Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2002
Publication Date: 1/8/2002
Citation: Kato, S., Jenkins, M., Fogarty, E.A., Bowman, D.D. 2002. Effects of freeze-thaw events on the viability of Cryptosporidium parvum oocysts in soil. Journal of Parasitology. 88(4):718-722.
Interpretive Summary: Cryptosporidium parvum (crypto for short) is a pathogenic microorganism associated with water, and has been the cause of several large-scale outbreaks of human gastrointestinal illness world-wide. The infective form of crypto that is in the environment is the oocyst, a multi-walled spherical capsule that contains four infections agents. The oocyst of crypto is known to be resistant to environmental pressures, and can survive in soil in the field for many months. The source of crypto in the environment and in watershed is new-borne animals such as calves on dairy farms and cattle herds. The objective of this study was to determine the effects of freeze-thaw cycles on the inactivation of crypto oocysts in water and in various soil types and under conditions of various levels of soil moisture ranging from dry, moist, and nearly saturated. This work was a cooperative effort between the Agriculture Research Center, J. Phil Campbell, Sr., Natural Resource Conservation Center, and Cornell University. Experiments indicated that rates of inactivation of oocysts were greater in soil than in water and greater in dry soil than in moist or wet soil. Oocysts subjected to freeze-thaw cycles had inactivation rates not significantly different from those oocysts subjected to -10 ºC (14 ºF) under static conditions. The results indicated that 99% of oocysts exposed to soils that are frozen at -10 ºC (14 ºF) will become inactivated within 50 days whether or not freeze-thaw cycles occur. Information from this study is important to managers of municipal watersheds, extension agents, and ranchers themselves where soils are frozen or experience freeze-thaw cycles during the winter months, and who can improve old or develop new management practices designed to prevent crypto from contaminating surface waters.
Technical Abstract: The effects of freeze-thaw events on the inactivation of Cryptosporidium parvum oocysts in soil were examined. Oocysts were inoculated into distilled water in microfuge tubes or into small chambers containing soil whose water content was maintained at 3%, 43%, or 78% of container capacity. The chambers and tubes were then embedded in three soils from different aspects of a hillside landscape (Experiments 1 and 2), and three distinct soil types (Experiment 3) and frozen at -10°C. Containers were thawed every three days for a period of 24 hr in one to nine freeze-thaw cycles over a period of 27 days (Experiments 1 and 2), and one to five freeze-thaw cycles over 15 days (Experiment 3). Oocysts viability was measured using the fluorescent dyes 4'6-diaminidino-2-phenylindole (DAPI) and propidium iodide (PI). Oocyst rates of inactivation were, on the whole, greater in soils than in water, and greater in dry soil than in moist and wet soils. Soil type showed no effect on oocyst inactivation rates. Oocysts subjected to freeze-thaw cycles had inactivation rates not significantly different from oocysts subjected to -10ºC under static conditions. Our results indicated that 99% of oocysts exposed to soils that are frozen at -10ºC will become inactivated within 40 to 50 days whether freeze-thaw cycles occur or not. If freeze-thaw cycles occur in rapid succession, rates of inactivation may increase and shorten the days to reach 99% inactivation.