Feline Toxoplasmosis from Acutely Infected Mice and the Development of Toxoplasma Cysts*

Abstract

The development of Toxoplasma cysts was studied in mice inoculated with tachyzoites by several routes. After 1-30 days of infection, murine tissues were examined microscopically, and portions of whole carcasses were fed to mice and cats. The feces of the cats were examined for oocyst shedding. Cyst-like structures containing distinct PAS[periodic acid-Schiff]-positive granules were first seen after 3 days of infection with tachyzoites, and became numerous by 6 days. Argyrophilic walls were first seen after 6 days, and became numerous by 16 days of infection with tachyzoites. Prepatent periods to oocyst shedding (PPO) were either short (3-10 days), or long (19-48 days). The short PPO was found only in cats that had ingested mice infected for 3 days or longer, and was related to the development of PAS-positive granules in T. gondii, and to high (60-100%), oral infectivity rates for cats. The long PPO followed the ingestion of mice infected for only 1-2 days, and was related to tachyzoites without distinct PAS-positive granules and low (.ltoreq. 32%) infectivity for cats. The long PPO followed ingestion of oocysts and the parenteral inoculation of tachyzoites, bradyzoites or sporozoites. Use of the short PPO as a criterion for detecting cysts in tissues, showed that numerous cysts developed in mice 5 days after inoculation with tachyzoites, 7-9 days after inoculation with cysts, and 9-10 days after inoculation with oocysts. Cysts developed faster and more frequently in the brain and muscle than in lungs, liver, spleen and kidneys of mice inoculated with tachyzoites.