Investigating the suitability of dogs and pigs as animal models for blastocystosis

Abstract

Blastocystis is an ubiquitous intestinal protozoon with a controversial pathogenesis and can colonise a wide range of species including humans, mammals, birds and reptiles. In the literature these species are collectively referred to as "Blastocystis", and that will be the convention throughout this document as well. In humans, it has been linked with non-specific gastrointestinal signs, non-gastrointestinal signs such as urticaria and recently irritable bowel syndrome. Major hurdles to determining the clinical significance/pathogenesis of Blastocystis are diagnostic limitations as it is a morphologically and genetically diverse organism, poor understanding of host-parasite relationship and the lack of a representative model to fulfil Koch's postulates.The principal aim of the project was identifying a suitable candidate for modelling human blastocystosis. This was defined as being a natural host of Blastocystis, meaning that the species had a high prevalence of Blastocystis infection with a predominant host-adapted subtype/s (ST) and the ability to harbour STs in common with humans. Once a candidate was identified, host/parasite interactions (e.g. intestinal location and pathology of Blastocystis, mucosal immune response) would be characterised to determine if this was comparable to the human infection.Dogs were identified as promising candidates as, in Brisbane, Queensland they have been shown to have a high Blastocystis prevalence and can harbour similar STs as in-contact humans. We utilised Polymerase chain reaction (PCR) to investigate the molecular epidemiology of Blastocystis in dogs in different geographical regions. We found a prevalence of 2.5%, 1.3% and 24% in dogs from Brisbane, a Cambodian village and stray dogs from India respectively. Stray dogs in India carried a diverse range of STs including 1, 4, 5 and 6, while dogs from Brisbane and Cambodia only carried ST1 and ST2 respectively. These differences in Indian dogs compared to the Brisbane/Cambodia dogs could reflect a closer proximity of the dogs to humans and other animals and their faeces. We concluded that dogs are unlikely natural hosts for Blastocystis but rather are transiently and opportunistically infected with a diversity of STs.iii Blastocystis is commonly reported in pigs, with most harbouring ST5 or ST1 and zoonotic transmission of ST5 from pigs has been proposed, therefore they were chosen as the next candidate. We studied the molecular epidemiology of pigs in two different geographic/environmental settings and also if they were zoonotic reservoirs by testing incontact humans for Blastocystis. Utilising PCR, it was found that Blastocystis prevalence in pigs from Southeast Queensland (SEQ) intensive piggeries and a Cambodian village was 76.7% and 45.2%, respectively, with all positive pigs harbouring ST5. 7.1% of pigs harboured ST1 and/or ST3 in addition to ST5, which are the two most common STs in humans. A minority (13.9%) of SEQ piggery staff harboured ST5, which is otherwise rare in humans, indicating possible zoonotic/r...