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The epidemiology of Sarcocystis in Western Australia

Savini, Giovanni (1994) The epidemiology of Sarcocystis in Western Australia. PhD thesis, Murdoch University.

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This work was designed to determine the prevalence of infection with Sarcocystis in a variety of domestic and non-domestic animals in Western Australia (WA), identify factors associated with infection, examine new diagnostic methods and to investigate the effect of experimental infection in animals which had naturally been exposed to the parasite.

Climate and management practices were found to significantly affect the prevalence of infection with Sarcocystis. The dry climatic conditions of WA, in particular the fluctuations in daily ambient temperature were found to be the major climatic factor limiting the prevalence of infection.

As the prevalence of infection in the State was relatively low, it was possible to determine which management factors were predisposing to the infection and which were protecting it from spread. The analyses of these factors revealed that owning pet animals did not necessarily imply infection with Sarcocystis in the herds. However, feeding these dogs raw meat and allowing their contact with domestic farm animals significantly increased the likelihood of infection with S. cruzi in cattle. Inadequate disposal of bovine carcases was also found to enhance the risk of infection with Sarcocystis in cattle. This may also account for the high proportion of foxes (42%) found to pass sporocysts in their faeces. Foxes, therefore may play an important role in spreading Sarcocystis in WA.

Fifty two percent of cattle were found to be infected with Sarcocystis. Sarcocystis cruzi was found to be the most predominant species (99%) in WA cattle. It would appear that bovine sarcocystosis in WA usually occurs as a chronic or low grade infection. However, the postal survey highlighted the fact that nearly one third of the abortions occurring annually in infected herds could be associated with the presence of this parasite.

An isolate of S. cruzi from WA was shown to be highly pathogenic to pregnant cows even though they had previously been exposed to a natural infection. Conversely, isolates of S. tenella/arieticanis were not able to induce signs of acute disease in naturally infected ewes. However, reproductive failure and premature birth were observed in both cattle and sheep. A meront found in the CSF of a newborn calf suggested that Sarcocystis could invade the foetus, however in this work, as congenital sarcocystosis was not seen, it is likely that abortion occurred as a result of a systemic effect upon the dams.

A new ELISA, that used antigens produced from merozoites of S. cruzi grown in vitro, was developed and tested to diagnose infection in naturally infected animals and to study the kinetics of different classes of immunoglobulins in cattle and sheep after experimental infection. This assay proved to be a useful and reliable tool for general sero-epidemiological investigations. It was also able to detect increases in the levels of IgG in animals naturally infected with Sarcocystis and challenged with S. cruzi, earlier and more accurately than did existing serological assays.

Merozoites of S. cruzi grown in vitro retained their ability to infect the natural intermediate host complete their life cycle in vivo and induce an immune response. When merozoites of S. cruzi grown in vitro, were inoculated into pregnant ewes reduced weight gain, premature deliveries and an increase perinatal mortality were observed. Meront-like organisms were also seen in several organs of the infected ewes. This result indicates that in nature the failure of S. cruzi\o infect sheep may be related to some impediment on the development of this species during the first phase of its life cycle in the host.

In conclusion, the findings of this thesis have shown that infection of Western Australian sheep and cattle with the suitable species of Sarcocystis may limit the production and performance of these animals.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Dunsmore, John and Robertson, Ian
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