Severity of tissue damage caused by different Borrelia strains 30 days post infection. Each histopathologic parameter was assessed and scored separately by a semi-quantitative criteria-based scoring method . The average scores were used to reflect the overall severity for each group of mice. The differences in disease severity between B31 and SZ infected mice were significant in brain, heart, liver and spleen tissues (p < 0.05). The asterisks indicate samples whose values are statistically significantly different between B31 and SZ (*, P <0.05; **, P <0.01; ***, P <0.001). Error bars indicate standard deviations.