Small vessels showing inflammatory cell infiltrates were invariably observed in the vicinity of intact Toxoplasma tissue cysts within the brains of mice congenitally infected with the protozoan. Lymphocytes were observed in intimate contact with the luminal aspect of the endothelium, penetrating into the thickened basal lamina and in the perivascular area, which also contained macrophages and neutrophilic granulocytes. Rarely, lymphocytes were observed attached to the outer membrane of the host neurone which contained a Toxoplasma tissue cyst and within the inflammatory infiltrate associated with a disintegrating cyst. An hypothesis is presented which combines these morphological observations to explain the events associated with tissue cyst associated recrudescence of latent Toxoplasma infection in mouse brain.

A Besnoitia species of the teiid lizard Ameiva ameiva (L.), from north Brazil was established in laboratory mice and hamster by the intraperitoneal inoculation of bradyzoites in the tissue cysts. In the lizards all the cyst wall layers were closely apposed. In the mice the layers of the wall were distinguishable, and ultrastructurally the inner cytoplasmic layer contained either a tight network of endoplasmic reticulum or packed mitochondria or both. These components were less frequent or sparse in the inner cytoplasmic layer of cysts in the lizard. The only animals available for experiments in attempts to indicate the definitive host of the parasite were 3 kittens of the domestic cat and a juvenile specimen of the snake Boa constrictor raised in captivity. No evidence of infection could be detected in these animals after feeding them with the tissues of mice harbouring cysts with very large number of bradyzoites.

The persistence of Toxoplasma gondii tissue cysts in organs of cats (definitive host) and rodents (intermediate hosts) was studied. Nine cats, 12 rats, and 12 mice were fed T. gondii oocysts and their organs were digested in pepsin and then bioassayed for bradyzoites in mice. Of 9 cats killed 37 or 51 days after feeding 102 (2 cats), 103 (3 cats) or 104 (4 cats) oocysts of the VEG strain, tissue cysts were found in each cat; in the tongue of 9, in the heart of 5, in the brain of 4, and in the eyes of 1 cat. The dose had no effect on the distribution of tissue cysts in cats. Twelve rats were each fed 105 oocysts of the VEG strain of T. gondii and killed 21, 29, 64 or 237 days later. At each time-period, 11 tissues of 3 rats were pooled and bioassayed in mice. Tissue cysts were found in the brain, skeletal muscle, heart and kidneys of rats at each killing time; in the lungs, intestines, and mesenteric lymph nodes in 3 of 4 instances; in the tongue, liver, and eyes in 2 instances and in the spleen in 1 instance. Also, using the same procedures and sampling the same 11 tissues as used for rats, tissue cysts were seen in all organs except in the tongue and liver of 3 mice killed on day 82 after feeding the VEG strain. In 9 mice (3 with each strain) fed oocysts of the ME-49, GT-1, or P89 T. gondii strain and killed 62–130 days later, tissue cysts were found consistently only in the brain. Thus, in rats and mice, most tissue cysts were found in the brain and rarely in the tongue. This was in marked contrast to the distribution of tissue cysts in cats.