Computational image analysis reveals the structural complexity of Toxoplasma gondii tissue cysts

Toxoplasma gondii is an obligate intracellular parasite infecting up to one third of the human population. The central event in the pathogenesis of toxoplasmosis is the conversion of tachyzoites into encysted bradyzoites. A novel approach to analyze the structure of in vivo -derived tissue cysts may be the increasingly used computational image analysis. The objective of this study was to quantify the geometrical complexity of T. gondii cysts by morphological, particle, and fractal analysis, as well as to determine if and how it is impacted by parasite strain, cyst age, and host factors. Analyses were performed on 31 images of T. gondii brain cysts of four type-2 strains (the reference Me49 strain and three local isolates, named BGD1, BGD14, and BGD26) using ImageJ software package. The parameters of interest included diameter, circularity, relative particle count ( RPC ), fractal dimension ( FD ), lacunarity, and packing density ( PD ). Although cyst diameter varied widely, its negative correlation with RPC was observed. Circularity was remarkably close to 1, indicating that the shape of the brain cysts was a perfect circle. RPC , FD, and PD did not vary among cysts of different strains, age, and derived from mice of different genetic background. Conversely, lacunarity, which is a measure of heterogeneity, was significantly lower for BGD1 strain vs. all other strains, and higher for Me49 vs. BGD14 and BGD26, but did not differ among Me49 cysts of different age, and derived from genetically different mice. This study is the first application of fractal analysis in describing the structural complexity of T. gondii cysts. Despite all the differences among the analyzed cysts, most parameters remained conserved. Fractal analysis is a novel and widely accessible approach, which along with particle analysis may be applied to gain further insight into T. gondii cyst morphology.