title: Quantitative estimation of the viability of Toxoplasma gondii oocysts in soil.
abstract: Toxoplasma gondii oocysts spread in the environment are an important source of toxoplasmosis for humans and animal species. Although the life expectancy of oocysts has been studied through the infectivity of inoculated soil samples, the survival dynamics of oocysts in the environment are poorly documented. The aim of this study was to quantify oocyst viability in soil over time under two rain conditions. Oocysts were placed in 54 sentinel chambers containing soil and 18 sealed water tubes, all settled in two containers filled with soil. Containers were watered to simulate rain levels of arid and wet climates and kept at stable temperature for 21.5 months. At nine sampling dates during this period, we sampled six chambers and two water tubes. Three methods were used to measure oocyst viability: microscopic counting, quantitative PCR (qPCR), and mouse inoculation. In parallel, oocysts were kept refrigerated during the same period to analyze their detectability over time. Microscopic counting, qPCR, and mouse inoculation all showed decreasing values over time and highly significant differences between the decreases under dry and damp conditions. The proportion of oocysts surviving after 100 days was estimated to be 7.4% (95% confidence interval [95% CI] = 5.1, 10.8) under dry conditions and 43.7% (5% CI = 35.6, 53.5) under damp conditions. The detectability of oocysts by qPCR over time decreased by 0.5 cycle threshold per 100 days. Finally, a strong correlation between qPCR results and the dose infecting 50% of mice was found; thus, qPCR results may be used as an estimate of the infectivity of soil samples.
authors.affiliation: Université de Lyon, Lyon, and CNRS, UMR5558, Laboratoire de Biométrie et Biologie Evolutive, Bâtiment Mendel, Université Lyon 1, Villeurbanne, France.
authors.forename: Maud
authors.initials: M
authors.lastname: Lélu
authors.forename: Isabelle
authors.initials: I
authors.lastname: Villena
authors.forename: Marie-Laure
authors.initials: ML
authors.lastname: Dardé
authors.forename: Dominique
authors.initials: D
authors.lastname: Aubert
authors.forename: Régine
authors.initials: R
authors.lastname: Geers
authors.forename: Emilie
authors.initials: E
authors.lastname: Dupuis
authors.forename: Francine
authors.initials: F
authors.lastname: Marnef
authors.forename: Marie-Lazarine
authors.initials: ML
authors.lastname: Poulle
authors.forename: Cécile
authors.initials: C
authors.lastname: Gotteland
authors.forename: Aurélien
authors.initials: A
authors.lastname: Dumètre
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Gilot-Fromont
mesh_terms: D000818:Animals; D001681:Biological Assay / Q000379:methods; D017931:DNA Primers / Q000235:genetics; D051379:Mice; D033661:Oocysts / Q000502:physiology*; D016133:Polymerase Chain Reaction / Q000379:methods*; D012987:Soil / Q000469:parasitology*; D015996:Survival Rate; D014122:Toxoplasma
chemical_list: D017931:DNA Primers; D012987:Soil