Stress-driven stage transformation of Neospora caninum

Neosporiosis, a serious disease caused by the apicomplexan intracellular parasite Neospora caninum, is considered to be one of the most economically important diseases in cattle. It is associated with potentially serious complications such as abortion, stillbirth and maternal infertility. To survive in fluctuating physiological and immunological environments, N. caninum has evolved a diverse set of regulatory mechanisms that govern various adaptive responses. The most intriguing paradigm in N. caninum adaptive evolutionary biology is its ability to alternate between two phenotypically and functionally distinct stages within the host. Recent research has reinforced the notion that N. caninum tachyzoite-bradyzoite stage switching on and off is correlated with its ability to form dormant cysts. Knowledge of the specific mechanisms that govern the dynamics of N. caninum phenotype switching enables a better understanding of the pathogenesis of the disease and effective control measures to be identified. Herein we review the available knowledge relating to various aspects of stage interconversion in N. caninum, with particular focus on the stress-related hypothesis presumed to be involved in this event. Finally, we put forward the postulation that N. caninum uses stage interconversion as an adaptive process to cope with the hostile environment within the host and to ensure its continuity in nature.