Abstract
Infectious diseases are contributing to the decline of endangered amphibians. We identified myxosporean parasites, Myxidium spp. (Myxosporea: Myxozoa), in the brain and liver of declining native frogs, the Green and Golden Bell frog (Litoria aurea) and the Southern Bell frog (Litoria raniformis). We unequivocally identified two Myxidium spp. (both generalist) affecting Australian native frogs and the invasive Cane toad (Bufo marinus, syn. Rhinella marina) and demonstrated their association with disease. Our study tested the identity of Myxidium spp. within native frogs and the invasive Cane toad (brought to Australia in 1935, via Hawaii) to resolve the question whether the Cane toad introduced them to Australia. We showed that the Australian brain and liver Myxidium spp. differed 9%, 7%, 34% and 37% at the small subunit rDNA, large subunit rDNA, internal transcribed spacers 1 and 2, but were distinct from Myxidium cf. immersum from Cane toads in Brazil. Plotting minimum within-group distance against maximum intra-group distance confirmed their independent evolutionary trajectory. Transmission electron microscopy revealed that the brain stages localize inside axons. Myxospores were morphologically indistinguishable, therefore genetic characterisation was necessary to recognise these cryptic species. It is unlikely that the Cane toad brought the myxosporean parasites to Australia, because the parasites were not found in 261 Hawaiian Cane toads. Instead, these data support the enemy-release hypothesis predicting that not all parasites are translocated with their hosts and suggest that the Cane toad may have played an important spill-back role in their emergence and facilitated their dissemination. This work emphasizes the importance of accurate species identification of pathogens relevant to wildlife management and disease control. In our case it is paving the road for the spill-back role of the Cane toad and the parasite emergence.
Highlights
Infectious diseases entering naıve populations are key threatening processes contributing to the precipitous global decline of biodiversity [1]
Myxosporean parasites in Australian endangered frogs A subset of Green and Golden Bell frog tadpoles (n = 38) from a semi-captive population in greater Sydney, New South Wales were submitted for necropsy as part of a routine health screening protocol required before animals from this facility could be translocated
Distinguishing exotic and invasive pathogens from native pathogens requires pre-existing knowledge of their historical distribution – a term ‘cryptogenic’ species has been coined to those species that we can not unambiguously place in either category [48]. This distinction is challenging for Myxidium spp. in Australian frogs where no historical baseline data exists and where an invasive host may have carried parasites to new places, because the enemy-release hypothesis predicts that not all parasites are translocated with their hosts [49,50]
Summary
Infectious diseases entering naıve populations are key threatening processes contributing to the precipitous global decline of biodiversity [1]. Circumstantial evidence suggests that myxosporean parasites are causing significant disease in amphibians and may be playing a role in their decline [6,7,8,9]. If myxosporean parasites of amphibians behave in a similar manner to those of fish, they would have a potential for dissemination outside of their original range and could be pathogenic in naıve populations [7,10]. The infamous Myxobolus cerebralis (Myxosporea) causes a whirling disease in salmonid fish and has been proven to have a devastating impact on wild and farmed fish populations in North America [11]. Myxobolus cerebralis was able to spread globally from Europe as the result of the translocations of infected fish and the cosmopolitan distribution of its aquatic invertebrate host – an oligochaete worm Tubifex tubifex [10,12]
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