Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2002
Publication Date: N/A
Interpretive Summary: Cryptosporidium, or Crypto as it is referred to in the popular press, is a pathogen that can threaten the quality of municipal drinking water. Dairies and new-borne calves have been identified as potential sources of this pathogenic microorganism. The transmissive form of Crypto is a microscopic sphere that contains and protects four infectious agents. Before these infectious spheres reach surface waters they are generally shed on soil in fecal material. This study's objective was to determine, under controlled laboratory conditions, how different soils, soil moisture, and soil temperature affect the length of time that these Crypto spheres can keep their infectious agents alive. Large numbers of Crypto spheres, that were purified from calf feces, were mixed with a clay soil, a silty soil, and a sandy soil. Each soil was kept either wet, moist or dry, and incubated at 4, 20, and 30ºC (39, 68, and 86ºF). The Crypto spheres were extracted from the soils and tested for ability to cause disease at 0, 22, 43, 84, and 156 days. Results indicated that survival of the infectious agents was greatest in the silty soil at 4ºC (39ºF). The soil moisture content had no effect. These results show that large numbers of Crypto spheres can remain infectious for months and threaten the quality of surface waters.
Technical Abstract: 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 30ºC), 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 X 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 4º and 20ºC, but not between 20º and 30ºC 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 20ºC; and at 30ºC 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.