1. Department of Molecular and Comparative Pathobiology, Johns Hopkins University, Baltimore, Maryland, USA
2. Research Animal Resources, Johns Hopkins University, Baltimore, Maryland, USA
3. W. Harry Feinstone Department of Molecular Microbiology and Immunology, The Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA
4. Department of Epidemiology and Biostatistics, Texas A&M University, College Station, Texas, USA