The cyst form of Toxoplasma gondii has been implicated as a cause of recrudescence of the latent infection in congenitally infected patients and in the immunocompromised host. A method was developed to evaluate the effect of drugs on the cyst form of the parasite and used to evaluate a variety of therapeutic agents. The most active compounds against the cyst form in vitro were arprinocid-N-oxide, azithromycin, and the hydroxynaphthoquinone 566C80.

The in vitro and in vivo activities of the hydroxynaphthoquinone 566C80 against the cyst form of Toxoplasma gondii were evaluated. In vitro treatment (100 ,Ig of 566C80 per ml for 3 days) of cysts isolated from brains of mice infected for 1, 2, 3, 4, or 9 months resulted in loss of viability of the cysts and did not reveal any influence of the duration of in vivo infection on sensitivity to the drug. In vivo experiments to determine the effect of prolonged treatment with 200 mg of 566C80 per kg of body weight per day on cysts in brains of CBA/Ca mice infected with strain ME49 revealed a steady and significant decline in the numbers of cysts compared with the numbers in untreated controls. Histopathology of brains from control mice revealed inflammatory infiltrates around capillaries and in the parenchymas and meninges which were consistently less evident in the brains of treated mice. In addition, cysts were rarely observed in treated mice, whereas extensive inflammation and large numbers of cysts were found throughout the entire brain in control mice infected for the same period. The reduction in the numbers of cysts was evident as early as day 5 of treatment but was more marked at 8 weeks of treatment. The numbers of cysts in the brains of Swiss Webster mice infected for 3 or 6 months also significantly decreased following treatment for 15 or 30 days with the same dose of 566C80. Our results indicate that 566C80 has excellent activity against cysts of T. gondii both in vivo and in vitro and that sensitivity of the cysts to 566C80 is not affected by the duration of the infection in vivo.

The pathological changes, host-parasite relationship and structure of the tissue cysts in the brains of mice chronically infected with four different strains of Toxoplasma gondii were examined by light and electron microscopy. In mice infected with the mouse-adapted ME49 strain for 4, 8, 12, 16 and 25 weeks, the pathological changes consisted of moderate to severe meningitis and cuffing of blood vessels by inflammatory cells. At 4 weeks post-infection (p.i.), lymphocytes were the major cell type, but at later time points, plasma cells predominated. Large numbers of cysts were observed at between 4 and 12 weeks p.i., with a decrease being seen at 16 weeks p.i. Microglial nodules, many containing tachyzoites or bradyzoites, were present at all time points. In contrast, the three strains isolated from patients with the acquired immune deficiency syndrome (AIDS) resulted in no meningitis in two cases (DEY, DAG) and in mild meningitis in one case (WIL), although all three showed some cuffing of blood vessels. In addition, only very low numbers of cysts and nodules were observed. Ultrastructurally, the cysts of all four strains were seen to be located within host cells. The cysts of the ME49 strain differed from those of the other strains in that a proportion contained immature and dividing bradyzoites at all time points, whereas those of the other strains contained only mature bradyzoites. From the observation of nodules with parasites and cysts with immature zoites, it would appear that the ME49 strain may result in an unstable chronic infection with a continuous turnover of cysts, a feature that should be taken into consideration when this strain is used as an experimental model of chronic toxoplasmosis.