Cryptosporidium parvum oocyst inactivation in three soil types at various temperatures and water potentials

The interaction between soil types, temperature, and soil water potential may have differential effects on the survival of Cryptosporidium parvum oocysts in the terrestrial environment. We examined the effects of three soil types (a silty clay loam, silt loam, and loamy sand), three temperatures (4, 20, and 30 °C), and three soil water potentials (−0.033, −0.5 and −1.5 MPa) on the inactivation kinetics of oocysts. Sentinel chambers were filled with air-dried and sieved soil, brought to the appropriate soil water potential, and inoculated with 2×106 freshly purified oocysts. The inoculated chambers were buried in the same bulk soil at the appropriate water potentials and incubated at one of the three temperatures. Triplicate chambers were removed from the bulk soil on days 0, 22, 43, 84 and 156. Sentinel oocysts were extracted, and assayed for potential infectivity by the dye permeability method. Oocysts suspended in sterile distilled water and incubated with the sentinel chambers were used as controls for the effect of temperature. The soil water potentials investigated did not affect oocyst inactivation at any temperature or with any of the three soil types. Rates of oocyst inactivation increased significantly between 4 and 20 °C, but not between 20 and 30 °C with the exception of oocysts incubated in the silty clay loam. Oocyst survival appeared to be significantly greater in the silt loam soil than in the two other soil types when incubated at 20 °C; and at 30 °C oocyst survival was significantly less in the silt clay loam than in the other two soil types. Rates of sentinel oocyst inactivation at all three soil water potentials were significantly lower than the control oocysts in water at the three test temperatures. Thus oocyst survival in soil was not affected by the water potentials between −0.033 and −1.5 MPa; it was affected by soil texture; but temperature appeared to be the factor most affecting oocyst survival. In the critical ambient range of temperature in temperate climates oocysts may survive for months in agricultural soil, and pose a threat to surface waters.