In 2009, a survey was conducted in Reunion Island to determine the renal leptospiral load in black rats trapped in the field. The concentration of leptospires in kidney tissue was calculated using qPCR. Our results showed high inter-individual variations of renal bacterial load in naturally infected black rats (mean = 8.27 ± 4.72 log-genome copies per mg kidney tissue). The objective of this study was to model the renal leptospiral load in 50 naturally infected black rats as a function of sex, age, and weight. Statistical analysis by sex showed that, in naturally infected males, the renal leptospiral load was correlated with weight (p-value = 0.032). Moreover, our model showed that weight and sex were significant explanatory variables for the renal leptospiral load in naturally infected young black rats (R2 = 0.953). Laboratory experimentation could not replicate naturally acquired infection, but field studies also present many limitations. Our study is the first attempt to explain individual variations in the renal leptospiral load in naturally infected reservoir animals but further research is needed.