Limitations of pulsed-field gel electrophoresis for the routine surveillance of Campylobacter infections.

To the Editor—Olsen et al. [1] recently provided compelling evidence that an infected food handler was the source of an outbreak of Campylobacter infections at a school luncheon. Their thorough review of events leading to this outbreak and their comparison with the only other reported transmission of Campylobacter species by a food handler reasonably explained both how the outbreak occurred and why such outbreaks are apparently so rare. The investigation of this outbreak was greatly aided by the use of pulsed-field gel electrophoresis (PFGE) to distinguish outbreak-associated cases from epidemiologically unrelated cases in the community. National standardized methods could allow investigators to link seemingly unrelated outbreaks to a common food source, as has been done with outbreaks of Salmonella and Shigella species and of Escherichia coli O157:H7. However, the usefulness of PFGE for this purpose does not justify the authors’ recommendation that Campylobacter species be added to the national molecular subtyping network, PulseNet. The usefulness of PulseNet as a national surveillance system to enable rapid detection and investigation of outbreaks is based on the routine submission and subtyping of isolates, with epidemiologic follow-up of cases, as we have demonstrated for both E. coli O157:H7 and Salmonella enterica serotype Typhimurium [2, 3]. Since there are relatively few Campylobacter outbreaks each year, the likelihood of PulseNet being used to identify a common source for these reported outbreaks would be very small. In particular, PulseNet would have contributed nothing to the outbreak investigation reported by Olsen et al. [1]. To be useful in improving surveillance of Campylobacter infections, molecular subtyping would need to be applied to all sporadic isolates, not just to known outbreaks. Because Campylobacter species are the most commonly reported cause of bacterial diarrhea, the additional workload to incorporate Campylobacter species into PulseNet would be large, rather than minimal as suggested by Olsen et al. [1]. In 1994, the Public Health Laboratories of the Minnesota Department of Health began to evaluate the use of PFGE to subtype Campylobacter isolates. Fifteen isolates from 2 unrelated outbreaks that occurred in 1993 were tested initially. The PFGE patterns for these outbreaks were different, and all isolates within each outbreak matched. To determine whether molecular subtyping could enhance outbreak detection for Campylobacter species, all isolates submitted to the Minnesota Department of Health in 1994 were tested by PFGE using Sma1 and were grouped by exact match for epidemiologic analysis following methods used for both E. coli O157:H7 and Salmonella Typhimurium [2, 3]. Isolates were obtained from 673 (72%) of 941 cases of Campylobacter infections reported among Minnesota residents in 1994. These 673 isolates were grouped into 248 distinct PFGE patterns, 74% of which were represented by only 1 or 2 isolates. Routine epidemiologic methods identified 2 outbreaks and 9 other case clusters involving 4% of all isolates. Use of PFGE revealed 8 more temporal clusters involving 9% of all isolates. Cases that could not be linked with other cases by PFGE pattern, time, and geographic location accounted for 87% of reported isolates. In this instance, use of PFGE increased the identification of clusters that suggested potential outbreaks; however, the large degree of diversity in PFGE patterns among isolates limits the usefulness of PFGE for outbreak detection when resources are scarce. In a recent survey of retail chicken products in Minnesota, Campylobacter isolates were subtyped by restriction fragment length polymorphism (RFLP) analysis of the flagellin gene amplified by polymerase chain reaction (PCR) [4]. Twelve distinct subtypes were recovered from 13 chicken products positive for Campylobacter species; however, up to 3 subtypes were identified per product. The effect of this Campylobacter strain variability on chicken was demonstrated during the 1998 investigation of a large Campylobacter outbreak that occurred in Minnesota as a result of cross-contamination from raw chicken to chopped lettuce. Of 30 confirmed cases associated with this outbreak that were subtyped by PCR-RFLP, 3 distinct RFLP patterns were detected along with 3 untypeable isolates. This variability on the product limits the potential for molecular subtype–specific surveillance of Campylobacter infections to identify potential common sources among sporadic cases. PulseNet provides the backbone for a foodborne disease surveillance system that has the potential to improve the sensitivity, specificity, and timeliness of outbreak detection and investigation. However, to reach this potential, all states need to have a system to collect isolates from clinical laboratories, subtype them when they are received, and perform epidemiologic follow-up of individual cases. This type of rapid investigation of E. coli O157:H7 and Salmonella species is not currently conducted in most states. Rather than try to fit pathogens such as Campylobacter into PulseNet, it would be prudent to invest in the public health infrastructure needed to improve PulseNet surveillance for established priorities, such as E. coli O157:H7 and Salmonella Typhimurium.