Trypanosoma rangeli--in vitro metacyclogenesis and fate of metacyclic trypomastigotes after infection to mice and fibroblast cultures.

High metacyclogenesis was induced when freshly-isolated Trypanosoma rangeli from humans were grown in a modified liver-infusion-tryptose medium and transferred into the medium overlaid on mouse fibroblasts at 27 degrees C in a 5% CO2 atmosphere. Such in vitro-generated metacyclic trypomastigotes could induce a significantly high and constant parasitemia in both ICR and SCID mice for a period of about a week but thereafter the parasitemia gradually decreased. Histological examination could not detect any tissue-forms of T. rangeli in various organs of SCID mice. On the other hand, two long-maintained stocks of T. rangeli produced lower metacyclogenesis and only latent parasitemia in both strains of mice. When these populations were incubated in fibroblast cultures at 37 degrees C in a 5% CO2 atmosphere, only trypomastigotes survived for two to three weeks without proliferation, while other forms, mainly epimastigotes, soon began to swell and degenerate. Electron microscopy showed that most surviving trypomastigotes had the basket-like conformation of the kinetoplasts. This is characteristic of the non-dividing trypomastigote stage of T. cruzi, and suggests that T. rangeli trypomastigotes may survive long periods in the blood without proliferation.