The introduction of enteric multiplex PCR into several diagnostic microbiology laboratories in 2014 resulted in a fivefold increase in Shigella notifications in Victoria. Culture-positive cases increased from an average of 100 per year ( 2000 to 2013) to 276 in 2014, mainly due to greater test sensitivity along with enhanced reflexive culture. An additional 185 cases were notified without culture confirmation. The Shigella PCR test is not specific where a positive PCR result indicates the presence of Shigella or Enteroinvasive E. coli (EIEC). PCR positive results without culture confirmation has led to discrepancies in the reporting of cases for national surveillance and uncertainties for public health management of this notifiable condition.
To determine the impact of CIDT on the surveillance and response to Shigella we reviewed Victorian case and testing data from 2012 to 2016. Results from 2016 indicate a total of 600 notified cases; 130 culture-positive only, 135 both PCR and culture-positive and 335 Shigella/EIEC PCR positive only. The most common risk factors for acquisition were overseas travel (n=322) and male to male sexual contact (n=128) of which 72% and 13% respectively were diagnosed with PCR alone. The difference in testing methodology for these two groups is possibly due to the clinics they attend and the laboratories used by these clinics. Shigella is a potential CARAlert organism; reportable when multidrug resistance (MDR) is detected (resistant to three of Ampicillin, ciprofloxacin, co-trimoxazole or ceftriaxone/cefotaxime). Twenty one cases of MDR were notified during this five year period increasing from zero in 2012 to nine in 2016.
Reflexive culture is imperative for Shigella/EIEC PCR positive cases. In 2016 only 130 of 600 cases (22%) notified were diagnosed by culture alone. While PCR testing increases the sensitivity for detecting cases it diminishes the ability to define, serotype, sequence and determine the resistance patterns when culture is not performed.