Use of a sentinel system for field measurements of Cryptosporidium parvum oocyst inactivation in soil and animal waste

A small-volume sentinel chamber was developed to assess the effects of envi ronmental stresses on survival of sucrose-Percoll-purified Cryptosporidium parvum oocysts in soil and animal wastes. Chambers were tested fur their ab ility to equilibrate with external chemical and moisture conditions. Sentin el oocysts were then exposed to stresses of the external environment that a ffected their viability (potential infectivity), as indicated by results of a dye permeability assay, preliminary laboratory experiments indicated tha t temperatures between 35 and 50 degrees C and decreases in sop water poten tial (-0.003 to -3.20 MPa) increased oocyst inactivation rates. The effects of two common animal waste management practices on oocyst survival were in vestigated on three dairy farms in Delaware County, N.Y., within the New Yo rk City watershed: (i) piling wastes from dairy youngstock (including neona tal calves) and (ii) spreading wastes as a soil amendment on an agricultura l field. Sentinel containers filled with air-dried and sieved (2-mm mesh) y oungstock waste or field soil were wetted and inoculated with 2 million ooc ysts in an aqueous suspension and then placed in waste piles on two differe nt farms and in soil within a cropped field on one farm. Controls consisted of purified oocysts in either phosphate-buffered saline or distilled water contained in sealed microcentrifuge tubes. Two microdata loggers recorded the ambient temperature at each field site. Sentinel experiments were condu cted during the fall and winter (1996 to 1997) and winter (1998). Sentinel containers and controls were removed at 2- to 4-week intervals, and oocysts were extracted and tested by the dye permeability assay. The proportions o f potentially infective oocysts exposed to the soil and waste pile material decreased more rapidly than their counterpart controls exposed to buffer o r water, indicating that factors other than temperature affected oocyst ina ctivation in the waste piles and soil. The effect of soil freeze-thaw cycle s was evident in the large proportion of empty sentinel oocysts. The potent ially infective sentinel oocysts were reduced to <1% while the proportions in controls did not decrease below 50% potentially infective during the fir st field experiment. Microscopic observations of empty oocyst fragments ind icated that abrasive effects of soil particles were a factor in oocyst inac tivation. A similar pattern was observed in a second field experiment at th e same site.