Efficacy of prophylaxis dose and duration on host defense against the deadly chytrid fungus Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd), an aquatic pathogenic fungus, is
responsible for the decline of hundreds of amphibian species worldwide and
negatively impacts biodiversity globally. Prophylactic exposure to the
metabolites produced by Bd can provide protection for naïve tree frogs and
reduce subsequent Bd infection intensity. Here, we used a response surface
design crossing Bd metabolite prophylaxis concentration and exposure
duration to determine how these factors modulate prophylactic protection
against Bd in Pacific chorus frog (Pseudacris regilla) tadpoles (5x5
surface design) and metamorphs (3x3 surface design). We exposed
individuals every weekday to one of five Bd metabolite concentrations or a
water control for one to five weeks, after which all animals were
challenged with live Bd to evaluate their susceptibility. Exposure to the
Bd metabolite prophylaxis reduced Bd load and prevalence compared to the
control for both the tadpoles and metamorphs. Increasing Bd metabolite
prophylaxis concentration did not confer additional protection for either
life stage, but increasing duration of exposure did benefit metamorphs by
decreasing Bd prevalence but not Bd load. On average, control tadpoles and
metamorphs had 66.2% and 99.4% higher Bd loads, respectively, than
tadpoles and metamorphs exposed to any Bd metabolite prophylaxis.
Additionally, Bd metabolite prophylaxis reduced Bd prevalence relative to
controls in both tadpoles (20.5 vs 56.3%, respectively) and metamorphs
(21.9 vs 87.5%, respectively). The efficacy of short-term exposures of
relatively low concentrations of Bd metabolites at reducing Bd infections
suggests that this approach has the potential to be scaled up to field use
to aid in disease mitigation and conservation. Our results, combined with
additional research on Bd metabolite prophylaxis for other amphibian
species, suggest that this strategy may represent a broadly useful tool to
protect at-risk amphibian populations.