| Summary: | Since at least 2001, routine post mortems of deceased penguins from Penguin Island, Western Australia, have been conducted at Murdoch University Veterinary Hospital (MUVH). In late 2011 and early 2012, a cluster of 12 cases presented with similar and characteristic gross and microscopic changes, namely birds in good body condition with hepatomegaly and splenomegaly, multifocal hepatic and splenic necrosis and numerous, small, 1-2μm diameter protozoan parasites within the necrotic foci. A review of earlier reports in the MUVH archive identified isolated similar cases from 2006 and 2008, which had been provisionally diagnosed as Avian Malaria, that is, Plasmodium spp. infection. An investigation was established in order to: a) definitively identify the parasite causing the mortalities, with the additional aims of: b) evaluating the live population of Penguin Island Little Penguins for the presence of parasitaemia, and c) investigating the possibility that another bird species present on Penguin Island might represent a reservoir of infection. Ninety-four blood smears were made from 79 individual Little Penguins collected from winter to summer of 2012 and in the early spring of 2013. One smear identified intraerythrocytic organisms consistent with the blood stages of an apicomplexan parasite, for example, merozoites or early gametocytes of Haemoproteus or Plasmodium, or merozoites or sporozoites of Babesia. Fifty-one blood smears were made from 51 Bridled Terns (Sterna anaethetus) captured during two visits to the island in November 2012 and March 2013, with no parasites detected in these smears. Electron microscopy of the protozoan parasite identified it as belonging to the phylum Apicomplexa. Further identification to the level of genus was not possible. Molecular identification of the parasite using Polymerase Chain Reaction (PCR) methodology gave inconsistent results. PCR performed by an independent laboratory identified a novel Haemoproteus spp. organism in 4 of 10 cases from this group; however, these results could not be replicated in our laboratory. Additional PCR using a variety of primers aimed at detecting members of the Apicomplexa identified a parasite from the family Sarcocystidae, which was subsequently identified as Toxoplasma. Immunohistochemistry of formalin fixed tissues also identified Toxoplasma in the hepatic and splenic lesions. The distinctive mortalities which were observed in this group of penguins, and which have occurred sporadically since, appear to be attributable to a fulminant toxoplasmosis, with or without a concurrent haemoproteosis in some cases. The significance of the apparent polyparasitism in some of the birds is unknown, as the relative contribution of concurrent Haemoproteus infection to the lesion aetiopathogenesis cannot be quantified at this time. Though the clinical signs of infection are unknown, the gross and microscopic appearance at post mortem is sufficiently characteristic to allow a diagnosis to be made on these features. Definitive confirmation of infection may be made by immunohistochemistry or PCR.
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