Parasite Polymorphism May Serve to Enhance Fitness in Different Host Environments

Phenotypic expression of trans-sialidase (TS) by Trypanosoma cruzi (Silvio strain) is restricted to a subpopulation (20-30%) of broad trypomastigotes (TS⁺ parasites), while the larger subpopulation of slender trypomastigotes lacks TS expression (TS^- parasites). Rhodnius prolixus nymphs were infected with fractionated T. cruzi parasite populations (Silvio strain). Bugs ingesting TS^- parasites produced significantly higher parasite loads than bugs ingesting either TS⁺ parasites or unfractionated populations. The addition of 1.0 μg of TS monoclonal antibody (TCN-2)/ml of blood meal to TS⁺ populations increased parasite populations to levels comparable to those of TS populations. In contrast, the addition of exogenous TS to TS^- parasite blood meals significantly reduced parasite loads to levels comparable to bugs ingesting TS⁺ parasites. These results suggest that T. cruzi trypomastigote polymorphism may serve to enhance survival in different host environments.