The occult visceral phase of mammalian trypanosomes with special reference to the life cycle of Trypanosoma (Trypanozoon) Brucei

1. 1. We have reviewed the indirect evidence for suspecting the existence of an occult visceral phase in Trypanosoma (Trypanozoon) brucei. This evidence consists of:— 1.1. a) Inoculated trypomastigotes of Botswana strains (unlike “laboratory strains” adapted to rats) do not begin to divide and multiply immediately they reach the blood; neither do their numbers affect the height of infection nor the length of prepatent period. They are removed from circulation and are replaced by a new population with different morphology and behaviour. 1.2. b) This new population consists of agranular trypomastigotes which are flushed into the blood at 72 hours; they do not divide until they have been in the blood for some hours. 1.3. c) Trypomastigotes of the first flush (72 hours) and of the relapse (192 hours) we call “long-narrow” forms, they are succeeded by “long-flat” forms which are also agranular under phase contrast, but undergo binary fission. 1.4. d) The long-flat forms develop granules and are removed from the blood. 1.5. e) Multinucleate forms are seen occasionally in the peripheral circulation but their numbers are increased by immuno-suppression, they are also found in smears of internal organs. 1.6. f) Amastigotes and sphaeromastigotes (the “latent bodies” of earlier workers) have been seen in smears of internal organs; most of them show signs of degeneration. 1.7. g) The evidence of headache and associated hydrocephalus in sleeping sickness, also the swelling of the choroid plexus in experimentally infected rats, is reviewed. 2. 2. We have also reviewed the direct evidence of occult stages in different species of the genus Trypanosoma. In all subgenera of mammalian trypanosomes, with the exception of Duttonella and Nannomonas, amastigote and/or sphaeromastigote forms have been found. In Schizotrypanum these forms are intracellular, in Herpetomonas and Megatrypanum they are intravascular. In Trypanosoma (Trypanozoon) brucei the amastigote forms are found in large masses in the choroid plexus. 3. 3. We have proposed, as a working hypothesis, the life cycle shown in Fig. 3 for T. brucei in the vertebrate host. On this hypothesis we base our explanation of the synchronism of the initial infection and of the remission, also our view of the fundamental difference between Rhodesian and Gambian strains. 4. 4. In conclusion, we have discussed the following points:— 1. a) We think that the choroid plexus is the most important but not necessarily the only site for t