Variance in Salmonella Serogroups and Serotypes between consistently high and low Incident Rate Counties in Florida, 2005 - 2010

BACKGROUND:  Florida has the highest number of Salmonella infections reported annually.   In addition, incidence rates are 1.9 times higher than the national average of 15.2 cases per 100,000 population. Previously reported data identified eleven counties with consistently high (CHC) (>48.4 cases/100,000 population) and eleven counties with consistently low (CLC) (<17.1 cases/100,000 population) incidence rates of salmonellosis from 2005-2010. Overall incidence rate ratio (IRR) between CHCs and CLCs was 2.8, and across demographic characteristics the IRR were similar.  The purpose of this analysis was to compare Salmonella serogroups and serotypes reported from CHCs and CLCs.

METHODS:   Reported cases of Salmonella infections acquired in CHCs and CLCs of Florida from 2005-2010 were aggregated by county and reported serogroups and serotypes were compared.

RESULTS:   From 2005-2010, CHC reported 6,399 cases of which 1,654 (n=26%) had serogroup and 306 (5%) had serotype testing performed. Serogroups D1 (26%), C2 (23%), and B (17%) represented nearly two-thirds of CHC isolates. Serotypes Javiana (39%) and Newport (17%) were most frequently isolated in CHC. CLC reported 7,929 cases of which 4,974 (63%) had serogroup and 259 (3%) had serotype testing performed. Serogroups B (37%), D (20%), and C (11%) also represented two-thirds of isolated for CLC cases. However, serogroup B isolates were far more common in CLC than CHC and D1 and C2 serogroups were less than 10% of all Salmonella isolates in CLC. Only serotype IV 50:z4,z23:-, formerly designated as serotype Flint, (24%) represented more than 10% of the serotyped isolates for CLC.

CONCLUSIONS:  CHC and CLC in Florida varied considerably in reported Salmonella isolate serogroup and serotype results. CLC had significantly more samples serogrouped than CHC and may have resulted in some bias towards rare strains being serotyped more frequently. Serotype IV 50:z4,z23:- (serogroup Z) is an exceedingly rare Salmonella strain nationally, as a result of this rarity this may have increased likelihood of being serotyped. The larger percentage of serogroups D1 and C2 and serotypes Javiana and Newport could indicate favorable environmental conditions in CHC resulting in increased rates compared to CLCs.  Serotypes Javiana and Newport have been associated with reptile exposures, compared to typical foodborne serotypes like Enteritidis, Typhimurium, and Heidelberg, and finding these environmental associations between strains and high incidence rates could help reduce incidence and morbidity. A more robust protocol for serotyping a larger proportion of reported Salmonella isolates could aid in determining the effects certain Salmonella strains drive increasing rates of salmonellosis in CHC.