Detection, transport, and retention of Toxoplasma gondii oocysts in saturated sandy porous media: Influence of electrolytes and natural organic matter in flow-through systems

Authors: PULLANO, CHRISTIAN - Clemson University; GHORBANI, MASHA - Aix-Marseille University; MUTTY, TIM - Clemson University; GOUASMIA, SOHIB - Aix-Marseille University; L'OLLIVIER, CORALIE - Marseille University Hospital Timone; Dubey, Jitender; DUMÈTRE, AURÉLIEN - Aix-Marseille University; DARNAULT, CHRISTOPHE - Clemson University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2025
Publication Date: 4/13/2026
Citation: Pullano, C., Ghorbani, M., Mutty, T., Gouasmia, S., L'Ollivier, C., Dubey, J.P., Dumètre, A., Darnault, C. 2026. Detection, transport, and retention of Toxoplasma gondii oocysts in saturated sandy porous media: Influence of electrolytes and natural organic matter in flow-through systems. PLOS ONE. 21(4). Article e0347130. https://doi.org/10.1371/journal.pone.0347130.
DOI: https://doi.org/10.1371/journal.pone.0347130

Interpretive Summary: Food safety remains a worldwide problem. Among the pathogens transmitted from animals to humans, coccidia are a group of protozoans (single celled parasites) that affect human health and economic losses to farmers of massive magnitude. Coccidia in general have an environmentally resistant stage (oocyst) excreted in feces of certain (definitive) hosts. Humans and animals become infected by ingesting food and water contaminated with oocysts. Controlling transmission is technically difficult because commonly used disinfectants are ineffective in killing oocysts and oocysts can survive for many months even in harsh climates. The coccidia includes members of the genus Toxoplasma, Cyclospora, and Sarcocystis that can cause severe illness in humans. Toxoplasma is one of the most studied parasites because it infects one-third of humanity and virtually all warm blooded animals. Efficient and sensitive methods have been developed for the detection of the oocysts in the environment. Prior to redirection of USDA’s food safety research program, ARS scientists contributed to a research consortium seeking to understand how oocysts are transported in ground water and soil. Findings present a better understanding of the mechanisms and factors that influence the transmission of this zoonotic pathogen in soils and receiving waters. These results will be of interest to parasitologists and biologists. This research was completed before redirection of Toxoplasma research at ARS.

Technical Abstract: Understanding the transport and retention of Toxoplasma gondii oocysts through soils and into ground and surface water is essential for determining the risk this parasite poses to water resources and human health worldwide. We studied here how various naturally occurring groundwater solutions containing different types of organic compounds (fulvic and humic acids) and electrolytes (NaCl, MgCl2, CaCl2) at different concentrations can affect the transport and retention of oocysts in engineered-saturated silica sand columns subjected to continuous flow to simulate the movement of groundwater through an aquifer. Breakthrough curve results from the qPCR analysis were then compared to non-reactive tracer tests to determine parameters that govern the transport of oocysts in saturated porous media. Though breakthrough of oocysts was observed in all tested solutions, higher ionic strength and ion valency resulted in greater oocyst retention. When both organic matter and electrolyte solutions were added to the systems, the electrolyte solutions displayed a far greater influence on parasite retention when compared to the influence of the organic matter alone. Collectively, this study demonstrates the pivotal role of soil groundwater solution chemistry in both the transport and retention of this important zoonotic parasite.