A Pseudo-Outbreak: Newly Implemented Gastrointestinal PCR Panels Reveal Elevated Baseline Levels of Yersiniosis in Nebraska Particularly during Late 2015

BACKGROUND: As rapid multiplexed, molecular testing platforms such as Gastrointestinal PCR Panels (GPPs) become more widely used, communicable disease surveillance and corresponding incidence rates previously calculated only on the basis of more traditional diagnostic testing methods such as culture, might be affected. Studies have documented increased rates of enteric pathogen detection by GPPs because of increased sensitivity compared with culture. In November 2015, Nebraska observed an apparent cluster of non-plague yersiniosis cases. In response, we investigated cases to establish if an outbreak had occurred or if the observed increase was surveillance artifact resulting from increased use of GPPs.

METHODS: Data from yersiniosis case investigations were compiled for Nebraska from 2005 through 2015 to establish annual numbers of incident cases by onset date and enumerate cases by diagnostic method (GPP only, culture only, or both [GPP positive test preceding reflex culture]). Additionally, we analyzed 2015 patient demographics and food histories, and data on laboratories actively reporting Yersinia enterocolitica during 2015 to establish number using GPPs by month of implementation.

RESULTS: During 2015, 17 yersiniosis cases were reported in Nebraska, representing a 425% increase above baseline annual number of reported cases during 2005–2014 (median/year = 4, range 3–7); no common exposures were identified. Of these, 7 (41.2%) were reported by culture alone and 10 (58.8%) by GPP (5 by GPP only and 5 by GPP with subsequent reflex culture). Of the latter, 60% (6/10) were detected either in November or December. Of 57 laboratories which had provided either culture- or molecular-dependent enteric disease reports in Nebraska during 2015, 7 (12.3%) reported these 17 yersiniosis cases, of which 3 had used GPP. Overall, 8 laboratories reported implementing GPP at some point during 2015. Of these, 6 (75%) had only implemented GPP during September or later.

CONCLUSIONS: Ten 2015 yersiniosis cases were diagnosed in Nebraska by a relatively small number of laboratories (n = 3) using a GPP. These cases would otherwise likely have been missed by traditional surveillance based solely on culture-dependent diagnostic methods. Our findings suggest this observed increase resulted from increased GPP usage. As GPP use further increases, some enteric disease incidence baseline numbers will inevitably change. Public health officials must be prepared to closely analyze surveillance data to determine whether observed changes in disease incidence are the result of true increases (i.e. outbreaks) or changes in diagnostic methodology and a transition to molecular testing platforms in clinical laboratories.