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Toxoplasma gondii is a coccidian parasite with the cat as its definitive host but any warm-blooded animal, including humans, may act as intermediate hosts. It has a worldwide distribution where it may cause acute and chronic toxoplasmosis. Infection can result from ingestion either of tissue cysts in infected meat of intermediate hosts or oocysts found in cat faeces via contaminated water or food. In this review, we highlight how the oocyst and sporocyst walls sustain the persistence and transmission of infective T. gondii parasites from terrestrial and aquatic environments to the host. We further discuss why targeting the oocyst wall structure and molecules may reduce the burden of foodborne and waterborne T. gondii infections.

Toxoplasma gondii is a zoonotic protozoan parasite that can cause morbidity and mortality in humans, domestic animals, and terrestrial and aquatic wildlife. The environmentally robust oocyst stage of T. gondii is fundamentally critical to the parasites success, both in terms of its worldwide distribution as well as the extensive range of infected intermediate hosts. Despite the limited definitive host species (domestic and wild felids), infections have been reported on every continent, and in terrestrial as well as aquatic environments. The remarkable resistance of the oocyst wall enables dissemination of T. gondii through watersheds and ecosystems, and long-term persistence in diverse foods such as shellfish and fresh produce. Here, we review the key attributes of oocyst biophysical properties that confer their ability to disseminate and survive in the environment, as well as the epidemiological dynamics of oocyst sources including domestic and wild felids. This manuscript further provides a comprehensive review of the pathways by which T. gondii oocysts can infect animals and people through the environment, including in contaminated foods, water or soil. We conclude by identifying critical control points for reducing risk of exposure to oocysts as well as opportunities for future synergies and new directions for research aimed at reducing the burden of oocyst-borne toxoplasmosis in humans, domestic animals, and wildlife.

Toxoplasma gondii oocysts are responsible for food- and water-borne infections in humans worldwide. They are resistant to common chemical disinfectants, including chlorinated products, presumably due to the structure and molecular nature of the oocyst wall but also the sporocyst wall. In this study, we used fluorescence microscopy and transmission electron microscopy to characterise the structure of both the oocyst and sporocyst walls, exposed to household bleach. Bleach removed the outer layer of the oocyst wall and the outer layer of the wall of sporocysts exposed due to rupture of the oocyst wall. The loss of the outer sporocyst wall layer was associated with a decrease in its autofluorescence, which can be linked to the degradation of dityrosine cross-link proteins, and loss of Maclura pomifera lectin-reactive glycoproteins. This study suggests that the inner layers of the oocyst and sporocyst walls are the main structures responsible for the resistance of the parasite to household bleach.

Abstract: Clinical toxoplasmosis in humans has been epidemiologically linked to the consumption of drinking water contaminated by Toxoplasma gondii oocysts. We evaluated killing of T. gondii oocysts after ultraviolet (UV) or ozone treatments by bioassay in mice and/or cell culture. A 4-log inactivation of the oocyst/sporozoite infectivity was obtained for UV fluences >20mJcm−2. In contrast, oocysts were not inactivated by ozone with an exposure (Ct) up to 9.4mgminl−1 in water at 20°C. In conclusion, UV treatment can be an effective disinfection method to inactivate T. gondii oocysts in drinking water, but ozone did not show promise in this research. [Copyright &y& Elsevier]

Abstract: Recent outbreaks of waterborne toxoplasmosis have stimulated the development of sensitive methods to detect Toxoplasma gondii oocysts in samples suspected to be contaminated. The immunomagnetic separation (IMS) have been standardised to detect waterborne protozoa, but it did not exist for Toxoplasma oocysts. In this study, we describe two monoclonal antibodies (mAbs 3G4 and 4B6) produced against the oocyst wall, and the incorporation of mAb 3G4 in an IMS procedure. We found that an indirect IMS method gave better mean recoveries than a direct one (69.4% and 25.2%, respectively). Dissociation of oocyst_magnetic bead complexes was greatly improved by using a 2% aqueous H2SO4 solution instead of a 0.1 N HCl solution (82.8% and 17.4%, respectively). With these parameters, mean recoveries of less than 1000 oocysts ranged from 44.6% to 82.9%, depending on incubating temperature and buffe r. Age of oocysts (1 or 12 months old) does not influence IMS performances. Results of this study indicate that the described IMS is an efficient technique to recover Toxoplasma oocysts. [Copyright &y& Elsevier]

We describe the use of a cesium chloride (CsCl) gradient as an improvement for the purification of Toxoplasma gondii oocysts from concentrated suspensions. After concentration by sucrose flotation, this technique gives a >96% recovery of very pure unsporulated or sporulated oocysts, but requires “fresh” oocysts (≤10 weeks of age). This material is suitable for biochemical and immunological analyses of environmentally resistant T. gondii oocysts. [Copyright &y& Elsevier]

Detection of Toxoplasma gondii oocysts in environmental samples is a great challenge for researchers as this coccidian parasite can be responsible for severe infections in humans and in animals via ingestion of a single oocyst from contaminated water, soil, fruits or vegetables. Despite field investigations, oocysts have been rarely recovered from the environment due to the lack of sensitive methods. Immunomagnetic separation, fluorescence-activated cell sorting, and polymerase chain reaction have recently shown promising use in detection of protozoa from complex matrices. Such procedures could be applied to T. gondii detection, if studies on the antigenic and biochemical composition of the oocyst wall are completed. Using such methods, it will be possible to assess the occurrence, prevalence, viability and virulence of T. gondii oocysts in environmental matrices and specify sources of human and animal contamination. [Copyright &y& Elsevier]