Invasive animals and the Island Syndrome: Parasites of feral cats and black rats from Western Australia and its offshore islands

Introduced animals impact ecosystems due to predation, competition and disease transmission. The effect of introduced infectious disease on wildlife populations is particularly pronounced on islands where parasite populations are characterised by increased intensity, infra-community richness and prevalence (the “Island Syndrome”). This thesis studied parasite and bacterial pathogens of conservation and zoonotic importance in feral cats from two islands (Christmas Island, Dirk Hartog Island) and one mainland location (southwest Western Australia), and in black rats from Christmas Island. The general hypothesis tested was that Island Syndrome increases the risk of transmission of parasitic and bacterial diseases introduced/harboured by cats and rats to wildlife and human communities. To investigate the Island Syndrome, necropsies were performed on feral cats and black rats and the macro parasites identified were collected and quantified to ascertain parasite prevalence, infra-community richness and intensity. On Christmas Island, it was determined that 92% of feral cats and 84% of rats harboured helminth parasites with an infra-community richness of 0-6, and 0-7, species in cats and rats, respectively. A high intensity (number of individual parasites recovered per host) was observed for some parasite species. These findings demonstrated that three epidemiological characteristics (high prevalence, infra-community richness and intensity/abundance) conformed to the characteristics of the Island Syndrome. However, contrary to the Island Syndrome hypothesis, a high regional richness of parasites was observed on Christmas Island, with nine species of helminth recorded in cats and 10 species in rats). The parasite community characteristic observations were repeated on Dirk Hartog Island, which also exhibited the same three characteristics of Island Syndrome (high prevalence, infra-community richness and intensity/abundance), but where no difference in regional richness was observed compared with the mainland environment. Specifically, the overall prevalence was significantly higher (p≤0.01) on Dirk Hartog Island (100%) compared to southwest WA (79.6%), as was mean infra-community richness (p≤0.001) (3.61±1.41 on Dirk Hartog Island and 1.57±1.29 from southwest WA). For those parasite species occurring on Dirk Hartog Island and in southwest WA, the prevalence and abundance was found to be significantly higher on Dirk Hartog Island than the southwest WA (p≤0.019 and p≤0.003, respectively). These findings suggest that not all facets proposed by the Island Syndrome hypothesis apply to all island environments, particularly for parasite communities harboured by invasive species. Parasites of both zoonotic and conservation significance were detected in the cats and rats from both islands and from mainland Western Australia. Pathogenic bacteria of public health importance were identified; two species of Bartonella in rats (Bartonella phoceensis and an unidentified Bartonella species) on Christmas Island, two species Bartonella in cats (B. henselae and B. koehlerae) from southwest Western Australia, and Leptospira interrogans from both cats and rats on Christmas Island. The presence of Trypanosoma in cats and rats (from all three locations) and Leishmania (Christmas Island only) were investigated, with neither of these vector-borne protozoans identified at any of the locations. In summary, this thesis presents new data pertaining to parasite community structures in two invasive mammalian pest species of global importance following their introduction to islands, and the potential relationship between their parasite community structures and parasite biology, prevailing physiographic factors and faunal biology. The observations suggest that cats and rats are important in contributing to and maintaining artificially elevated parasite species’ richness within both insular and mainland environments. The findings also highlight potential threats that invasive animals pose with respect to disease transmission to susceptible ecological communities, in particular insular ecosystems, as reservoir hosts for parasitic and bacterial organisms.