BACKGROUND: Vibriosis incidence in the US is increasing. Illness is more commonly reported among residents of coastal areas where shellfish consumption and seawater exposure is more common. In 2016, Vibrio case reports in Colorado increased substantially, prompting an assessment of the impact of culture-independent diagnostic tests (CIDTs) on surveillance. Vibrio culture requires selective media and often has not been included in standard stool testing, whereas CIDT panels most often adopted by clinical laboratories in Colorado include Vibrio. In Colorado, Vibrio-positive CIDT results (where reflex culture was negative or not performed) have been classified as suspect cases and investigated since 2014.
METHODS: We analyzed demographic, exposure, and laboratory data of Vibrio cases reported 2012-2016 and compared culture-confirmed with CIDT-positive suspect cases. We compared culture-confirmed cases reported from laboratories that continue to culture with those that have switched to CIDTs.
RESULTS: During 2012–2016, 65 culture-confirmed and 12 CIDT-positive suspect cases were reported. The incidence of confirmed cases per 100,000 increased from 0.19 in 2012‐2013 to 0.24 in 2014-2015 to 0.35 in 2016 and the incidence of suspect cases increased from 0 in 2012-2013, to 0.02 in 2014 to 0.05 in 2015 to 0.15 in 2016. The mean age of culture-confirmed and CIDT-positive suspect cases was similar (45 and 52, respectively, p=0.24), while confirmed cases were more often male as compared to suspect cases (77% vs. 25%, p<0.001). Among cases with a positive stool specimen (n=60), culture-confirmed cases were more likely than CIDT-positive suspect cases to report exposure to raw shellfish (89% vs. 25%, p<0.001). Culture-confirmed cases from clinical laboratories that have adopted CIDTs have increased from 1 per year in 2012-2013 to 4 per year in 2014-2015 to 12 in 2016. In contrast, culture-confirmed cases from clinical laboratories that continue to perform culture have been stable, with an average of 9 cases reported per year. Reflex culture was performed for 21 (91%) CIDT-positive results; Vibrio isolates were recovered from 11 (52%).
CONCLUSIONS: In Colorado, culture-confirmed Vibrio cases are increasing. This is likely due to an increase in testing associated with CIDTs. Isolate recovery for Vibrio is low and could be due to inadequate use of specialized media, organisms dying in transit, and/or the detection of non-viable organisms. Suspect cases reported less raw shellfish exposure. Additional information is needed to assess the value of complete case and environmental health investigations of CIDT-positive Vibrio cases given competing public health priorities.