Use of Cobalt-60 Irradiation for Inactivation of Protozoan Cysts

NON-TECHNICAL SUMMARY: This project addresses food safety with respect to protozoan contaminants on produce. Importation of produce from countries that have high incidences of certain protozoal diseases provides a mechanism for increased prevalence of these pathogens in the US to occur. The purpose of the project is to determine how effective irradiation of produce is for inactivating protozoal parasites that are infectious to humans.

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APPROACH: A calibrated cobalt-60 source will deliver varying doses of irradiation to Giardia lamblia cysts in water. Cysts will be inoculated into gerbils and infection will be assessed. The most likely killing doses will then be used to irradiate several types of produce that have been artificially contaminated with Giradia cysts. Cysts will be recovered from the produce and inoculated in gerbils, and infection with the parasite will be assessed. Similar testing will be performed with acidified sodium chlorite; final evaluation of the viability of treated cysts will use an animal model. Some microorganisms can recover from irradiation damage and regain infectivity. This phenomenon will be studied with Giardia cysts, and the window of time during which cellular repair occurs will be determined.

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PROGRESS: 2000/10 TO 2005/09<BR>
Numerous trials were conducted to discover an efficient method for decontamination of fresh produce or water that contains cysts of the human pathogen, Giardia lamblia, and thus reduce incidence of giardiasis (a zoonosis) in humans. Overall results of the project pinpointed specific parameters for complete killing of cysts of Giardia lamblia, i.e., doses of 0.5 kilo-Gray or higher, using gamma-irradiation. This dose rendered cysts incapable of initiating infections in animal model systems or in vitro systems. Produce (such as lettuce, tomato, and strawberry) or pure water that contained known concentrations of cysts was decontaminated when irradiated with 0.5 kilo-Gray cobalt-60. Doses of 0.25 kilo-Gray resulted in reduction of pathogen but not a complete kill in every case. Doses lower than 0.25 kilo-Gray are not useful for decontamination of water or food. It was also determined that vital stains and in vitro excystation are not accurate for determination of inactivation of Giardia cysts, but bioassays that use animal models yield reliable data. Other methods underestimate viability of irradiated pathogens and should not be used when determining parameters for disinfection of food destined for consumption by humans. It was determined that gerbils that are orally inoculated with cysts will become infected by 6 days post-inoculation and excrete viable cysts. At higher doses, irradiation produces cellular damage in Giardia cysts that is visible with electron microscopy. Cellular damage is mainly seen as degranulation and vacuolization of cytoplasm. Comet assays indicated that initial cellular damage at lower irradiation doses might be in disruption of DNA as seen by migration of broken strands in an electrophoretic field (i.e., before cellular damage is directly visible with electron microscopy).
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IMPACT: 2000/10 TO 2005/09<BR>
Import of contaminated produce, destined for consumption by humans, into the United States is an important problem. Several protozoal diseases of humans are transmitted via contaminated food and water. Some food-borne outbreaks in the US have been traced to importation of the parasite on uncooked food. In recent years, an additional concern has been transportation of gastrointestinal parasites to some types of US crops by an influx of migrant agricultural workers. Gamma-irradiation causes damage in many bacterial and protozoal species and therefore is potentially useful for treatment of contaminated food. Recommendations of doses that decontaminate three types of produce that contain cysts of Giardia lamblia is useful for reducing the chances of food-borne giardiasis in the US. Knowledge of the cellular level at which damage occurs is essential in evaluating safety of irradiation with respect to mutation