Was infectious syphilis being misclassified in remote Australian outbreaks? Evidence that informed modification of the national case definition.

Objective: To assess the ability of the national case definition to identify infectious syphilis during an outbreak affecting predominantly Aboriginal and Torres Strait Islander people in a remote Australian region.

Methods: A retrospective case series study of all non-congenital syphilis cases in the region notified between 1 January 2009 and 31 December 2012 was performed. The national infectious syphilis case definition was compared with an expanded case definition derived from experienced clinician assessment and the definition proposed in the Interim Guidelines for the Public Health Management of Syphilis Outbreaks in Remote Populations in Australia from the Communicable Diseases Network Australia (CDNA).

Results: Two hundred and forty syphilis cases were notified, of which 44 (18.3%) were symptomatic. The national case definition classified 106 (44.2%) cases as infectious, compared with 182 (75.8%) using the clinician-derived expanded case definition and 165 (68.8%) by the interim guidelines case definition. Seven confirmed and 6 probable cases were diagnosed as a result of contact tracing of probable infectious cases identified using the expanded case definition.

Conclusions and implications: The national case definition for infectious syphilis applied in this remote Australian outbreak underestimated infectious cases when compared with experienced clinicians' evaluation by up to 76 cases (42%) and was inadequate to monitor the magnitude of a syphilis outbreak in such a setting. This may compromise surveillance and resource allocation decisions, and could reduce the capacity to interrupt transmission and contain an outbreak. A revised national case definition, informed by this analysis, was released by CDNA in July 2015.