Environmental transmission of Toxoplasma gondii: Oocysts in water, soil and food

Abstract

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 parasite's 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.

Keywords: Food; Oocyst; Soil; Toxoplasma gondii; Transmission; Water.

Fig. 1

Structure of a sporulated Toxoplasma gondii oocyst and molecular composition of the walls enclosing the sporozoites (red crescent structures). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

A Toxoplasma gondii oocyst transmission ‘tree’. The flux of environmental transport and infection with T. gondii starts with oocysts shed in cat feces that contaminate soil and/or water, and are subsequently transmitted to hosts (intermediate, paratenic and definitive). Green ovals and arrows represent different sources/scenarios of contamination/infections caused by ingestion of oocysts; pink ovals and arrows represent scenarios of transmission via bradyzoites (tissue cysts); and purple ovals and arrows represent human-to-human transmission caused by vertical (congenital), transfusional, or organ transplantation infections. Water is represented in blue with blue arrows depicting infection or contamination transmitted directly from water sources to hosts. Soil is represented in brown with brown arrows depicting infection or contamination transmitted directly from sources of soil to hosts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)