Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 10, 2000
Publication Date: N/A
Interpretive Summary: Infections by the single celled parasite Toxoplasma gondii cause mental retardation and loss of vision in children and abortion in livestock. Humans become infected by ingesting T. gondii (bradyzoites) in uncooked meat and by ingesting food and water contaminated with the resistant stage of Toxoplasma (oocyst). A scientist at the Beltsville Research Center has found that a cat can shed millions of oocysts after ingesting as few as 1 bradyzoite. These findings will be of interest to biologists, parasitologists, veterinarians and public health workers.
Infectivity of bradyzoites of the VEG strain of Toxoplasma gondii was compared in cats and mice. For this, tissue cysts were separated from brains of infected mice using a Percoll gradient and bradyzoites were released by incubation in acidic pepsin solution. After filtration through a 3-æm filter, bradyzoites were counted and diluted 10-fold in RPMI tissue culture medium. Dilutions estimated to have 1, 10, 100, and 1,000 bradyzoites were fed to cats and inoculated into mice, orally or subcutaneously (s.c.). Three experiments were performed. In Experiment 1, 2 of 2 cats fed 1,000 bradyzoites, 1 of 2 cats fed 100 bradyzoites, 1 of 4 cats fed 10 bradyzoites, and 1 of 4 cats fed 1 bradyzoite shed millions of oocysts; 1,000 bradyzoites were infective to all 4 inoculated mice s.c., but not to 4 mice inoculated orally, and 100 bradyzoites were infective to 2 of 4 mice injected s.c. but not to 4 mice inoculated orally. All 16 mice e(8 oral, 8 s.c.) injected with 1 or 10 bradyzoites were negative for T. gondii. In Experiment 2, 1 of 4 cats fed 10 counted bradyzoites shed oocysts; the same inocula was not infective to 4 mice injected s.c. In experiments 3, 3 of 4 cats fed 1,000 bradyzoites shed oocysts and the inocula was infective to 10 of 10 mice s.c., and 4 of 10 mice orally, 4 of 4 cats fed 100 bradyzoites shed oocysts and the inocula was infective to 6 of 10 mice s.c. and 0 of 10 mice orally; 10 bradyzoites were not infective to cats and mice. Results indicate that bradyzoites are more infective to cats than to mice and cats can shed millions of oocysts after ingesting just a few bradyzoites.