Evaluation of PFGE Cluster Investigations
Salmonella
Previous analysis of MDH Salmonella PFGE cluster data from 2001 through 2007 were published in an Emerging Infectious Diseases article entitled “Salmonella enterica Pulsed-Field Gel Electrophoresis Clusters, Minnesota, USA, 2001–2007.” In brief, clusters of >4 case isolates were more likely to be solved than clusters of 2 case isolates. In addition, clusters in which the first 3 case isolates were received at MDH within 7 days were more likely to be solved than were clusters in which the first 3 case isolates were received over a period >14 days.
This published study was recently updated with 5 additional years (2008-2012)of data, using the same methodology as the paper referenced above. The results of analyses using the cumulative data are presented here.
During 2000 – 2012, the median number of Salmonella isolates subtyped per year was 607 (range, 507 -743 isolates) (Figure 3). The median number of Salmonella clusters per year was 53 (range, 39 – 74 clusters). The median number of confirmed Salmonella outbreaks per year was 7 (range, 4 – 11 outbreaks). There were no statistically significant temporal trends in the proportion of Salmonella clusters that resulted in identification of a confirmed outbreak (p = 0.25).
Clusters of common Salmonella serotypes (Newport, Heidelberg, and Montevideo) and uncommon serotypes were both significantly more likely to be solved than clusters of the very common serotypes (Enteritidis and Typhimurium) (Table 1). In the previously published analysis, uncommon serotypes had an elevated odds ratio but did not reach statistical significance.
The probability of a cluster being solved increased significantly as the number of cluster cases increased. Clusters of 3, 4, and ≥5 cases were significantly more likely to be solved than clusters of 2 cases (Table 1). In the previously published analysis, clusters of >3 cases had an elevated odds ratio but were not significantly more likely to be solved when compared to clusters of 2 cases.
Clusters in which the first 3 cluster case isolates were received at the public health laboratory on the same day or with-in 7 days were significantly more likely to be solved than clusters in which the first three cluster case isolates were received during a period of more than 14 days (Table 1).
Investigating all 618 Salmonella PFGE clusters resulted in a cluster investigation positive predictive value (percentage of clusters investigated that were solved) of 14% (Table 2). Investigating PFGE clusters of >3 cases resulted in a cluster investigation positive predictive value of 22%; 20% of all Salmonella isolates were part of clusters involving >3 cases. Investigating PFGE clusters of >4 cases resulted in a cluster investigation positive predictive value of 29%; 15% of all Salmonella isolates were part of clusters involving >4 cases. Investigating PFGE clusters in which the first 3 cluster case isolates were received at the public health laboratory within 7 days resulted in a cluster investigation positive predictive value of 42%; 10% of all Salmonella isolates were part of clusters of this nature. Investigating Salmonella PFGE clusters with ≥4 cases or clusters in which the first 3 cluster case isolates were received at the public health laboratory within 7 days would have resulted in a cluster investigation positive predictive value of 31% (Table 2); 17% of all Salmonella isolates were part of clusters meeting these criteria.
Conclusion
The cumulative data indicate that clusters of >3 case isolates were significantly more likely to be solved than clusters of 2 case isolates. However, a bigger jump in percentage of clusters solved occurred with clusters of >4 cases. Clusters of common and uncommon PFGE subtypes were significantly more likely to be solved than clusters of very common PFGE subtypes (S. Typhimurium and S. Enteritidis), suggesting a greater need for additional molecular subtyping demonstration with the very common serotypes. Greater success in solving clusters also was observed with clusters in which the first 3 case isolates were received at the public health laboratory within 7 days.
Recommendation
If resources do not permit interviewing of all S. enterica cases, or investigation of all PFGE clusters, our data suggest that prioritizing clusters with >4 cases (or >3 cases if resources permit), and those in which the first 3 case isolates were received at a public health laboratory within 7 days, is a reasonable alternative. One caveat, however, is that these data are based on the MDH system, which includes interviewing of every case with a detailed questionnaire as soon as it is reported. It is not known how these data would (or would not) extrapolate to other systems. Another caveat is that by limiting investigation to clusters with >4 cases or clusters with densities <7 days, some outbreaks will go undetected.
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Abstract
We determined characteristics of Salmonella enterica pulsed-field gel electrophoresis clusters that predict their being solved (i.e., that result in identification of a confirmed outbreak). Clusters were investigated by the Minnesota Department of Health by using a dynamic iterative model. During 2001–2007, a total of 43 (12.5%) of 344 clusters were solved. Clusters of >4 isolates were more likely to be solved than clusters of 2 isolates. Clusters in which the first 3 case isolates were received at the Minnesota Department of Health within 7 days were more likely to be solved than were clusters in which the first 3 case isolates were received over a period >14 days. If resources do not permit investigation of all S. enterica pulsed-field gel electrophoresis clusters, investigation of clusters of >4 cases and clusters in which the first 3 case isolates were received at a public health laboratory within 7 days may improve outbreak investigations.