Abstract
Bullfrog farming and trade practices are well-established, globally distributed, and economically valuable, but pose risks for biodiversity conservation. Besides their negative impacts on native amphibian populations as an invasive species, bullfrogs play a key role in spreading the frog-killing fungus Batrachochytrium dendrobatidis (Bd) in the natural environment. Bullfrogs are tolerant to Bd, meaning that they can carry high infection loads without developing chytridiomycosis. To test the potential of bullfrog farms as reservoirs for diverse and virulent chytrid genotypes, we quantified Bd presence, prevalence and infection loads across approximately 1,500 farmed bullfrogs and in the water that is released from farms into the environment. We also described Bd genotypic diversity within frog farms by isolating Bd from dozens of infected tadpoles. We observed individuals infected with Bd in all sampled farms, with high prevalence (reaching 100%) and high infection loads (average 71,029 zoospore genomic equivalents). Average outflow water volume from farms was high (60,000 L/day), with Bd zoospore concentration reaching approximately 50 million zoospores/L. Because virulent pathogen strains are often selected when growing in tolerant hosts, we experimentally tested whether Bd genotypes isolated from bullfrogs are more virulent in native anuran hosts compared to genotypes isolated from native host species. We genotyped 36 Bd isolates from two genetic lineages and found that Bd genotypes cultured from bullfrogs showed similar virulence in native toads when compared to genotypes isolated from native hosts. Our results indicate that bullfrog farms can harbor high Bd genotypic diversity and virulence and may be contributing to the spread of virulent genotypes in the natural environment. We highlight the urgent need to implement Bd monitoring and mitigation strategies in bullfrog farms to aid in the conservation of native amphibians.
Highlights
The international wildlife trade facilitates introductions of invasive species and pathogens that threaten native communities
Bullfrogs play an important role in the dynamics of amphibian chytridiomycosis, a disease caused by the fungus Batrachochytrium dendrobatidis (Bd) that has been linked to amphibian declines worldwide[21]
We hypothesized that because of the high Bd tolerance in bullfrogs[23,26], isolates from bullfrogs would be more pathogenic to a native Brazilian host species than those isolated from native frogs
Summary
The international wildlife trade facilitates introductions of invasive species and pathogens that threaten native communities. Bullfrog farming emerged as an alternative to overharvesting native amphibian species[4] This practice continues to contribute to the current global amphibian crisis by facilitating biological invasions[5,6,7,8,9,10,11]. Bullfrog farms may promote continuous spillover of pathogenic zoospores into native anuran communities and propogation of virulent Bd genotypes, but previous studies have not quantified pathogen outflow from bullfrog farms or compared the virulence of genotypes carried by farmed and native host species under controlled experimental conditions. We hypothesized that because of the high Bd tolerance in bullfrogs[23,26], isolates from bullfrogs would be more pathogenic to a native Brazilian host species than those isolated from native frogs. Our results provide important information about the dynamics of Bd in the amphibian trade that should be used to guide actions directed at conserving native anurans in Brazil and elsewhere
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