Use of the Historical Limits Method to Detect Increases in Primary and Secondary Syphilis, Arizona 2011-2014: An Exploratory Study.

BACKGROUND: Increasing numbers of reported primary and secondary (P&S) syphilis cases in the United States suggest the need for improved surveillance methods. An outbreak detection method using reported syphilis test results, which can be counted before the conclusion of a syphilis case investigation, could lead to timelier outbreak detection.

METHODS: The historical limits comparison method was used to compare the number of positive rapid plasma reagin results reported during 2011-2014 with data for the preceding 3 years. An outbreak alert was generated when the monthly count of positive rapid plasma reagin quantitative results was greater than the historical mean plus 2 standard deviations for 2 consecutive months.

RESULTS: Three outbreak alerts occurred during 2011-2014. The first alert occurred in December 2012 in Maricopa County (Phoenix area). Primary and secondary cases subsequently increased from 10 in January 2013 to 15 in March followed by 5 months of consecutive increases. A second alert was generated for Maricopa County in May 2014. Primary and secondary cases increased from 29 in May to 42 in July 2014. Reported cases remained elevated for approximately 7 months after the second alert. In December 2013, an outbreak alert occurred for Pima County (Tucson area). The number of reported P&S syphilis cases in Pima County increased from 6 in February to 15 in March. Counts of reported cases remained elevated for approximately 6 months after the alert.

CONCLUSIONS: Use of historical limits comparison method based on syphilis laboratory results can provide an outbreak alert before increases in reported cases of P&S syphilis.