eLife assessment
For decades it has been accepted that only the growth-arrested "stumpy" form of Trypanosoma brucei can infect the arthropod vector, the Tsetse fly, but this was recently challenged by a demonstration that - under artificial conditions that are known to enhance infectivity - the proliferative "slender" form can also establish Tsetse infections. The infectiousness of the two forms is a fundamental question in trypanosome biology and epidemiology, concerning both infection dynamics and parasite differentiation. The authors of the current study provide independently compelling evidence that without artificial enhancement, the "stumpy" form is indeed much more infective for Tsetse than the slender form. The authors suggest that this is probably also true in the wild, but the precise reason for the discrepancy in results between the two laboratories remains unclear.