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Epidemiology poster session 4: Methodological aspects: Outbreak evaluation
P1-S4.11 The use of the historical limits method of outbreak surveillance to retrospectively detect a syphilis outbreak among American Indians in Arizona
  1. M Winscott,
  2. A Betancourt,
  3. R Ereth
  1. Arizona Department of Health Services, Phoenix, USA


Background In April 2007, an Indian Nation located in Southern Arizona declared an outbreak of syphilis among its tribe members. The Arizona Department of Health Services Sexually Transmitted Diseases Control Program (ADHS STDCP) was first alerted by the tribe about the increase in syphilis cases during January 2007, 6 months after the occurrence of the first case in the outbreak. At the time, the ADHS STDCP did not have a method in place to monitor surveillance data for the detection of an outbreak of any sexually transmitted disease occurring within the state.

Methods In January 2009, the Arizona Department of Health Services STD Control Program developed a syphilis outbreak detection system based upon a Historical Limits Comparison Method (HLCM) to monitor reported syphilis-related labs. The Southern Arizona Indian Nation outbreak was then retrospectively evaluated using the state surveillance database and the HLCM outbreak detection system.

Results Retrospective analysis of the HLCM system of the syphilis outbreak in Arizona indicates that, had this system been in place at the outset of the outbreak, the ADHS STDCP would have been alerted to the increase in syphilis-related labs reported from the tribal Indian Health Service centers between 10 and 12 weeks earlier.

Conclusions The use of the HLCM system of syphilis outbreak detection in Arizona would have alerted the state health department earlier to an increase in syphilis cases occurring on the tribal lands in Southern Arizona. Earlier awareness of the increase in cases would have allowed for earlier intervention and collaboration with the tribe to control the outbreak. The ADHS STDCP has since instituted this method of outbreak detection among identified high morbidity or high risk surveillance sites for both syphilis and gonorrhoea.

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