Mississippi is highest in the nation in cases per 100,000 population for Gonorrhea and Chlamydia not Syphilis. For the first two though yeah for whatever reason Mississippi has almost double the national average. Syphilis it's right at the national average.
The most recent CDC data is from 2012
2012 Syphilis cases by state
2012 Gonorrhea cases by state
2012 Chlamydia cases by state