Host preferences inhibit transmission from potential superspreader host species

Host species that are particularly abundant, infectious, and/or infected
tend to contribute disproportionately to symbiont (parasite or mutualist)
maintenance in multi-host systems. Therefore, in a facultative multi-host
system where two host species had high densities, high symbiont
infestation intensities, and high infestation prevalence, we expected
interspecific transmission rates to be high. Instead, we found that
interspecific symbiont transmission rates to caged sentinel hosts were an
order of magnitude lower than intraspecific transmission rates in the
wild. Using laboratory experiments to decompose transmission rates, we
found that opportunities for interspecific transmission were frequent,
where interspecific and intraspecific contact rate functions were
statistically indistinguishable. But most interspecific contacts did not
lead to transmission events due to a previously unrecognized transmission
barrier: strong host preferences. During laboratory choice experiments,
the symbiont preferred staying on or dispersing to its current host
species, even though the oligochaete symbiont is a globally distributed
host generalist that can survive and reproduce on many snail species.
These surprising results suggest that when managing symbiont transmission,
identifying key host species is still important, but it may be equally
important to identify and manage transmission barriers that keep potential
superspreader host species in check.