Ecology of Cat-borne Parasitoses in Australia

Abstract

Cat-borne parasitoses have substantial impacts on livestock, wildlife and human health worldwide. Toxoplasma gondii, Sarcocystis gigantea and S. medusiformis are all cat-borne parasites that share similar biology and ecology, and cause the diseases toxoplasmosis and macroscopic ovine sarcocystosis. I aimed to study the ecology of these cat-borne parasitoses to develop a better understanding of what ecological factors influenced their ability to cycle within an ecosystem. However, it was first necessary to find a study ecosystem where these parasitoses thrived. Using abattoir surveillance data I mapped the occurrence of macroscopic ovine sarcocystosis in the skeletal muscles of sheep across South Australia. Sarcocystosis was highly clustered on Kangaroo Island compared to the South Australian mainland. Second, I investigated if Toxoplasma infection in sheep was associated with macroscopic ovine sarcocystosis to see if I could provide indirect evidence for the clustering of Toxoplasma infection in sheep on Kangaroo Island. Toxoplasma infection was highly prevalent in sheep on the island (56.8%) and was associated with macroscopic ovine sarcocystosis in the oesophagus, but not in skeletal muscles, at the animal- and farm-level. By surveying macropods on Kangaroo Island and the adjacent mainland, I showed that Toxoplasma infection was also higher in western grey kangaroos on the island (20.4%) than on the mainland (0%). This suggested that these parasitoses are well established and thrive on Kangaroo Island and that the island is an appropriate ecosystem in which to study the ecology of these cat-borne parasitoses. Pushing my mapping analyses further, I identified environmental characteristics positively associated with higher densities of sarcocystosis affected locations. The occurrence of sarcocystosis increased at locations with low soil pH and high clay content. I then examined the seroprevalence of Toxoplasma in rodents (Mus musculus and Rattus fuscipes), brushtail possums (Trichosurus vulpecula) and koalas (Phascolarctos cinereus) to explore the impact of the ecology of these species on their risk of infection. Toxoplasma seroprevalence in all species was found to be negligible, suggesting that the intermediate host’s lifespan, feeding ecology and niche influence the parasite’s ability to cycle. To investigate how much cat (Felis catus) abundance may explain the occurrence of these cat-borne parasitoses, I conducted a camera trap survey in both regions and estimated their relative abundance using a simultaneous standardised approach. Cat abundance on the island was estimated to be over ten times higher than that on the adjacent mainland. I suggest that high cat abundance is the primary reason for the high occurrence of cat-borne parasitoses in sheep and macropods on the island, although the ecology of the intermediate host likely influences the ability of the parasites to cycle in these populations. I recommend that the control of cats should be the most effective and acceptable intervention to control these two cat-borne parasitoses in ecosystems where they occur frequently.