Previously unreported factors influencing intracellular infection by Trypanosoma cruzi in vitro were studied. Use of different temperatures to influence growth of extracellular flagellates led to recognition of the effects of varied temperatures of incubation upon the course of intracellular infection by the parasite. As determined with Brasil strain T. cruzi, a consistent response occurred when cultures of various tissues inoculated with organisms from NNN medium were compared at incubation temperatures of 26, 33, and 38°C. Although extracellular flagellates multiplied and persisted in the fluid at low temperatures, intracellular injection was minimal; infection of cells was best established at 33°C, at which temperature leishmanial forms multiplied and developed into trypanosomes, and new cells became infected in a progressive fashion; at 38°C the degree of intracellular infection was intermediate, leishmanial forms multiplied but remained as rounded L.D. forms, and no progressive infection of new cells occurred.
Two other strains of T. cruzi exhibited very limited pathogenicity in vitro. Variation in infectivity for cells by different morphologic stages of the parasite was suggested from results with inocula of blood trypanosomes, and with NNN flagellates of increasing age. Action of unheated normal rat and guinea pig sera upon organisms grown on NNN, and the failure of convalescent rat serum to influence intracellular infection, was noted.
The course of infection in vivo was not demonstrably affected by attempts to alter temperatures of mice infected with Brazil strain T. cruzi. However, evidence is presented suggesting that the temperature effect upon intracellular development of T. cruzi may be a significant mechanism in the parasite-host cell relationship.