Natalie Holt sees reminders nearly everywhere of the serious toll a years-long syphilis outbreak has taken in South Dakota. Scrambling to tamp down the spread of the devastating disease, public health officials are blasting messages to South Dakotans on billboards and television, urging people to get tested. Holt works in Aberdeen, a city of about 28,000 surrounded by a sea of prairie, as a physician and the chief medical officer for the Great Plains Area Indian Health Service, one of 12 regional divisions of the federal agency responsible for providing health care to Native Americans and Alas…