Observations on the development and nature of pseudocysts and cysts of Toxoplasma gondii

Abstract

1. 1) Various opinions as to the mode of reproduction of Toxoplasma have been discussed. The general opinion exists that schizogony does not form part of the organism's life cycle as we know it, but there is considerable confusion regarding the nature of the membrane which is frequently to be demonstrated about masses of toxoplasms. Particularly confusing is the loose use of such terms as “pseudocyst” and “cyst” by many authors to describe almost any aggregation of toxoplasms in both acute and chronic infections. 2. 2) The behaviour of toxoplasms of three different, highly virulent strains was studied in mice, guinea-pigs, multi-mammate rats and canaries : no evidence of schizogonic division of Toxoplasma was seen in the study of the parasites in a wide variety of tissues from different animals at all stages of the acute disease. The sole mode of reproduction appears to be by longitudinal binary fission. On no occasion was any structure other than the membrane of the enclosing host cell seen to surround the masses of parasites in any tissue and at any stage of the acute infection. During such infections the host-cell membrane of heavily parasitized cells undergoes no apparent modification other than extreme stretching ; the resulting mass of parasites is essentially of a temporary nature and soon breaks down to liberate the toxoplasms. 3. 3) A study was made of the behaviour of Toxoplasma in asymptomatic, chronic infections using 11 strains of negligible pathogenicity in mice, rabbits and one young dog. There is an initial, low grade proliferation of the parasites (acute infection) — particularly within the cells of the lymphoid-macrophage system (e.g. peritoneal exudate). This phase is soon replaced by a much more prolonged period of binary fission in the deeper tissues. It is characterized by the formation of a highly elastic, tough membrane about the parasites and this was shown to be distinctly separate from that of any enclosing host cell. The resulting structures may persist in the host tissues for at least 5 years and probably longer. Their development was studied from the 8th day of infection up to 102 months in the central nervous system of mice and rabbits : similar bodies were studied in the lungs of mice and rabbits where their development was a constant feature of the chronic infection.They were sometimes found in the lumen of alveoli and bronchioles and were demonstrated in the lungs of a chronically infected dog. 4. 4) The terms “cyst” and “pseudocyst” are defined : evidence is offered to suggest that true cysts of Toxoplasma are produced in the chronic disease as distinct from the pseudocysts associated with the acute infection. 5. 5) The significance of the invariable presence of these cysts in association with the natural type of Toxoplasma infection (the chronic infection) is discussed — in particular the presence of the cysts in the lungs. It is suggested that they may play an important role in the natural transfer of the parasite from host to host.