MMP2 and infection: The relative expression levels of MMP-2 mRNA at 0 d, 2 d, 7 d, 14 d, and 28 d after infection in HSK mouse model were 1.00±0.00, 1.59±0.04, 2.02±0.15, 2.70±0.26, and 1.99±1.86, respectively.