and support the growing importance of incorporating
interactions between parasites in explaining population-level patterns of
host infection over space and time.