Studies on the prevalence and specific identity of Trypanosoma in introduced and native Australian and New Guinean (Sahulian) rodents

Native rodent species represent almost half of terrestrial mammalian extinctions in Australia since European colonisation in 1788. The rapid extinction of two native rodent species in Christmas Island was attributed to Trypanosoma lewisi, introduced with black rats during European colonisation. The aim of this study was to investigate the prevalence and specific identity of trypanosomes in endemic and exotic Sahulian (Australian and Papua New Guinean) rodents. We hypothesised that Sahulian rodents had lost their trypanosomes during initial colonisation of Sahul, and that T. lewisi was introduced with European settlement in 1788, contributing to native rodent declines in Australia. Our sub-hypothesis was that endemic rodents in an intact ecosystem without introduced Rattus species are not infected with T. lewisi while introduced rodents at urban sites are.
Small mammals were trapped at two study sites; an intact cool temperate rainforest in Dorrigo, and urban site in Wagga Wagga. Ten rodent tissue samples from an urban environment in Goroka, Papua New Guinea (PNG), were included in this study. Fleas were collected and morphologically identified. A polymerase chain reaction (PCR) protocol for vertebrate mitochondrial cytochrome b (cytb) and nested PCR for Trypanosoma species were performed on blood and tissue samples collected from rodents trapped in Wagga Wagga, rodents and antechinus trapped in Dorrigo, and rodent samples from PNG. DNA purification was performed on all cytb and PCR positive Trypanosoma samples, and specific identities confirmed with bidirectional Sanger sequencing.
T. lewisi was detected in black rats (n=2) in Wagga but not in native bush rats in Dorrigo (n=5). M. musculus (n=2) did not have trypanosomes, however both Antechinus stuartii (n=2) captured had a Trypanosoma sp. with 98-99% similarity to a species previously described in two haemadipsid leeches in Victoria, Australia. T. lewisi was detected in one rodent tissue from PNG. There was a significantly lower prevalence of infection of T. lewisi in native bush rats (0%, n=5) in forested habitat in Dorrigo than in introduced rats (100%, n=2) in an urban environment in Wagga Wagga (p=0.048). The findings from this study support the theory that T. lewisi was introduced with black rats subsequent to European colonisation, as it is significantly more prevalent in invasive rodents compared to the native rodents studied. Furthermore, this indicates the presence of barriers to dispersal of T. lewisi between introduced black rats and native bush rats. Understanding these barriers are essential in the management and mitigation of disease emergence in novel host species.