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A precise dose of gamma irradiation will be determined to attenuate Toxoplasma oocysts, as an initial step towards development of a vaccine to prevent cats from transmitting this food pathogen to meat producing animals. Although complete vaccine development exceeds the scope of this one-year project, ultimately we envision that vaccine baits for cats will be distributed on farms. However, vaccine baits for cats must not infect the meat of any non-target farm animals that may consume them, and this will first be investigated in mice. <P> Using the new Dubey strain of Toxoplasma, we will: <ol> <LI>Demonstrate the safety of irradiated oocysts in mice. Mice are an excellent proxy or model of food animals - like food animals, they are a natural intermediate host of Toxoplasma and are exquisitely susceptible to infection. Mice will consume irradiated oocysts, and four weeks later muscle and brain will be tested for the presence of encysted organisms using PCR.(Experiment 1) <LI> Establish the minimum dose of gamma irradiation of oocysts to prevent them from inducing cats to shed more Dubey strain oocysts.(Experiment 2) <LI>Demonstrate the efficacy of irradiated Dubey strain oocysts to vaccinate cats. Vaccinated cats and unvaccinated controls will be challenged by feeding them tissues of mice infected with a different strain of Toxoplasma. Cat feces will be monitored for fecal excretion of organisms using routine parasitological exams and PCR. (Experiment 3)
<P> Approach: In Experiment 1: The Dubey strain of Toxoplasma gondii oocysts will receive precise levels of gamma irradiation. Oocysts of each irradiation level will be administered to mice by gavage. The minimum irradiation dose that prevents infection of mice will be determined by PCR testing of mouse tissues 1 month after administration of oocysts. In Experiment 2: The Dubey strain of Toxoplasma gondii ocysts will receive precise levels of gamma irradiation. Oocysts of each irradiation level will be administered orally to cats, and thereafter cat feces will be monitored daily to detect excretion of Toxoplasma organisms. The minimum dose of irradition that prevents oocysts from inducing patent infections in cats will be determined, using fecal flotation examinations. In Experiment 3: Cats vaccinated against Toxoplasma gondii using the optimal protocol, as previously determined by the results in Experiments 1 and 2, will be challanged by ingesting the carcasses of mice that have been infected with an unrelated strain of Toxoplasma (Me49). Thereafter, cat feces will be examined daily using fecal flotation and coprologic PCR for the presence of Toxoplasma organisms. Results will be statistically compared with a positive control group of unvaccinated cats.